Teensy 4.0 - No 3.3v (PMIC_ON_REQ)

ct1305

Member
Hi all,

Had a weird situation that I am trying to better understand.

One of our Teensy 4.0 units was not powering on:

5V input via micro USB
0V on 3V3 rail
0V on Pin 4 (EN) of TLV75733P

No Short to GND on the 3V3 rail. Power cycling the 5V input did not change the above results, it seemed that PMIC_ON_REQ was not going high to enable the TLV75733P.

Using benchtop PSU, I drove pin 4 of the TLV75733P high. 3V3 rail was then created, and the led on the board was flashing twice (2 Blinks = NXP JTAG Not Responding) indicating improper start-up.

Power cycling 5V input again, the Teensy is now working perfectly fine? TLV75733P pin 4 is going high and the 3V3 rail is created at start-up. When measuring the PMIC_ON_REQ I was doing so on PIN 4 of the TLV75733P, rather then the 100K R7.

Is there any explanation for this? Could it have been a poor solder joint on Pin 4 that I have inadvertently "Fixed" when probing with the multimeter (Seems a little unlikely as it was probed multiple times before driving it high with benchtop PSU)

The On/Off pin is not connected to anything, the teensy is directly soldered on top of an Audio Board.

Thanks in advance !
 
I'm guessing you've already seen section "Power Up Sequence" on this page.


Difficult to guess what happened with your board. Perhaps pin 4 on the voltage regulator was the issue? Normally U4 is quite strong due to the solder on the bottom thermal pad.

But without knowing the USB_CAP and SNVS_IN voltages, difficult to say whether the RT1062 chip got to the powerup step where it should send the ON_REQ signal to the voltage regulator.

If this is one of the very old Teensy 4.0 which had 2 separate 3-pin BAT54C diodes for D1 & D2 rather a single 6-pin BAS40-05V, I would also look at those diodes. The 3 pin diodes were not nearly as strong as the 6 pin diode, so those 3 pin diodes can be damaged if the board is handled in certain ways. If D1 is just barely touching and becomes disconnected, you could get ~2.4V on USB_CAP without a path to power up SNVS_IN. Lack of SNVS power before the externally applied 3.3V would be one explanation that would seem to fit the facts you described. Perhaps while measuring voltage at U4 the D1 diode might have also been inadvertently touched?

We've also heard cases on this forum where no powerup turned out to be the F1 fuse or L2 inductor physically broken off. On Teensy 4.0 they're on the bottom side and they're taller than most of the other bottom side parts. If the bottom side scrapes against a surface, those 2 parts are the most likely to break off.

But this is all just guesswork from limited info. If you see the problem again, try to get measurements of the other powerup lines like USB_CAP and SNVS_IN and SNVS_CAP (all have test points on the bottom side) as these can help pinpoint where in the 8 step power up sequence things went wrong.
 
Hi Paul,

Appreciate the response! Apologies for the lack of info, as the board is directly soldered to the top of an audio board probing the components underneath is a touch difficult. I will work towards replicating the issue just to better my knowledge on the circuitry and start-up procedure.

Thanks for the info regarding the spots to check if this was to happen again on another unit.
 
I'm guessing you've already seen section "Power Up Sequence" on this page.


Difficult to guess what happened with your board. Perhaps pin 4 on the voltage regulator was the issue? Normally U4 is quite strong due to the solder on the bottom thermal pad.

But without knowing the USB_CAP and SNVS_IN voltages, difficult to say whether the RT1062 chip got to the powerup step where it should send the ON_REQ signal to the voltage regulator.

If this is one of the very old Teensy 4.0 which had 2 separate 3-pin BAT54C diodes for D1 & D2 rather a single 6-pin BAS40-05V, I would also look at those diodes. The 3 pin diodes were not nearly as strong as the 6 pin diode, so those 3 pin diodes can be damaged if the board is handled in certain ways. If D1 is just barely touching and becomes disconnected, you could get ~2.4V on USB_CAP without a path to power up SNVS_IN. Lack of SNVS power before the externally applied 3.3V would be one explanation that would seem to fit the facts you described. Perhaps while measuring voltage at U4 the D1 diode might have also been inadvertently touched?

We've also heard cases on this forum where no powerup turned out to be the F1 fuse or L2 inductor physically broken off. On Teensy 4.0 they're on the bottom side and they're taller than most of the other bottom side parts. If the bottom side scrapes against a surface, those 2 parts are the most likely to break off.

But this is all just guesswork from limited info. If you see the problem again, try to get measurements of the other powerup lines like USB_CAP and SNVS_IN and SNVS_CAP (all have test points on the bottom side) as these can help pinpoint where in the 8 step power up sequence things went wrong.
Update regarding this:

Further review of the board showed some left over flux on pins 5 & 6 of the BAS40-05V. (Potentially shorting these pins out)

Measuring the button cell battery attached it is dead (<1v but cannot sustain any load). Following the power up sequence, Step 2, it would be possible that VDD_USB_CAP was being sunk by the attached button cell or damage due to the potential current draw? This presents the possibility that this was the initial cause of failure.

However

When I bench tested the teensy (In my initial post), prior to driving pin 4 of the TLV75733P high, the button cell battery was disconnected. Power cycling the unit when the button cell battery was disconnected did not render any results. Only when the 3V3 rail was created (NXP JTAG Not Responding - 2 flashes) then power cycled did the boot up sequence worked correctly and has continued to work flawlessly.

Paul is there a potential that the RT1062 chip (VDD_USB_CAP or VDD_SNVS_IN) could have been damaged due to the direct connection to the dead button cell? And as such all subsequent attempts to power on the teensy would not get passed step 2 (even when the source of that problem, the dead button cell, was removed?) Then manually powering the TLV75733P it has somehow corrected the problem (or I am just lucky that providing 3V on VDD_SNVS_IN has fixed it somehow)

Obviously a lot of hypotheticals from my side to work out the end result, as I am not quite knowledgeable enough and didn't test other sections of the boot up sequence when the chip wasn't turning on. Definitely a lesson learnt for me on inspecting the boards a bit more when soldering more then a few of them.

Maybe it would be worth recreating this problem again (Joining pins 5 & 6 of the BAS40-05V) and seeing if this same issue occurs again

Thanks again

(Attached very poor photo of the flux/debris on pins 5 & 6)
 

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