arkader
Posts: 5
Joined: Tue Aug 07, 2018 11:52 am

(boot problem) Pi 3b+ and 3.3v regulator

Wed Aug 15, 2018 6:34 pm

Hi,

It seems there's a problem with the 3.3v regulation on the raspberry pi 3b+ according to these threads :
viewtopic.php?t=213225
viewtopic.php?t=213053
viewtopic.php?f=44&t=214713

Is there something that can be done about it ? Is it something a firmware update can fix ?

Thanks,

hippy
Posts: 3586
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: (boot problem) Pi 3b+ and 3.3v regulator

Wed Aug 15, 2018 7:43 pm

arkader wrote:
Wed Aug 15, 2018 6:34 pm
It seems there's a problem with the 3.3v regulation on the raspberry pi 3b+ according to these threads ...
It is not entirely clear what problem you are alluding to so it might be best to spell it out.

From what I can tell the claim is that the 3V3 pin on the GPIO cannot source enough current for add-ons when using the Pi 3B+. That seems odd to me because the 3V3 for the GPIO comes from the MxL7704 PMIC according to the circuit diagram and that should be able to deliver up to 1.5A, considerably more than previous boards.
arkader wrote:
Wed Aug 15, 2018 6:34 pm
Is there something that can be done about it ? Is it something a firmware update can fix ?
I wouldn't have thought so but it might depend on what the actual problem is.

arkader
Posts: 5
Joined: Tue Aug 07, 2018 11:52 am

Re: (boot problem) Pi 3b+ and 3.3v regulator

Thu Aug 16, 2018 12:16 am

Hi,

Thanks for your reply,

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. As soon as the green led starts blinking I can plug my screen back and everything is fine. From there I can sudo reboot or whatever, it works perfectly.

If I shutdown however, then it won't power back on (unless I unplug the screen of course).

The problem doesn't happen with the raspberry 3b, only with the b+ (tried with 2 screens and 3 different power sources)


From these other threads and a bit of googling it looks like the problem is not specific to waveshare screens, and happens with other gpio hat devices (hence the "regulator must be the issue" consensus)... the one thread marked as "solved" doesn't provide a solution, the guys basically changed the design of their hardware hat so that it takes its 3.3v from the 5v pins with its own regulator instead of using the one from the pi.


Because there's no green activity at all I guess I cannot change anything on my SD card to make it work, that's why I wonder if the regulator is indeed the problem, if hardware-savvy people know about the issue and can reproduce it, and if a firmware update could solve it...

LTolledo
Posts: 532
Joined: Sat Mar 17, 2018 7:29 am

Re: (boot problem) Pi 3b+ and 3.3v regulator

Thu Aug 16, 2018 12:25 am

My RPi3B+ desktop (this unit am using to post to this forum) has an I2S DAC clone plugged to its GPIO port.

as an added test I also have an OSOYOO 3.5" touchscreen and connected it to my other RPi3B+ lying around
the setup as follows
  • RPI3B+, no heatsinks (I took it out from it's FLIRC gen2 case)
    2.5" 1.0 TB HDD boot (bus powered from RPi3B+'s USB port, Eluteng SATA to USB 3.0 adapter)
    no microSD card
    OSOYOO 3.5" touchscreen plugged to RPi3B+'s GPIO
    5V 3.0A PSU
    wireless mouse and keyboard dongle
    WiFi connection to network


It booted....
"Don't come to me with 'issues' for I don't know how to deal with those
Come to me with 'problems' and I'll help you find solutions"

Some people be like:
"Help me! Am drowning! But dont you dare touch me nor come near me!"

hippy
Posts: 3586
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: (boot problem) Pi 3b+ and 3.3v regulator

Thu Aug 16, 2018 1:09 am

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.

arkader
Posts: 5
Joined: Tue Aug 07, 2018 11:52 am

Re: (boot problem) Pi 3b+ and 3.3v regulator

Thu Aug 16, 2018 12:49 pm

Thanks for your replies,

@LTolledo, thanks for your test, maybe I should try with another pi 3b+ just in case mine is the problem... osoyoo doesn't seem to be making 3.2" screens so I'm kinda stuck with waveshare...

@hippy I see.. I'll try to find information on timings, but I guess in that case sourcing the 3.3v from elsewhere is probably the best solution... my project uses a teensy board as well, with a couple free pins remaining... maybe I can take 3.3v from there and it'll work.
In the mean time I wrote a mail to the manufacturer just in case, thanks again for your info

arkader
Posts: 5
Joined: Tue Aug 07, 2018 11:52 am

Re: (boot problem) Pi 3b+ and 3.3v regulator

Fri Aug 17, 2018 7:35 pm

An update for the future googlers of this problem, here's the answer I got from Waveshare customer service :

"Hello,

The problem is the 3.3V pins connection. If the 3.3V pins didn't connected, the 3B+ could start normally, we have update for new batch. Very sorry for the problem.

Best regards
Waveshare Service Team
"

As the reply was a bit ambiguous and since I had 2 waveshare screens I decided to sacrifice one and just cut the pins 1 and 17 with a dremel (pin 1 can be cut as it's exposed on the corner, as for pin 17 I drilled carefully on the side expose the metal then cut it with pliers), and I confirm the 3b+ boots correctly!

(note: maybe the 3.3v is used to power the touchscreen, so if you need the touchscreen I can't guarantee that solution will work for you... )


Thanks again to everyone who took the time to help here :)

hippy
Posts: 3586
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: (boot problem) Pi 3b+ and 3.3v regulator

Sat Aug 18, 2018 10:09 am

arkader wrote:
Fri Aug 17, 2018 7:35 pm
I decided to sacrifice one and just cut the pins 1 and 17 with a dremel (pin 1 can be cut as it's exposed on the corner, as for pin 17 I drilled carefully on the side expose the metal then cut it with pliers), and I confirm the 3b+ boots correctly!
That does seem to prove that add-ons connected to the MxL7704 3V3 rail can affect or load that in a manner which prevents a Pi 3B+ from booting. Severing that connection allows booting to proceed.

The unanswered question is why, what the exact cause is. That is perhaps something the Foundation and MaxLinear might want to look into.

Thanks for testing.

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