arkader wrote: ↑
Thu Aug 16, 2018 12:16 am
the exact problem is that when I plug a waveshare touchscreen on the pi 3b+, it won't boot... red light is on, but no act (seems like the sd card isn't even read).
While it's stuck in this state, I can unplug my screen, the green light will start blinking, and the rpi will start booting.
My guess would be, with the the screen plugged in, it is affecting or loading 3V3 or some other voltage rail from the MxL7704 PMIC such that it doesn't complete its power-up sequence, doesn't release the SoC from reset. With the screen removed it can successfully reach a 'power good' state so the SoC is released from reset and boots up.
The reason it works on 3B but not 3B+ could be because the power sequencing or timing may be different. That could explain why using a separate 3V3 regulator from 5V on another add-on board allowed booting as described. A separately regulated 3V3 would come on almost as soon as 5V did, and disconnecting its 3V3 from the MxL7704 rail would mean it could never affect nor load it.
If it is a sequencing or timing issue it might not be considered a problem attributable to the 3B+ as much as the screen not being designed for the sequencing and/or timing the MxL7704 on the 3B+ has. If that is the case then 'which is to blame for things not working' could probably be batted back and forth forever.
The above is entirely speculation. To diagnose the issue further one would likely need sequencing and timing information for the 3B power rails and signalling and the same for a 3B+ without the screen. Repeated for when the screen is attached. Knowing which voltages the screen uses would probably help, as would having a circuit diagram of that.
I imagine most Pi users won't have the equipment to measure any of that. The people who designed or manufacture the screen may however be able to do so. It would also be worth scouring Google to see if there is anything already published in that respect.
Similarly for any other add-on which won't allow a Pi 3B+ to boot-up.