Quentin Santos

Obsessed with computers since 2002

I have always been fascinated with computers. Nowadays, I mostly use Rust, but I started out with a QuickBASIC book from the local library when I was in elementary school. I also got a Master in computer science from ENSL and a PhD in cryptography from ENS.

Contact me at qsantos@qantos.fr.

Latest articles

  • The USB interposer
    In a previous article, I claimed that there were no good alternatives to the RP2350-USB-A board. To accept a USB keyboard or other device, a USB board has to present itself as a USB host. It does this by pulling down the two USB data pins. The USB device then signals itself by having a data pin pulled up. The RP2350-USB-A has a pull-up resistor on a data line, forcing it to act as a USB device. This means it is not possible to use that board as a keyboard pass-through, for instance. Unless you modify it. I looked at… Read more: The USB interposer
  • My first PCB: AIC3204 eval board
    I had some fun with the Adafruit TLV320DAC3100. This is a small board built around the (TLV320)DAC3100 audio chip. “DAC” stands for “Digital-to-Analog Conversion”; that is, it takes some audio signal (here, I²S) and converts it to the analog equivalent (differential for speakers, single-ended for headphones). Microcontrollers typically don’t have analog output, and getting analog circuitry right can be tricky. With something like the DAC3100 board, you can just tell the chip what sound you want to produce, without worrying about interference, and the chip will do the rest. The other side would be “ADC”, or “Analog-to-Digital Conversion”. Most microcontrollers… Read more: My first PCB: AIC3204 eval board
  • A few spikes in Pico cos()
    A picture is worth a thousand LLM prompts: Horizontal value is the input value x for cosf(x), ranging from 0 to 2π. The orange line shows the result of evaluating cosf(x), which looks exactly as expected. The blue line shows the time it took to evaluate cosf(x) for each x, in nanoseconds. I discovered this while investigating why my main loop would occasionally stall for whole milliseconds (that’s hundreds of thousands of cycles for my RP2350!), causing USB HID reports to be dropped. After a while, I figured out that the culprit was this code (slightly simplified): This generates a… Read more: A few spikes in Pico cos()
  • The RP2350 is too fast
    I had a few hours of “fun” figuring out why my RP2350 could use my ILI9341 display, but not the ST7796 one. To be more specific, displaying stuff with the first display worked flawlessly, while the ST7796 would display a single frame correctly, and then glitch. Depending on the exact code running, the glitches would include: Changing completely unrelated code would change the exact glitch, most likely because it slightly changed the timing at which the display code ran, making diagnostic more difficult. On the opposite, just displaying the output of time_us_64() help significantly, since I could immediately tell whether… Read more: The RP2350 is too fast
  • Asynchronous I²C in C
    Context I have a RP2350 and an SSD1306 module. I want to use the former to display stuff on the latter. The most common modules are connected over I²C: The simple way to use such a display looks like this: Basically, to refresh the SSD1306, we need to send an I²C message with a command (first byte) and data (the rest of the buffer). The command 0x40 is “Set Display Start Line”. Specifically, 0x40 means “draw from line 0”, 0x41 means “draw from line 1”, and so on until 0x7F which means “draw from line 63”. Each byte that follows… Read more: Asynchronous I²C in C

All articles