This project didn't start with a grand plan to build a multi-function tester. It started with a disappointment.

I was working on the ESP32-C3 version of my SINAD indicator. The ADC performance was… not great. So I thought: "What if I try the RP2040? Maybe its ADC is better suited for this kind of measurement." https://nobcha23.hatenablog.com/entry/2025/06/10/211826

The RP2040's ADC turned out to be slightly better – but still not enough for serious measurement work. I realized that for real accuracy, I would need an external ADC. And that meant dealing with I2S, which turned out to be its own can of worms.

So there I was: a handful of RP2040 boards sitting on my desk, no clear use for them, and a bit of frustration. https://nobcha23.hatenablog.com/entry/2026/04/01/194146

That's when I thought: "Why not turn them into something useful? A simple tester, maybe? Just to put these boards to work."

And that's how R909-tester began – not as a brilliant idea, but as a way to salvage some boards and learn something along the way. It turned out to be a fun project after all.
https://chitose6thplant.fc2.page/r909-tester/