babbleberry/rpi4-osdev

no data on UART with raspi 400

dodecahedral-champion opened this issue · 4 comments

dodecahedral-champion commented

Hello,

I was having trouble getting UART1 to respond on my raspi 400, so I tried your code to check if I was missing something obvious; but there is still no output. :-(

Some things I checked:

  • I used the serial console of the stock raspbian build to verify that the UART is electrically connected properly (the console was usable)
  • I used /proc/iomem in raspbian to verify that the raspi 400 does list its UART at the same memory locations as the raspi 4.
  • I added some UART output to your part5-framebuffer example to verify that the kernel does boot up and does execute the relevant instructions (the framebuffer output was visible).
  • I checked that core_freq_min=500 and enable_uart=1 were included in the config.txt

It's possible that there is some other obscure difference between the raspi 4 and raspi 400, so I am considering splashing out on a raspi 4B to test with....

I wonder if you have any ideas?

dodecahedral-champion commented

That diagram seems to be using wiringpi's pin numbering scheme (http://wiringpi.com/pins/), which is similar-yet-different to the Broadcom one. On https://pi4j.com/1.4/pins/rpi-4b.html, they list the same pinout for the 4B model. Very confusing, but at least the pinouts are the same.

I will have another think today and then decide whether to buy a 4B for testing.

dodecahedral-champion commented

Well I had a strange experience with the raspi4B. At first I seemed to have gone backwards: the same card I used to successfully test your part5 example on the raspi400 refused to work (in addition to the uart not working). I reimaged the card from scratch and walked through baby steps, again starting with raspbian to be sure that at least somebody was able to talk on the uart. In the end the 4B could successfully run your part4 example on the 4B, and... [drum roll] ...the same card also works on the raspi400!

I tried working backwards to determine if there was some obscure difference between the two configurations, but I've just finished using the exact same config.txt and it still works.

So I guess we can chalk this one up to "mysterious gremlins" and close the issue. 😩

babbleberry commented

That is very strange indeed. I also find the "mysterious gremlins" answer somewhat unsatisfying. It does happen though! I'm happy for you to close the issue - do keep in touch with how you get on with the RPi400.

All the best,
Adam

dodecahedral-champion commented

I suspect the gremlins are in the SD card, I've just had another error that was magically corrected by redoing from start. I'll try another card brand maybe. Anyway I can get UART input & output on the 400, plus framebuffer output, so everything looks OK so far.