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All The Ways I Broke My Website

from Ian's Blog [alt+shift+b] in technology

This post is a living diary of all the times I messed up something with my website in a funny way. I value those who have the confidence to own their mistakes and share the learning with others, and so this is me doing just that! That Time I Accidentally Made a Tarpit That Time I Accidentally Made Really Large Headers That Time I Accidentally Made a Tarpit Back to Top A "tarpit" is an unofficial term used in computing to describe an intentionally slow response to a request. In these modern times many people are using tarpits as a way to combat the relentless theft of data by AI companies, although there's little to no evidence of that actually being in any way effective. I don't use tarpits, at least not intentionally, but there was that one time when I accidentally created a tarpit and trapped all visitors in it. As I've shared previously, I refuse connections from IP addresses that are blocked or belong to a blocked subnet, and I enforce this firewall during the TCP handshake. The logic here is straightforward: there's no reason to waste resources doing a TLS handshake, accepting an HTTP request, and then rejecting the connection if I already know I'm going to reject it at the earliest step. At the time, the code worked like this: the HTTP server would repeatedly call the Accept() function below expecting a new connection. I've added some comments to help explain the logic. func (l *firewallListener) Accept() (net.Conn, error) { // Accept the connection from the TCP listener. This blocks until there is a connection to accept or the listner was closed. conn, err := l.l.AcceptTCP() if err != nil { return conn, err } // Separate the IP address out from the remote address (which includes the port) ip := utils.SocketStringToIPAddress(conn.RemoteAddr().String()) if ip == nil { return nil, nil } // Check if it's blocked, if so close the connection and return a refuseError if IsBlocked(ip, true) { conn.Close() return nil,...
2 weeks ago

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What's New in TLS Inspector 3.0

On March 30th 2016 the first commit of TLS Inspector, back then called Certificate Inspector, was made - and just shy of two weeks later version 1.0 was released. Now, 10 years later, TLS Inspector celebrates its 10th year, and I've got some exciting changes to share with you all. A Not-So-Quick History The original idea for TLS Inspector came about when I was travelling on a passenger ferry between Vancouver Island and mainland Vancouver. The operator, BC Ferries, provided free WiFi at the time, which was useful as cell service is spotty out in the water. However, this free WiFi wasn't working right, as all TLS Connections were failing with untrusted certificates (even though no captive portal was being presented). I looked in the App Store for any app that could help see what was going on, and found nothing but a one, very limited app filled with in-app-purchases. "I could probably write something like this myself", I thought, and so began TLS Inspector. In 2017 the app caught the attention of cryptologist Kenn White, who reached out to me and provided invaluable feedback. After a short and intensive development cycle, TLS Inspector graduated from a small proof-of-concept into the tool it is today, with release 1.5 being the first where the app gained widespread use. Originally named "Certificate Inspector", the app was renamed to "TLS Inspector" after DigiCert issued a cease-and-desist over the use of the name. In many ways, I felt that this was a good thing in the end, as the new name better reflects what the app is truly about - inspecting the entire TLS connection - not just certificates. Modernization In August 2023 I began work on implementing support for DNS over HTTPS within the app, however this feature never came to realization. Instead, my attention shifted into focusing this new-found DNS energy towards a new app, DNS Inspector. Even though TLS Inspector never got this feature, I did gain one piece of valuable insight from the effort; the realization that TLS Inspector's codebase was quickly aging, and was at serious risk of being left behind. When TLS Inspector was first released, Swift was still a very new language, and those familiar with Swift's early days will recount the growing pains it had, where every year there were major breaking changes to the language and APIs. In addition, Swift did not offer functional interoperability with C for many years - this made using Swift in TLS Inspector anywhere from impossible to incredibly difficult. As a result, Objective-C was used. However by 2019 Swift has significantly matured, and so I was comfortable splitting the app into two components - a Swift-based UI and an Objective-C based backend. Despite its maturity, its C interoperability was still very new and, frankly, very bad, so this design made sense. Apple has been investing an immense amount of time and effort into evolving and growing Swift, and they could not be more clear that the future of Apple Development is exclusively on Swift - SwiftUI is a great example of this. DNS Inspector was my first official SwiftUI app, and served as a validation test for both me and the platform, to answer the question of "do I feel it's ready?", and the answer was yes. Today we finally mark the completion of our transition to Swift, as TLS Inspector 3.0 written 100% in Swift, including all interaction with C libraries. Three-Point-O Let's get one thing clear right away: the experience you've come to expect from TLS Inspector is not changing. This effort to completely rewrite the app was also done with a strong desire to not change how the app worked. I, like many of you, dislike when a tool you may rely on suddenly changes how it works. Tradespeople don't just wake up one morning to suddenly find that their hammer had a software update and now it works entirely differently, why should TLS Inspector? I'm making a point of stressing this because I'm being quite serious when I say that this update is a total rewrite of the application. Nearly every component you see below has been substantially to entirely rewritten. And no, before you ask, the phrase "biting off more than you can chew" means nothing to me. TLSUI Before I get into talking about what changes I made, I want to clarify what I mean when I talk about things like SwiftUI or a "Swift-based user interface", because those sound like the same thing. So, buckle up, because it's time to learn about UIKit. Ask anyone who'd developed a native desktop application for macOS (no, not Electron) and they will absolutely gush about how much they love AppKit, and you can count me among one of those people! When Apple was building iPhoneOS (which later became iOS), they naturally wanted to reuse what they could from OS X, AppKit being no exception. However, as countless designers will tell you, user interfaces designed for touch screens absolutely do not work for traditional keyboard-and-mouse PCs. Apple knew this, and forked AppKit over to UIKit so the two could diverge cleanly. UIKit was, and I suppose still technically is, how you make native user interfaces on iOS apps. Apple provides you with three ways to work with UIKit: Do it entirely in code by yourself Use Xcode Interface Builder (.nib and .xib) Use Storyboards Storyboards are just a fancy way of joining together xibs, so realistically 2 and 3 are the same thing. When referring to the "Swift-based User Interface" of TLS Inspector before version 3.0, I'm referring to option #3 above. TLS Inspector previously used storyboards with UIKit for the entire UI. Initially being written in Objective-C and later ported over to use Swift. SwiftUI, however, takes a different approach. SwiftUI is Apple's attempt at modernizing the user interface tooling provided to developers by dropping interface builders and storyboards, and going all-in on declarative interfaces. As mentioned earlier, DNS Inspector was a test for me to see if I felt that SwiftUI was ready for da big leagues (as we say in the biz). The answer was a confident mostly sorta kinda yes. It's very clear that Apple is pouring a lot of resources into building out SwiftUI, and with every iOS release it gets better! The problem is that with every iOS release Apple changes things, and I hate when they do that! Unlike when Apple releases a new version of The Swift Language, SwiftUI is a runtime framework, which makes supporting old iOS versions quite annoying. Although, to be fair, I'm sure people found AppKit annoying during its early years. I only hopped on the UIKit train after it had almost 10 years to mature. SwiftUI has a lot of extremes. On one hand, it makes creating List-based interfaces (which used UITableView before) such a breeze, but at the same exact time there's bizarre limitations where your best bet is to just abandon SwiftUI and fall back to UIKit elements. Anyways, enough history and preamble, let's finally talk about the new user interface: As I said about 8 billion words ago, the goal here was not to dramatically change TLS Inspector - just to give it a gussy up, at most. The Main View The main view of the app has been tidied up, primarily by removing the Tip section. Let's be real, you don't need to be reminded that you can inspect websites from other apps every single time you use TLS Inspector. The advanced inspection parameters can be accessed by tapping on the gear (or cog, if you speak metric). Response View The response views are largely unchanged aside from adding more information. The app now highlights the root certificate in the chain as well as performs a reverse DNS lookup on the remote address. The trust details view has been tidied up to provide more direct information about the trust situation. Additionally, specific trust failures will also have what I call "Safety Warnings", which highlight when common indicators-of-compromise have been detected, directing users to engage with Apple support or a trusted friend. A "quick exit" button will immediately exit the app. Certificate Information The certificate information screen has been somewhat reorganized, moving the alternate names right to the top, and improving support for new "subject-less" certificates such as those issued by Let's Encrypt. In addition, there's several new datapoints provided: The results from CRL or OCSP checks are now displayed The source of the certificate is displayed (if the certificate was sent by the server or found on the device) All X.509 extensions are displayed, even those not understood by the app TLSKit TLSKit is the name of the "Backend" of the app - this is a Swift Package that performs all of the inspection tasks, collects and presents the results. Previously TLS Inspector used CertificateKit, which was a classic Objective-C based framework for accomplishing the same goals. However, CertificateKit contained some of the oldest and (in my opinion) worst code of the app. It was a frequent source of crashes and locked me out from enjoying new features with Swift Packages since it used legacy code and legacy tools. TLSKit is more than just a Swfit-rewrite of CertificateKit, however, because I took this as a perfect oppertunity to go back to the drawing board and rethink how the backend should work given how the app works today. Remember, CertificateKit contained some of the oldest code, back from before I really knew what I was doing or what I wanted TLS Inspector to be. External Dependencies A major hurdle that proved as a total "Go/No Go" test for this rewrite was the ability to use external dependencies in a Swift package, namely Curl and OpenSSL. After doing some initial research it appeared as if Apple had finally added proper Foreign function interface (FFI) support in Swift. This means one could directly invoke C function calls in these libraries within Swift without the need for an Objective-C bridge. However, in my effort to utilize Swift Packages, I needed to find a way to link to precompiled static libraries. To solve this puzzle, I reached out on the Apple Support forums, and with help from the fine folks there we were able to untangle this mess and get things working. An exciting prospect I plan to work on after 3.0 has stablized is moving away from OpenSSL towards using WolfSSL. This will also unlock the ability to inspect QUIC connections, something not currently possible in the app. It Takes A Village To say this was a massive undertaking for just one guy who does this part time is an understatement, and it's clear that I never could have done this alone. I cannot wrap things up here without giving my sincerest of thank you to the following: David (aka Catfish_Man), for their incredible patience and guidance helping me with the Swift 6 transition. Quinn (aka "The Eskimo"), for repeatedly helping me get through difficult tooling challenges on the Apple developer forums. All of the people who helped test the app. You helped me catch some real nasty bugs early and provided such great feedback. ChatGP- nah just kidding. could you imagine though? lol.

11th Apr 2026 • 1 votes
Identifying Counterfeit AirPods Pro 2

Turns out buying Air Pods from eBay is risky, who knew? Well, clearly I didn't because I totally fell for one. I was looking to pick up some used AirPods Pro 2 to use while out on walks or whenever my usual over-ears wouldn't be comfortable. I found a decent-enough looking listing that was selling a "opened and used once, but otherwise new" set for only 30% off what Apple is asking. Seems fair, right? Wrong. What arrived in my postbox were counterfeits, and I'm left out of pocket (at least, until eBay processes my refund request). In the meantime, I went and did what I should've done from the start and just bought a genuine pair from a retailer, partly because I still actually want a pair, but now also because I want to compare the two side-by-side and show you all the differences. The Packaging The front of the counterfeit box is actually very well done. The printing is close enough to the real one that without a genuine to compare against you'd have a really hard time telling if its fake. The box is even embossed correctly. Remember that these were sold to me as having been opened, so the fact the box is a little smushed doesn't really mean anything. The rear of the boxes is where we start to begin to see differences, although nothing immediately telling of a fake without a genuine box to compare against. It's interesting that the fake box does not show the USB-C cable like the genuine one does. Note: I had already unsealed the fake box. The box arrived sealed and in plastic shrink-wrap. The genuine box did not come in plastic wrap. The bottoms of the boxes begin to show some of the key differences between a real and fake box. On the real box, Apple has printed all information to the box itself, including the serial number. The fake box uses stickers for everything. The contents of the box don't give away too many clues. I had unwrapped the included charging cable myself for reasons you'll see later, it was wrapped when I first opened it. The bud bits additional tips contain a few clues, notably the worse printing the size labels and the lack of the indentions in the packaging. The Charging Case The exterior of the case reveals no clues. The only visual difference is the difference in assembly location (which I wouldn't be comfortable saying identifies it as a fake), and the different typeface used for the labels. This is very hard to identify without a known-good comparison. However, while impossible to demonstrate with pictures, the feel of the fake case is notably worse than the real one. The fake case rattles when you shake it and sounds hollow when you tap it. The real one feels dense with no rattles. Inside the case looking at the serials we start to see some more key differences. While my phone's camera is doing no favours to the image, the printing quality on the fake pods is notably worse. In addition, note the lack of input and output power requirements The Individual Pods This is where the counterfeits really begin to standout. Whoever made this case did a really good job, but whoever did the pods did not. Genuine AirPods have vents on and around the body. These aren't just for style, and are actual grills. Fake AirPods, like mine, just use styled plastic fillers. In addition, the pre-installed tip should include its size embossed into the rubber Lastly, a very important thing to check, the serial number of each individual pod should be unique - it should not match the serial of the charging case. These are three individual products each with their own serials. Apple should label all 3 on the box, instead the only label the charging case. The Hard Evidence Taking the serial number of the charging case, M6WYX2HJKR, and putting it into Apple's warranty check tool, I am presented with irrefutable evidence of the forgery: USB-C, you say?

14th May 2025 • 1 votes
Securing My Web Infrastructure

Securing My Web Infrastructure A few months ago, I very briefly mentioned that I've migrated all my web infrastructure off Cloudflare, as well as having built a custom web service to host it all. I call this new web service WebCentral and I'd like to talk about some of the steps I've taken and lessons I've learned about how I secure my infrastructure. Building a Threat Model Before you can work to secure any service, you need to understand what your threat model is. This sounds more complicated than it really is; all you must do is consider what your risks how, how likely those risks are to be realized, and what the potential damage or impact those risks could have. My websites don't store or process any user data, so I'm not terribly concerned about exfiltration, instead my primary risks are unauthorized access to the server, exploitation of my code, and denial of service. Although the risks of denial of service are self-explanatory, the primary risk I see needing to protect against is malicious code running on the machine. Malicious actors are always looking for places to run their cryptocurrency miners or spam botnets, and falling victim to that is simply out of the question. While I can do my best to try and ensure I'm writing secure code, there's always going to be the possibility that I or someone else makes a mistake that turns into an exploitable weakness. Therefore, my focus is on minimizing the potential impact should this occur. VPS Security The server that powers the very blog you're reading is a VPS, virtual private server, hosted by Azure. A VPS is just a fancy way to say a virtual machine that you have mostly total control over. A secure web service must start with a secure server hosting it, so let's go into detail about all the steps I take to keep the server safe. Network Security Minimizing the internet-facing exposure is critical for any VPS and can be one of the most effective ways to keep a machine safe. My rule is simple, no open ports other than what is required for user traffic. In effect this only means one thing: I cannot expose SSH to the internet. Doing so protects me against a wide range of threats and also reduces the impact from scanners (more on them later). While Azure itself offers several of ways to interact with a running VPS, I've chosen to disable most of those features and instead rely on my own. I personally need to be able to access the machine over SSH, however, so how do I do that if SSH is blocked? I use a VPN. On my home network is a WireGuard VPN server as well as a Dynamic DNS setup to work-around my rotating residential IP address. The VM will try to connect to the WireGuard VPN on my home network and establish a private tunnel between them. Since the VM is the one initiating the connection (acting as a client) no port must be exposed. With this configuration I can effortlessly access and manage the machine without needing to expose SSH to the internet. I'm also experimenting with, but have not yet fully rolled out, an outbound firewall. Outbound firewalls are far, far more difficult to set up than inbound because you must first have a very good understanding of what and where your machine talks to. OS-Level Security Although the internet footprint of my VPS is restricted to only HTTP and HTTPS, I still must face the risk of someone exploiting a vulnerability in my code. I've taken a few steps to help minimize the impact from a compromise to my web application's security. Automatic Updates First is some of the most basic things everyone should be doing, automatic updates & reboots. Every day I download and install any updates and restart the VPS if needed. All of this is trivially easy with a cron job and built-in tooling. I use this script that runs using a cron job: #!/bin/bash # Check for updates dnf check-update > /dev/null if [[ $? == 0 ]]; then # Nothing to update exit 0 fi # Install updates dnf -y update # Check if need to reboot dnf needs-restarting -r if [[ $? == 1 ]]; then reboot fi Low-Privileged Accounts Second, the actual process serving traffic does not run as root, instead it runs as a dedicated service user without a shell and without sudo permission. Doing this limits the abilities of what an attacker might be able to do, should they somehow have the ability to execute shell code on the machine. A challenge with using non-root users for web services is a specific security restriction enforced by Linux: only the root user can bind to port at or below 1024. Thankfully, however, SystemD services can be granted additional capabilities, one of which is the capability to bind to privileged ports. A single line in the service file is all it takes to overcome this challenge. Filesystem Isolation Lastly, the process also uses a virtualized root filesystem, a process known as chroot(). Chrooting is a method where the Linux kernel effectively lies to the process about where the root of the filesystem is by prepending a path to every call to access the filesystem. To chroot a process, you provide a directory that will act as the filesystem root for that process, meaning if the process were to try and list of contents of the root (/), they'd instead be listing the contents of the directory you specified. When configured properly, this has the effect of an filesystem allowlist - the process is only allowed to access data in the filesystem that you have specifically granted for it, and all of this without complicated permissions. It's important to note, however, that chrooting is often misunderstood as a more involved security control, because it's often incorrectly called a "jail" - referring to BSD's jails. Chrooting a process only isolates the filesystem from the process, but nothing else. In my specific use case it serves as an added layer of protection to guard against simple path transversal bugs. If an attacker were somehow able to trick the server into serving a sensitive file like /etc/passwd, it would fail because that file doesn't exist as far as the process knows. For those wondering, my SystemD service file looks like this: [Unit] Description=webcentral After=syslog.target After=network.target [Service] # I utilize systemd's heartbeat feature, sd-notify Type=notify NotifyAccess=main WatchdogSec=5 # This is the directory that serves as the virtual root for the process RootDirectory=/opt/webcentral/root # The working directory for the process, this is automatically mapped to the # virtual root so while the process sees this path, in actuality it would be # /opt/webcentral/root/opt/webcentral WorkingDirectory=/opt/webcentral # Additional directories to pass through to the process BindReadOnlyPaths=/etc/letsencrypt # Remember all of the paths here are being mapped to the virtual root ExecStart=/opt/webcentral/live/webcentral -d /opt/webcentral/data --production ExecReload=/bin/kill -USR2 "$MAINPID" TimeoutSec=5000 Restart=on-failure # The low-privilege service user to run the process as User=webcentral Group=webcentral # The additional capability to allow this process to bind to privileged ports CapabilityBoundingSet=CAP_NET_BIND_SERVICE [Install] WantedBy=default.target To quickly summarize: Remote Access (SSH) is blocked from the internet, a VPN must be used to access the VM, updates are automatically installed on the VM, the web process itself runs as a low-privileged service account, and the same process is chroot()-ed to shield the VMs filesystem. Service Availability Now it's time to shift focus away from the VPS to the application itself. One of, if not the, biggest benefits of running my own entire web server means that I can deeply integrate security controls how I best see fit. For this, I focus on detection and rejection of malicious clients. Being on the internet means you will be constantly exposed to malicious traffic - it's just a fact of life. The overwhelming majority of this traffic is just scanners, people going over every available IP address and looking widely known and exploitable vulnerabilities, things like leaving credentials out in the open or web shells. Generally, these scanners are one and done - you'll see a small handful of requests from a single address and then never again. I find that trying to block or prevent these scanners is a bit of a fool's errand, however by tracking these scanners over time I can begin to identify patterns to proactively block them early, saving resources. Why this matters is not because of the one-and-done scanners, but instead the malicious ones, the ones that don't just send a handful of requests - they send hundreds, if not thousands, all at once. These scanners risk degrading the service for others by occupying server resources that would better be used for legitimate visitors. To detect malicious hosts, I employ some basic heuristic by focusing on the headers sent by the client, and the paths they're trying to access. Banned Paths Having collected months of data from the traffic I served, I was able to identify some of the most common paths these scanners are looking for. One of the more common treds I see if scanning for weak and vulnerable WordPress configurations. WordPress is an incredibly common content management platform, which also makes it a prime target for attackers. Since I don't use WordPress (and perhaps you shouldn't either...) this made it a good candidate for scanner tracking. Therefore, any request where the path contains any of: "wp-admin", "wp-content", "wp-includes", or "xmlrpc.php" are flagged as malicious and recorded. User Agents The User Agent header is data sent by your web browser to the server that provides a vague description of the browser and the device it's running on. For example, my user agent when I wrote this post is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:128.0) Gecko/20100101 Firefox/128.0 All this really tells the server is that I'm on a Mac running macOS 15 and using Firefox 128. One of the most effective measures I've found to block malicious traffic early is to do some very basic filtering by user agent. The simplest and most effective measure thus far has been to block requests that have no user agent header. I also have a growing list of bogus user agent values, where the header looks valid - but if you check the version numbers of the system or browser, nothing lines up. IP Firewall When clients start getting a bit too rowdy, they get put into the naughty corner temporarily blocked from connecting. Blocked connections happen during the TCP handshake, saving resources as we skip the TLS negotiation. Addresses are blocked 24 hours, and I found this time to be perfectly adequate as most clients quickly give up and move on. ASN Blocks In some extreme situations, it's necessary to block entire services and all of their addresses from accessing my server. This happens when a network provider, such as an ISP, VPN, or cloud provider, fails to do their job in preventing abuse of their services and malicious find home there. Cloud providers have a responsibility to ensure that if a malicious customer is using their service, they would terminate their accounts and stop providing their services. For the most part, these cloud providers do a decent enough job at that. Some providers, however, don't care - at all - and quickly become popular amongst malicious actors. Cloudflare and Alibaba are two great examples. Because of the sheer volume of malicious traffic and total lack of valid user traffic, I block all of Cloudflare and Alibaba's address space. Specifically, I block AS13335 and AS45102. Putting It All Together Summarized, this is the path a request takes when connecting to my server: Upon recieving a TCP connection, the IP address of the client is checked if it's either in a blocked ASN or is individually blocked. If so, the request is quickly rejected. Otherwise, TLS is negotiated, allowing the server to see the details of the actual HTTP request. We then check if the request is for a banned path, or has a banned user agent, if so the IP is blocked for 24 hours and the request is rejected, otherwise the request is served as normal. The Result I feel this graph speaks for itself: This graph shows the number of requests that were blocked per minute. These bursts are the malicious scanners that I'm working to block, and all of these were successful defences against them. This will be a never-ending fight, but that's part of the fun, innit?

17th Apr 2025 • 42 votes
Having a Website Used to Be Fun

According to the Wayback Machine, I launched my website over a decade ago, in 2013. Just that thought alone makes me feel old, but going back through the old snapshots of my websites made me feel a profound feeling of longing for a time when having a website used to be a novel and enjoyable experience. The Start Although I no longer have the original invoice, I believe I purchased the ianspence.com domain in 2012 when I was studying web design at The Art Institute of Vancouver. Yes, you read that right, I went to art school. Not just any, mind you, but one that was so catastrophically corrupt and profit-focused that it imploded as I was studying there. But thats a story for another time. I obviously didn't have any real plan for what I wanted my website to be, or what I would use it for. I simply wanted a web property where I could play around with what I was learning in school and expand my knowledge of PHP. I hosted my earliest sites from a very used Dell Optiplex GX280 tower in my house on my residential internet. Although the early 2010's were a rough period of my life, having my very own website was something that I was proud of and deeply enjoyed. Somewhere along the way, all that enjoyment was lost. And you don't have to take my own word for it. Just compare my site from 2013 to the site from when I wrote this post in 2024. Graphic Design was Never My Passion 2013's website has an animated carousel of my photography, bright colours, and way too many accordions. Everything was at 100% because I didn't really care about so much about the final product more as I was just screwing around and having fun. 2024's website is, save for the single sample of my photography work, a resume. Boring, professional, sterile of anything resembling personality. Even though the style of my site became more and more muted over the years, I continued to work and build on my site, but what I worked on changed as what I was interested in grew. In the very early 2010s I thought I wanted to go into web design, but after having suffered 4 months of the most dysfunctional Art school there is, I realized that maybe it was web development that interested me. That, eventually, grew into the networking and infrastructure training I went through, and the career I've built for myself. Changing Interests As my interests changed, my focus on the site became less about the design and appearance and more towards what was serving the site itself. My site literally moved out from the basement suite I was living in to a VPS, running custom builds of PHP and Apache HTTPD. At one point, I even played around with load-balancing between my VPS and my home server, which had graduated into something much better than that Dell. Or, at least until my internet provider blocked inbound port 80. This is right around when things stopped being fun anymore. When Things Stopped Being Fun Upon reflection, there were two big mistakes I made that stole all of the enjoyment out of tinkering with my websites: Cloudflare and Scaling. Cloudflare A dear friend of mine (that one knows I'm referring to it) introduced me to Cloudflare sometime in 2014, which I believe was the poison pill that started my journey into taking the fun out of things for me. You see, Cloudflare is designed for big businesses or people with very high traffic websites. Neither of which apply to me. I was just some dweeb who liked PHP. Cloudflare works by sitting between your website's hosting server and your visitors. All traffic from your visitors flows through Cloudflare, where they can do their magic. When configured properly, the original web host is effectively hidden from the web, as people have to go through Cloudflare first. This was the crux of my issues, at least with the focus of this blog post, with Cloudflare. For example, before Lets Encrypt existed, Cloudflare did not allow for TLS at all on their free plans. This was right at the time when I was just starting to learn about TLS, but because I had convinced myself that I had to use Cloudflare, I could never actually use it on my website. This specific problem of TLS never went away, either, as even though Cloudflare now offers free TLS - they have total control over everything and they have the final say in what is and isn't allowed. The problems weren't just TLS, of course, because if you wanted to handle non-HTTP traffic then you couldn't use Cloudflare's proxy. Once again, I encountered the same misguided fear that exposing my web server to the internet was a recipe for disaster. Scaling (or lack thereof) The other, larger, mistake I made was an obsession with scaling and security. Thanks to my education I had learned a lot about enterprise networking and systems administration, and I deigned to apply those skills to my own site. Aggressive caching, using global CDNs, monitoring, automated deployments, telemetry, obsession over response times, and probably more I'm not remembering. All for a mostly static website for a dork that had maybe 5 page views a month. This is a mistake that I see people make all the time, not even just exclusive for websites - people think that they need to be concerned about scaling problems without first asking if those problems even or will ever apply to them. If I was able to host my website from an abused desktop tower with blown caps on cable internet just fine, then why in the world would I need to be obsessing over telemetry and response times. Sadly things only got even worse when I got into cybersecurity, because now on top of all the concerns with performance and scale, I was trying to protect myself from exceedingly unlikely threats. Up until embarrassingly recently I was using complicated file integrity checks using hardware-backed cryptographic keys to ensure that only I could make changes to the content of my site. For some reason, I had built this threat model in my head where I needed to protect against malicious changes on an already well-protected server. All of this created an environment where making any change at all was a cumbersome and time-consuming task. Is it any wonder why I ended up making my site look like a resume when doing any changes to it was so much work? The Lesson I Learned All of this retrospective started because of the death of Cohost, as many of the folks there (myself included) decided to move to posting on their own blogs. This gave me a great chance to see what some of my friends were doing with their sites, and unlocking fond memories of back in 2012 and 2013 when I too was just having fun with my silly little website. All of this led me down to the realization of the true root cause of my misery: In an attempt to try and mimic what enterprises do with their websites in terms of stability and security, I made it difficult to make any changes to my site. Realizing this, I began to unravel the design and decisions I had made, and come up with a much better, simpler, and most of all enjoyable design. How my Website Used to Work The last design of my website that existed before I came to the conclusions I discussed above was as follows: My website was a packaged Docker container image which ran nginx. All of the configuration and static files were included in the image, so theoretically it could run anywhere. That image was being run on Azure Container Instances, a managed container platform, with the image hosted on Azure Container Registries. Cloudflare sat in-front of my website and proxied all connections to it. They took care of the domain registration, DNS, and TLS. Whenever I wanted to make changes to my site, I would create a container image, sign it using a private key on my YubiKey, and push that to Github. A Github action would then deploy that image to Azure Container Registries and trigger the container to restart, pulling the new image. How my Website Works Now Now that you've seen the most ridiculous design for some dork's personal website, let me show you how it works now: Yes, it's really that simple. My websites are now powered by a virtual machine with a public IP address. When users visit my website, they talk directly to that virtual machine. When I want to make changes to my website, I just push them directly to that machine. TLS certificates are provided by Lets Encrypt. I still use Cloudflare as my domain registrar and DNS provider, but - crucially - nothing is proxied through Cloudflare. When you loaded this blog post, your browser talked directly to my server. It's almost virtually identical to the way I was doing things in 2013, and it's been almost invigorating to shed this excess weight and unnecessary complications. But there's actually a whole lot going on behind the scenes that I'm omitting from this graph. Static Websites Are Boring A huge problem with my previous design is that I had locked myself into only having a pure static website, and static websites are boring! Websites are more fun when they can, you know, do things, and doing things with static websites is usually dependent on Javascript and doing them in the users browser. That's lame. Let's take a really simple example: Sometimes I want to show a banner on the top of my website. In a static-only website, unless you hard-code that banner into the HTML, doing this from a static site would require that you use JavaScript to check some backend if a banner should be displayed, and if so - render it. But on a dynamic page? Where server-side rendering is used? This is trivially easy. A key change with my new website is switching away from using containers, which are ephemeral and immutable, over to using a virtual machine, which isn't, and to go back to server-side rendering. I realized that a lot of the fun I was having back then was because PHP is server-side, and you can do a lot of wacky things when you have total control over the server! I'm really burying the lede here, but my new site is powered not by nginx, or httpd, but instead my own web server. Entirely custom and designed for tinkering and hacking. Combined with no longer having to deal with Cloudflare, has given me total control to do whatever the hell I want. Maybe I'll do another post some day talking about this - but I've written enough about websites for one day. Wrapping Up That feeling of wanting to do whatever the hell I want really does sum up the sentiment I've come to over the past few weeks. This is my website. I should be able to do whatever I want with it, free from my self-imposed judgement or concern over non-issues. Kill the cop in your head that tells you to share the concerns that Google has. You're not Google. Websites should be fun. Enterprise tools like Cloudflare and managed containers aren't.

27th Oct 2024 • 42 votes

More in technology

Inside a 1980s filter chip that uses switched capacitors

Sometimes it's easier to identify an IC with a microscope. While sorting a box of old ICs, CuriousMarc came across some Harris ICs labeled "F1-10-5", a mysterious part number that didn't show up in any databooks. Since unidentifiable ICs are useless, he gave me one to analyze. Conveniently, it was in a ceramic package, so I could open it up with a quick tap from a chisel. Under the microscope, the chip's most striking feature was a grid of square capacitors. With all those capacitors, I guessed that it was a switched-capacitor filter. The die provided another clue: the part number HF-10. With this information, we quickly found that the chip was Harris's version of the standard MF10 switched-capacitor filter chip.1 The Harris integrated circuit, labeled F1-10-5 (or maybe FI-10-5), with a 1985 date code. Photo courtesy of CuriousMarc. Switched-capacitor filters were a popular way to implement analog filters in the 1980s. Rapidly switching capacitors in and out of a circuit enabled the construction of single-chip filters that were easy to use and performed well. The MF10, introduced by National Semiconductor in 1981, provides two flexible filters on a chip; each filter acts as a low-pass filter, band-pass filter, or a high-pass filter. The filter's characteristics are simple to control with a few external resistors. The Harris HF-10 die under the microscope with the main functional blocks labeled. (Click for a larger image.) Since I had the chip under the microscope, I took the opportunity to analyze it more closely. The white lines are the metal wiring that connects the chip's circuitry. Under the metal layer are two layers of polysilicon (reddish) and the underlying silicon (gray). The top and bottom halves of the chip are mostly mirror images, corresponding to the chip's two filters. The distinctive reddish squares in the middle of the chip are 72 tiny capacitors, constructed from polysilicon. Above the capacitors, CMOS switches turn on and off at the clock frequency, switching capacitors in and out of the circuit. Each filter uses three operational amplifiers (op amps), outlined in red. At the right are the three outputs from the three op amps: high pass, band pass, and low pass. The control circuitry is on the left: clock level shifting, clock shaping, frequency ratio handling, startup circuitry, and current sinks to provide fixed currents to other parts of the chip. Around the edges of the silicon die, 20 hair-thin bond wires connect the die to its 20 external pins. The die has some interesting chip art: a Harris logo and an outline of Florida; Harris was headquartered in Melbourne, Florida. The initials on the die are presumably the engineers who designed the chip. Some interesting images from the die. Switched capacitor circuits The filter is based on switched-capacitor circuits. A switched capacitor can replace a resistor in certain circuits, as shown below. The switches are controlled by a clock signal; the switches alternately close in clock phase 1 and phase 2 (ϕ1 and ϕ2). In phase 1, the capacitor is charged to the input voltage. In phase 2, the capacitor passes charge to the output. By rapidly toggling the switches, charge is (almost) steadily passed to the output. The larger the capacitance, the more charge that is passed through. Likewise, a higher frequency passes more charge. It can be shown that the circuit matches a resistor with resistance of 1/(fC): a higher capacitance and frequency correspond to lower resistance. A switched capacitor can replace a resistor. Why would you replace a simple resistor with this complicated switching circuit? In an integrated circuit, resistors are inaccurate and inconveniently large, especially high-value resistors. Replacing a large resistor with a small capacitor saves space on the die. Moreover, it is easy to generate an extremely accurate clock frequency with an inexpensive quartz crystal, making the filter's frequency highly accurate. Finally, the equivalent resistance can be changed simply by changing the clock frequency, making it easy to tune or sweep the filter. On-chip capacitors are fairly inaccurate, with the capacitance typically varying by 20% from chip to chip due to variations in manufacturing conditions. However, this isn't a problem in the MF10 because the circuitry was designed to depend on the ratio between capacitances, which is stable. Specifically, the MF10 uses 72 identical square capacitors, which will have almost identical capacitances. Careful examination shows that some of the capacitors are separate, while others are connected in groups of 8 to form larger capacitors.2 This yields a highly accurate ratio of 8:1 between the grouped capacitors and the individual capacitors, even though the absolute capacitance will vary from chip to chip. Each capacitor is constructed from two layers of polysilicon,3 forming the plates of the capacitor, separated by a thin layer of insulating oxide that acts as the dielectric. I estimate that each capacitor square is 5 picofarads. The grid of capacitors in the MF10. I've added yellow lines to show how the capacitors are grouped. The switches are above and below the capacitors. This chip uses one more trick with switched capacitors: it inverts the voltage while acting as a resistor. In the switched-capacitor circuit below, there are four switches. The capacitor charges to the input voltage during phase 1, the same as before. But duing phase 2, note that the top plate of the capacitor is grounded, while the output comes from the bottom plate. If the capacitor was charged to, say, 1 volt, the top plate is 1 volt above the bottom plate. So if the top plate is grounded, then the bottom plate must be at -1 V. (This is the same idea as a charge pump.) This circuit turns out to yield a more accurate filter because some parasitic capacitances cancel out. By using four switches, the switched capacitor can invert the voltage. The op-amp integrator The heart of most analog circuits is the operational amplifier, or op-amp. An op-amp takes two inputs and amplifies the difference by many orders of magnitude. Normally, an op-amp is configured with negative feedback, which forces the two inputs to be essentially the same. Op-amps are useful not only for amplification, but for filtering, buffering, summing, and other tasks. A basic op-amp integrator. The filter chip uses op-amps as integrators, to integrate an input voltage over time. The circuit above shows a simple op-amp integrator. The input voltage produces a current that flows through the resistor and charges the capacitor, so the capacitor holds the integral of the input voltage over time. You might expect that the left side of the capacitor would become positive as it charges. However, the op-amp's feedback forces both inputs to ground, so instead the right side of the capacitor becomes negative. Thus, the output is the negative integral.4 The MF10 chip uses the circuit above, except the resistor is replaced with a switched capacitor. The capacitor across the op-amp is not switched, but consists of either 8 or 16 capacitors from the capacitor grid. The CMOS switches The CMOS switch is the technology that makes the switched-capacitor filter possible. A CMOS switch has a fairly low resistance (maybe tens of ohms) when closed and an enormously high resistance (hundreds of megohms) when open. This high resistance ensures that the charge doesn't leak out of the capacitors. A CMOS switch is constructed by combining an NMOS transistor and a PMOS transistor. The NMOS transistor and PMOS transistor are opposites. An NMOS transistor is good at pulling the output low, while a PMOS transistor is good at pulling the output high, so in combination they provide an effective switch. An NMOS transistor is turned on by a high voltage on the gate, while a PMOS transistor is turned on by a low voltage on the gate. Thus, a CMOS switch requires two control signals of opposite polarity, which is a minor inconvenience. A CMOS switch. The diagram above shows how a switch is implemented with an NMOS transistor and a PMOS transistor in parallel. When the control line is high, and the inverted control line is low, both transistors turn on, providing a path through the switch circuit. When the control line is low (and the inverted line high), the transistors turn off, opening the switch. The chip uses CMOS switches in pairs, with one switch on and the other off. This forms the equivalent of a toggle switch that connects either A or B to the output. This circuit is simply two CMOS switches, with separate control lines for each switch, as shown below. In the MF10, the switch toggles at the clock frequency. During one clock phase, the switch is connected to A, while the switch is connected to B during the other clock phase. The schematic on the right, below, is the same circuit, but reorganized to match the layout on the die. A double-throw CMOS switch. The photo below shows a CMOS switch on the die, constructed from two PMOS transistors and two NMOS transistors. The four control lines run horizontally in polysilicon, forming a transistor gate where they cross doped silicon. The upper PMOS and NMOS transistors are driven by the clock phase 1 (Φ1) signals, while the lower transistors are driven by the phase 2 signals. CMOS switches on the die. The metal layer was removed to show the transistors. One problem with switched-capacitor filters is that the clock can generate switching noise that appears in the chip's outputs. The MF10 uses several techniques to reduce clock noise. Each set of transistors is surrounded by two isolation rings: one positive and one negative. These block noise from traveling through the silicon substrate. Note that the rings have opposite polarity for the NMOS transistors and the PMOS transistors. The light tan region in the photo above is a second layer of polysilicon. This polysilicon is connected to ground, providing a shield layer over the switching circuits. For the photo above, I removed the metal layer with acid5 to make the transistors more visible. The photo below shows the original die, with the metal layer connecting the transistors. The small black circles are connections between the metal layer and silicon or polysilicon. The same CMOS switches, showing the metal layer. Putting it together: the state variable filter There are many ways of creating a filter. The MF10 chip uses a technique called the state variable filter, invented in 1967. This circuit acts as three filters, with high-pass, band-pass, and low-pass outputs. Moreover, the circuit is flexible since the frequency, the gain, and the filter quality (Q) can be varied independently. It uses three op-amps: one to sum signals and two for integration. By changing how the values are summed, the characteristics of the filters can be changed. The diagram below shows a simplified representation of a state variable filter. The mathematics behind a state variable filter is complicated, so I won't get into it. In short, the signal, the integral, and the double integral form the three state variables that define the state of the system. Simplified diagram of a state variable filter, with two integrators. Inspired by North Coast Synthesis. The block diagram below shows how the filter is represented in the MF10 datasheet.6 The diagram is similar to the diagram above, with three op-amps. However, the summing circuitry has been separated out. Moreover, the feedback paths are not shown explictly. Instead, resistors are connected between the chip's external pins (squares) to configure the filter as desired. The mode switch at the top allows the low-pass feedback to be controlled by an external pin (SA/B). Block diagram of one of the filter sections. Adapted from the datasheet. The schematic below is my reverse-engineered schematic of the filter, as implemented on the chip. It closely matches the block diagram, but fills in the details. In the block diagram, the summing circuit (circle) adds one signal and subtracts two signals. This summing circuit is implemented with the three switched capacitors on the left, which act as summing resistors. Note that one switch is grounded during phase 1, while the others are grounded during phase 2; switching the polarity implements addition versus subtraction. The top sum input is either feedback from the low-pass output or ground, selected by an input pin. A CMOS switch is used here, but the switch is static, not clocked, so it doesn't use protection rings and shielding like the other switches. My reverse-engineered schematic of one of the filters. Click this image (or any other) for a larger version. The integrators have switched capacitors on the inputs, acting as resistors. The integration capacitor is either 8 or 16 "squares" of capacitance, selected by a ratio selection pin. This controls the ratio between the clock frequency and the filter frequency, either 50:1 or 100:1.7 Although the integration capacitors are attached to a CMOS switch, the switch is static, so the capacitors act as regular capacitors, not switched capacitors. The op-amps The op-amps are fairly standard CMOS op-amps, built from about 35 transistors. (You might get a lower count if you try counting the transistors below, since some of the blocks are multiple transistors.) The op-amp transistors are much larger than the CMOS switch transistors (very bottom, center). On the die, each op-amp is split into two parts: the differential amplifier on the left and an additional amplification stage on the right. A large capacitor (pinkish) sits between the halves. My first thought was that this was the integration capacitor, but it is just a frequency compensation capacitor, common in many op-amps to stabilize the output. The op-amps also have large transistors next to the output pins; these transistors are functionally part of the op-amps, but located next to the pins to minimize resistance. One of the chip's op-amps. I removed the metal layer to make the transistors visible. One unusual feature of the op-amps is a low-power mode. Pulling a particular IC pin low causes the chip to stop filtering and enter a low-power mode, reducing power consumption by 70%. This is implemented by shutting down the "current mirror" circuits that provide fixed currents to the op-amps and other parts of the chip. The non-overlapping clock generator The MF10 chip is driven by external clock signals, one for each filter, with the frequency of the filter proportional to the clock frequency. The photo of the CMOS switches earlier showed that the clock drives four control lines for the switches. You might think that two control lines would be sufficient: the clock and the inverted clock. The problem is that it is very important to avoid having both switches closed at the same time, even for a moment, as that will short the inputs and corrupt the signals. Instead, the two switches have separate control lines that enforce a small gap between when one switch opens and the other one closes. This is implemented with the circuit below that takes an input clock signal and produces the four outputs that drive the switches. The circuit to generate non-overlapping clock signals. There is a delay between when gate A or B turns on and when the corresponding output changes. The idea behind the circuit is that a phase is blocked from going high until after the other phase goes low, with a pair of inverters providing additional delay. In more detail, suppose the input clock drops from high to low. Gate A will turn off, causing the phase 1 output (ϕ1) to drop after a few gate delays (A delay). Gate B can't turn on until ϕ1 goes low. After additional gate delays, ϕ2 goes high. The behavior is similar when the input clock goes high. Gate B turns off, causing ϕ2 to go low after a delay. This allows gate A to turn on, turning on ϕ1 after more delay. To summarize, after a phase is turned off, there is a delay before the other phase turns on, so the two phases never overlap. The clock-shaping circuitry is implemented with CMOS logic gates. The photo above shows this circuitry under the microscope, with the metal layer removed. The rectangular blocks are doped silicon that forms transistors. The darker regions on the left are NMOS transistors and the lighter regions on the right are PMOS transistors. A CMOS gate consists of NMOS and PMOS transistors working together. The PMOS transistors are larger because PMOS transistors are slightly less efficient than NMOS transistors. The dark circles are contacts between the silicon and the metal layer on top. The copper-colored lines are not metal but a special type of silicon called polysilicon. When a polysilicon line crosses doped silicon, it forms the gate of a transistor. The pinks and greens are due to thin-film interference from a thin layer of oxide that didn't completely dissolve; the silicon is actually gray. The ternary input A weird feature of the chip is the input pin that selects the ratio between the input clock and the filter frequency. In effect, this is a digital input with three values. Tying the pin to the high supply voltage selects a 50:1 ratio. Tying the pin to the midpoint between the supply voltages selects a 100:1 ratio. Pulling the pin to the low supply voltage stops the filter and puts the chip into a low-power mode.8 To handle the three-level input, the input goes through two separate buffers, one that transitions at a lower voltage and one that transitions at a higher voltage. Thus, the two buffers separate the middle signal level. Each buffer consists of a special inverter feeding into a regular inverter. Before explaining the special inverters, I'll review how a regular CMOS inverter works. A CMOS inverter is constructed from a PMOS transistor and an NMOS transistor. When the input is high, the NMOS transistor turns on and pulls the output to ground. When the input is low, the PMOS transistor turns on and pulls the output high. Thus, the input signal is inverted. A CMOS inverter is constructed from a PMOS transistor and an NMOS transistor. In the die photo, you can see the four PMOS transistors (light gray) and four NMOS transistors (darker), forming four inverters. When a polysilicon line (copper-colored) crosses a doped silicon region, it forms the gate of a transistor. For this picture, I dissolved the metal layer in acid so the transistors are visible. The metal layer connected the transistors to complete the wiring of the inverters: it connects the two "out1" contacts to "in2" and connects the two "out2" contacts to the rest of the chip. For the second buffer, "out3" connects to "in4" and so forth. The four inverters that handle the ternary input. I flipped the image to make the orientation better. In this circuit, the length of the transistor gates is varied to make the inverters activate at different voltage levels. Six of the transistor gates are normal (orange arrows); the PMOS gates are wider (in the vertical direction) than the NMOS gates because PMOS transistors are inherently weaker. However, two of the transistor gates are unusually long (horizontal direction, red), making the transistors weak since the current must travel a longer distance. The inverter on the left has a weak PMOS transistor. If the input is high or low, the inverter will operate normally. But if the input is in the middle, both transistors will partially turn on. Since the PMOS transistor is very weak, the NMOS transistor will "win", pulling the output low. Thus, the leftmost inverter treats a medium-level input as a 1, outputting a 0. The third inverter is the opposite; the NMOS transistor has a long, winding gate, so it is weak. In this case, a medium-level input will partially turn on both transistors, but the PMOS transistor will "win", pulling the output high. To summarize, the two inverters have opposite behavior for a middle-level signal, allowing the three input levels to be distinguished. Since the output from a special inverter may be weak, the output goes to a normal inverter to amplify the signal. Conclusions Like most semiconductor companies, Harris has a complicated history. Harris started way back in 1895 as a printing press company. Harris moved into high technology in the 1950s and 1960s, acquiring various radio and electronics companies. In particular, Harris entered the IC business in 1967, when it acquired Radiation, Inc., renaming it Harris Semiconductor a few years later. (We've encountered some Radiation modules in Apollo systems, but I haven't written about them yet.) Harris got out of the semiconductor business in 1999, spinning off Intersil, which was later acquired by the Japanese semiconductor firm Renesas. In 2019, Harris merged with L3 Technologies to become L3Harris, the eighth-largest defense contractor in the US. As for switched-capacitor filters, they have lost popularity as filtering is now more easily done in the digital domain. Texas Instruments acquired National Semiconductor (and the MF10) in 2011; TI's website shows the MF10 as active but expensive and out of stock, so it's probably no longer being manufactured. State variable filters are still used in the synthesizer world both because of their flexibility and because they provide low-pass, band-pass, and high-pass filters in one unit. For more, follow me on Bluesky (@righto.com), Mastodon (@[email protected]), or RSS. Thanks to CuriousMarc for providing the IC. AI statement: Despite the presence of the em dash, no AI was used in the writing of this article (details). Notes and references Once we found the "HF-10" part number, a search turned up a National Semiconductor databook that confirmed that the Harris HF-10 was a direct replacement for the National Semiconductor MF10. It remains a mystery why the Harris chip is externally labeled "F1-10-5" rather than "HF-10". This format doesn't resemble other Harris part numbers. I would suspect a military part number, but it is completely different from the military formats that I've seen on other chips, such as JM38510 numbers or NSN numbers. ↩ You might wonder why the larger capacitors are formed by connecting eight smaller capacitor squares, rather than making one capacitor that is eight times as big. The reason is to get better matching between the two capacitor sizes. A capacitor that is eight times as large won't have exactly eight times the capacitance due to factors such as the behavior of the electric field around the edge of the capacitor, inaccuracies that may make the capacitor slightly larger or smaller than desired, or etching variability around the edges. By building larger capacitors out of identical smaller capacitors, the values can match very well, up to ±0.01% according to The Art of Analog Layout. (With laser trimming, matching of ±0.001% is possible, but that is much more accuracy than the MF10 required.) ↩ Most chips from this era have a single layer of polysilicon, so I was surprised to find two layers in this chip. I've seen two layers of polysilicon before, in the MK4116 DRAM chip and AMD's LANCE Ethernet chip. In both cases, the second layer of polysilicon was used for storage devices. ↩ A standard op-amp integrator is an inverting integrator, and the output is negative. However, the MF10 uses the four-switch switched capacitor that inverts the input voltage. The two negatives cancel out, so the MF-10's integrator is a non-inverting integrator. See Introducing the MF10: A Versatile Monolithic Active Filter Building Block for details. ↩ To remove the metal layer, I used Whink rust stain remover (1.5-3.5% HF) to remove the oxide layer and hydrochloric acid to dissolve the metal. I applied Whink for 20 minutes and HCl for 16 minutes in total. I alternated each chemical for about 3 minutes each, applying a few drops at a time. I examined the die under the microscope after each application to gauge the progress. I stopped at this point since the metal was removed and the underlying transistors were visible. Moreover, the silicon became differentially stained, with NMOS transistors significantly darker than PMOS transistors. Some more Whink would probably improve the appearance of the die, but the risk is that the polysilicon might get removed, which would be bad for reverse engineering. In other words, I'd rather stop too early than destroy the features that I want to see. ↩ For reference, the full block diagram of the chip is below, from the datasheet. Block diagram of the MF10 from the Texas Instruments datasheet.  ↩ The filter frequency of the MF10 can be set to either the clock frequency divided by 50 or divided by 100. You might wonder where these ratios come from, since the capacitors on the chip are in 8:1 or 16:1 ratios, not 50:1 or 100:1. The formula for a switched-capacitor integrator is that the filter frequency is the clock frequency divided by 2π times the capacitor ratio. (This can be derived from the op-amp integrator formula and the equivalent resistance of a switched capacitor.) It turns out 2π×8 is 50.27 and 2π×16 is 100.5, providing the 50 and 100 values. Note that these values aren't exactly 50 and 100; they are off by 0.5%. Curiously, the datasheet specifies that the typical frequency error is ±0.2%, significantly smaller. I suspect that the explanation is that the capacitor ratio is not precisely 16:1, due to stray capacitance in the wiring and other factors, and the designers ensured that these factors tweaked the ratio in the desired direction. ↩ I suspect that the ternary input pin was used because the chip didn't have enough physical pins for all the functions they wanted. Note that the two filters are entirely independent, even with separate clocks, except for the 50/100 ratio control and the A/B mode control. I'm sure that these two functions would have independent control pins if the chip had pins available. They could have used a standard 24-pin package for the chip rather than the somewhat unusual 20-pin package, but maybe they had a motivation for avoiding a much larger 24-pin package. ↩

5 hours ago • 1 votes
Radxa's Q8B has 2x the performance and expansion of the Pi 5

There was a time I'd look at a board like the Radxa Dragon Q8B (at left, above) and be like, "there's no way I'd spend $209 on an SBC with 8 gigs of RAM". But we're in 2026, and seeing the 8 gig Raspberry Pi 5 going for almost the same amount, I figured I'd give it a shot. On paper, the Q8B beats the Pi 5 in pretty much every way. A lot of that is thanks to this Snapdragon 8cx Gen 3 chip, which is the same chip I tested on Microsoft's Windows Dev Kit 2023.

yesterday • 1 votes
Three years later

Reflections on October 7th

2 days ago • 1 votes
The Sting

The Sting belongs in the pantheon of films I'm deeply embarrassed to have not watched earlier. Not just because it's a great film — and it is — but because it is so incredibly my shit that I feel retroactively spurned for not having watched it sooner.

2 days ago • 1 votes
It's a Gas!

If everything worked as well as the product called Evapo-Rust, the world would be a much better place. That’s just one of the many lessons learned during my recent — successful! — project to transform my old, nonfunctioning gasoline-powered generator into something much better.

3 days ago • 1 votes
📚 BoredReading

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