burtyb
Posts: 36
Joined: Sat Mar 05, 2016 4:19 pm

3v3 drops on Pi4 reboot

Sun Jul 07, 2019 9:10 pm

My HAT uses the 3v3 on the Pi GPIO for the EEPROM and an I/O Expander.

On Pi <4 the 3v3 rail was permanently live meaning the Pi the HAT was on could be rebooted and it wouldn't reset the I/O Expander but on the Pi4 when I reboot the 3v3 power is cut resetting the I/O Expander.

Is there any way to keep the 3v3 rail live whilst rebooting a Pi4?

Technocolour
Posts: 86
Joined: Thu Jul 04, 2019 6:23 pm

Re: 3v3 drops on Pi4 reboot

Tue Jul 09, 2019 5:06 pm

That got me curious for what kind of caps one would need to buffer it, so I measured it with my Rigol scope, and it goes dark for just shy of 60ms.

dp11
Raspberry Pi Engineer & Forum Moderator
Raspberry Pi Engineer & Forum Moderator
Posts: 21
Joined: Thu Dec 29, 2011 5:46 pm

Re: 3v3 drops on Pi4 reboot

Tue Jul 09, 2019 6:20 pm

If you need 3.3v to stay up then the best way is to create it from the 5v rail.

hippy
Posts: 6717
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: 3v3 drops on Pi4 reboot

Tue Jul 09, 2019 10:47 pm

One guess would be that GLOBAL RESET is asserted to deliver a reboot, which pulls the PMIC GLOBAL_EN low, which kills all power. Hence the glitch on 3V3. If that's the case and the PMIC 'shorts 3V3 to 0V' when deactivated you are probably going to need more than just capacitors to ride that out.

trejan
Posts: 1112
Joined: Tue Jul 02, 2019 2:28 pm

Re: 3v3 drops on Pi4 reboot

Wed Jul 10, 2019 11:31 am

Technocolour wrote:
Tue Jul 09, 2019 5:06 pm
That got me curious for what kind of caps one would need to buffer it, so I measured it with my Rigol scope, and it goes dark for just shy of 60ms.
Doesn't sound like a good idea to do that from this post. The PMIC won't startup if it sees 3.3V.

hippy
Posts: 6717
Joined: Fri Sep 09, 2011 10:34 pm
Location: UK

Re: 3v3 drops on Pi4 reboot

Wed Jul 10, 2019 12:34 pm

Good point. The caps would have to discharge before the re-boot could happen. It could work with a 'perfect diode' -

Code: Select all

  ___
 |   |
 | P |--- 3V3 ---o )--|>|---.---> 3V3 HAT
 | M |                     _|_
 | I |                     ===
 | C |                      |
 |___|         .-o )--------^---> 0V HAT
           0V _|_
But it would be easier to add a 3V3 regulator powered from 5V as previously suggested.

That also seems to be the solution for HAT's which prevent a Pi 3B+ booting.

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