Category: Electronics

  • Custom mechanical keyboard: OS-specific custom RGB lighting with QMK

    Jun 30, 2024
    Electronics
    22 minutes
    qt.ax/kbl

    My old Corsair keyboard has been struggling recently. It has some weird issues, either in hardware or firmware, that cause it to sometimes go crazy and randomly “press” the wrong keys, forcing me to pull out my backup keyboard until the lunacy1 passes. On top of that, managing it requires Corsair’s bloated, Windows-only iCUE software or a reverse-engineered alternative like ckb-next, which isn’t fun for a Linux user like me, and even with ckb-next, the customization is limited.

    So I figured I’d get a new keyboard. I have a few simple requirements:

    1. It should be a 100% keyboard because I use the numpad quite a bit for number entry, e.g. to manage my personal finances;
    2. It should have a backlight since I often use my computer at night in relative darkness, and while I can touch type just fine, being able to see the keyboard is nice;
    3. It should have tactile mechanical switches, but not the obnoxious clicky ones. For reference, my old keyboard has Cherry MX browns, which I liked; and
    4. It should have properly programmable and customizable firmware. QMK is the popular option, so I searched for keyboards supporting that, and failing that, at least keyboards with proper first-party Linux support.

    As it turned out, I couldn’t find any prebuilt mechanical keyboards that ticked all the options and were in stock, so I figured I might just get into the custom mechanical keyboard scene and build my own. Thus began a journey that saw immense frustration and nerd-sniping…

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  • Microsecond Accurate Time Synchronization on LAN with PTP

    Jan 26, 2023
    Sysadmin, Electronics
    16 minutes
    qt.ax/ptp

    Last time, I built a stratum 1 NTP server with a PPS signal from a GPS receiver, synchronizing my server’s clock to within 10 microseconds of UTC. However, NTP was designed to synchronize clocks within a few tens of milliseconds over the Internet, and I’d be lucky to achieve millisecond accuracy on a LAN. I mentioned that PTP was the alternative that could achieve accuracy in the sub-microsecond range. Well, this time I’ll be setting up PTP between my server and my PC with the hardware timestamping on the ConnectX-3s.

    If you are following along at home, don’t despair if your hardware can’t do timestamping or PTP. I will also attempt to set up PTP with software timestamping later for my other devices.

    Naturally, I first turned to the gpsd documentation, since that was a decent reference for setting up NTP with the PPS signal. Well, this is what it says for PTP with hardware timestamping:

    Sadly, theory and practice diverge here. I have never succeeded in making hardware timestamping work. I have successfully trashed my host system clock. Tread carefully. If you make progress please pass on some clue.

    That didn’t sound encouraging at all. “Oh well, I guess I am on my own here,” I thought to myself. “How bad could digging through a few man pages and random online documentation be? Worst case, there is the source code, right?”

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  • DIY a Stratum 1 NTP Server with a Serial Port

    Jan 24, 2023
    Electronics, Sysadmin
    17 minutes
    qt.ax/pps

    These days, it seems like everyone is posting about turning Raspberry Pis into a stratum 1 NTP server by hooking up a cheap GPS module, most often the GT-U7 u-blox 7 clone with a PPS (pulse-per-second) signal output, whose rising edge indicates exactly the start of a second.

    While this seems like a cool idea, it suffers from one flaw—while the Raspberry Pi itself almost certainly has very accurate time, getting accurate time to the rest of the network would be problematic. This is because the Ethernet adapter on Raspberry Pis before the Pi 4 was hooked up via USB, and the polling nature of USB introduces jitter, preventing the accurate signal from reaching the rest of the network. Unfortunately, I only have a Raspberry Pi 3 model B in my possession, which suffers from the problem.

    Now, I could have gotten a Raspberry Pi 4, but those aren’t priced sanely at the moment and it would be just an exercise in copying. Instead, I looked at the various alternatives. The traditional way of doing this kind of thing involves hooking up a GPS receiver into a serial port, which generates an interrupt. If the PPS signal is delivered to the DCD (data carrier detect) signal (as described in RFC 2783), then the in-tree Linux driver pps_ldisc is able to do the timestamping in kernel mode for the highest possible accuracy.

    I found out that my server’s X570 motherboard came with a serial port header (labelled COM). This meant that I could buy some fancy GPS receiver with a serial port and hook it up. Unfortunately, those aren’t priced sanely either, so I decided to build my own with the GT-U7 module and a driver module for RS-232 (the common serial port standard).

    This was late last year. I ordered the components on AliExpress and they all arrived in January, so I finally started this project.

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