STM32U5G9 + ST7701SN --- LP DSI works, HS video does not
Hi everyone,
I'm having a very annoying issue with an STM32U5G9ZJ and a Newhaven NHD-4.0-480480AF-MSXP-CTP display using an ST7701SN over 2-lane MIPI DSI.
The basic communication works, but I cannot get any video onto the display.
What works
The panel responds correctly to LP short DSI commands:
- Reset works.
- ST7701 registers can be read.
- The initialisation sequence runs successfully.
0x29Display ON works.0x23All Pixels On makes the display white.
So the panel and the LP DSI communication are working.
What doesn't work
As soon as I try to send RGB888 video over HS DSI, the display stays black.
I've tested this both using the LTDC/framebuffer and the internal pattern generator. The latter was to rule out the framebuffer/LTDC completely.
The pattern generator starts successfully and the DSI registers show the video engine and both data lanes entering an active HS state, but the panel still stays black.
I'm using:
- 2 DSI lanes
- RGB888
- Burst video mode
- 480 × 480
- 480 packet size
- Continuous clock
- Automatic clock lane control disabled
I've also checked the following:
- DSI PLL/configuration
- LTDC and framebuffer
- DSI PHY status
- DSI error/status registers
- PCB routing and continuity
- DSI lane mapping
- Tried swapping the lanes (didnt expect this to work but tried anyways)
- ST7701 initialisation
- DSI internal pattern generator
- Continuous clock / DPCC
Unfortunately I don't have a MIPI analyser or high-speed scope, so I can't currently see what is actually being transmitted on the HS lanes.
At this point I'm wondering whether I'm hitting a known STM32U5G9 DSI HS/PHY issue, or whether there's something specific about the ST7701SN that I'm missing.
Has anyone successfully driven an ST7701/ST7701S/ST7701SN from a U5G9, or come across a similar situation where LP commands work but HS video doesn't?
I have sucessfully created a PCB for the STM32F4 series and a Midas MIPI display, I utilised the same routing technique and ensured impedance control. I am more certain it is a software issue given the symptoms.
Any suggestions for what I should investigate next would be greatly appreciated. I am pulling my hair out at what I expect will be a silly thing I have missed.
Thanks in advance!
