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Thread: Teensy 4.0 Mysterious Power-Up Behavior

  1. #1

    Teensy 4.0 Mysterious Power-Up Behavior

    I've used several T4.0 boards over the past few months with no issues, but the new one I just received is behaving mysteriously. On power-up, the LED flashes three times and then stops (about 4 seconds). The board is not recognized by the Teensy loader (with one exception: see below). I found that, coincident with the end of the 3rd flash, the 3.3v output is dropping from 3.3v to about 1.3v.

    Unfortunately, the T4 is now soldered to my custom PCB, and I forgot to test the T4 before installing it. The behavior is the same when powering the T4 from the USB port and when powering from my external power supply (5v to the Vin pin). The 3.3v outputs from the T4 are not connected to anything, so there's no overcurrent situation. My external power supply shows only about 30-40mA total, dropping to about 10mA when the flashing stops.

    I did get the Teensy loader to recognize the board if I plug in the USB cable and press the Program button within the 4 second "timeout". But then I wasn't able to download my code ("No Teensy boards were found on any USB ports of your computer"). But...somehow I was able to download the code once, which seems to work correctly (I think, having tested much). But clearly something is wrong, so I'd like to get to the bottom of it. My other T4-based PCBs, with a similar but not identical design, continue to work perfectly, including downloads.

    Does this ring a bell with anyone? Thanks!

  2. #2
    Senior Member+ defragster's Avatar
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    2nd thing ... A recent T4 posted as damaged in shipping, some 3 of the bottom smd's gone

    1st would be always remember to power test on unwrapping

    Now soldered ... check for shorts or unexpected behavior on anything connected

    How close is 4 seconds? If power on/off pin is grounded about 5 seconds it goes to sleep and 3.3v pin should then go 0

    Watch that power on/off pin with meter on start, it should hit 3v and stay up ... also the 3.3v

  3. #3
    Thanks very much, @defragster. I thought I had considered everything you mentioned, but I didn't check carefully enough. I have a switch on the Power On/Off pin, and due to a discrepancy between the switch's schematic symbol and the footprint, the pin was tied to ground. Once I removed the switch, everything started working as expected. Thanks!!

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