Background:
I bought a DPI LCD made by Waveshare. I set it up according to the wiki (downloaded overlays, copied to /boot/overlays, updated /boot/firmware/config.txt) and connected it to my Pi 400 using GPIO adapter. Everything worked fine - both the terminal and desktop. I played with it for a bit and turned everything down.
Next morning, when I turned 400 on again, display wasn't working correctly anymore: http://saya.wyobraz.net/1000024486.mp4
At first I thought the display broke down. I also have Pi 1 (Debian 10) and Pi 3 (Debian 11), so I tried connecting it to them (with and without GPIO adapter) - everything worked fine (Pi 1 was connected using the 'Buster" approach from Waveshare's wiki, Pi 3 was also using overlays).
Next thing I checked were GPIO pins on Pi 400. I've ran "gpiotest", which reported no problems. Then I toggled each GPIO using "raspi-gpio set" and checked if the voltages were ok - also no problems there. Then I connected SPI e-paper display, and it worked ok (although it's most likely not using all GPIO pins to work).
VSYNC on the oscilloscope: http://saya.wyobraz.net/2025-08-24_12.13.41.jpg
HSYNC on the oscilloscope: http://saya.wyobraz.net/2025-08-24_12.14.28.jpg (on 5us peaks were looking fine, but I didn't take picture)
PCLK on the oscilloscope: http://saya.wyobraz.net/2025-08-24_12.17.00.jpg (1us is my limit)
Switching configuration on 400 from overlays to Buster-style (with dpi_timings and that stuff) results on the display showing one frame and then going all black.
I also tried making config.txt on 400 match the one from Pi 3, but it also didn't help:I checked output of "raspi-gpio get" on Pi 3 and Pi 400 with Debian 12 and 11 and I got this: http://saya.wyobraz.net/raspi-gpio_get.png
"level" values seem to vary constantly, so it's probably not relevant. What I found strange was the presence of "pull=UP" and "pull=DOWN" on Pi 400, which was not present on Pi 3.
Does anyone have any other ideas what might be worth checking? Maybe my 400's DPI controller, if something like this exists, broke down?
I bought a DPI LCD made by Waveshare. I set it up according to the wiki (downloaded overlays, copied to /boot/overlays, updated /boot/firmware/config.txt) and connected it to my Pi 400 using GPIO adapter. Everything worked fine - both the terminal and desktop. I played with it for a bit and turned everything down.
Next morning, when I turned 400 on again, display wasn't working correctly anymore: http://saya.wyobraz.net/1000024486.mp4
At first I thought the display broke down. I also have Pi 1 (Debian 10) and Pi 3 (Debian 11), so I tried connecting it to them (with and without GPIO adapter) - everything worked fine (Pi 1 was connected using the 'Buster" approach from Waveshare's wiki, Pi 3 was also using overlays).
Next thing I checked were GPIO pins on Pi 400. I've ran "gpiotest", which reported no problems. Then I toggled each GPIO using "raspi-gpio set" and checked if the voltages were ok - also no problems there. Then I connected SPI e-paper display, and it worked ok (although it's most likely not using all GPIO pins to work).
VSYNC on the oscilloscope: http://saya.wyobraz.net/2025-08-24_12.13.41.jpg
HSYNC on the oscilloscope: http://saya.wyobraz.net/2025-08-24_12.14.28.jpg (on 5us peaks were looking fine, but I didn't take picture)
PCLK on the oscilloscope: http://saya.wyobraz.net/2025-08-24_12.17.00.jpg (1us is my limit)
Switching configuration on 400 from overlays to Buster-style (with dpi_timings and that stuff) results on the display showing one frame and then going all black.
I also tried making config.txt on 400 match the one from Pi 3, but it also didn't help:
Code:
dtparam=audio=oncamera_auto_detect=1display_auto_detect=1dtoverlay=vc4-kms-v3dmax_framebuffers=2arm_64bit=1disable_overscan=1[pi4]arm_boost=1[all]dtoverlay=vc4-kms-v3ddtoverlay=vc4-kms-DPI-7inchdtoverlay=waveshare-7dpi"level" values seem to vary constantly, so it's probably not relevant. What I found strange was the presence of "pull=UP" and "pull=DOWN" on Pi 400, which was not present on Pi 3.
Does anyone have any other ideas what might be worth checking? Maybe my 400's DPI controller, if something like this exists, broke down?
Statistics: Posted by cremmy — Sun Aug 24, 2025 12:22 pm — Replies 6 — Views 160