Kicking the CAN #5 — Hunting the Bug (RPi4)

2026-03-05

One day later ....

The Claude Code instance responsible for the RPi4 was also very quickly convinced that everything was correct on its software side. To be sure, it also reviewed the code on the Nucleo board but concluded here as well that the code was correct.

After that, it had its first insight. It explained to me that there was a jumper on the Nucleo board that short-circuits RX and GND, and that this jumper needed to be removed. That was definitely the first step in the right direction. The RX and TX voltages then rose to 3.3V as expected.

But ..... still NO blinking.

After another unsuccessful attempt to swap sender and receiver roles, Claude Code had me measure the resistances to make sure the 120 ohm termination resistors were in place. I also had to measure the supply voltage. The 3.3V! still matched Claude Code's expectations. However, Claude found my own measurements at the SPI pins odd, as they occasionally deviated erratically from the expected values. I was therefore asked to carefully check all plug connections once more. They were secure.

Then it suggested I trace the bus voltage with the logic analyzer. From the screenshot of the recording, it determined that the bus voltage did not match expectations.

Claude Code then asked about the manufacturer of the breakout board and began examining the hardware specifications of the breakout board more closely, first on the associated wiki page and then in the GitHub repository.

And ... the second insight followed. The board had an internal voltage converter and therefore expected a 5V supply voltage.

Reconnected .... and .... BLINKING.