Teensy 3.6 on custom PCB does not always run program when not powered from USB

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Razon

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The Problem:
When powered from USB (PC or wall adapter) LED always turns ON.
When powered from my PCB, sometimes the LED doesn't turn on. It seems that if LED doesn't turn on, successive power cycles (4-5) will eventually make the LED turn ON. Once ON, if power is cycled with short dead times, the LED will come on every time. However, if I power it OFF and wait, I'm back to first case where it might not turn ON.

Things I've tried:
- Use a delay (1000) at beginning of program,
- Ensured the issue is repeatable with simple "turn built in LED ON" program (No waiting for serial or peripherals)
- Set USB type to "No USB"
- Temporarily added a 5V regulator (center of picture) on PCB, removed the 3.3V pin on teensy and supped power to the Vin pin instead. (asumed my 3.3V source was not good)

Any idea what's causing this?

Code: 
void setup() {
pinMode(13, OUTPUT);
}

void loop() {
digitalWrite(13, 1);
}

PCB has built in 3.3V voltage regulator and Normally powers the Teensy directly on the 3.3V pin. In the following picture a 5V regulator was added to supply power on Vin pin.
t3.6.JPG
 
View attachment Schematic.pdf

The circuit is simple, reading some inputs and controlling the outputs.
Inputs are opto-coupled or through comparator.
Outputs go to power switches, a display and and aux port.

FYI: I performed the tests with the display both connected and disconnected with same results.
The circuit is powered by a lab/bench adjustable power supply.

.....should see if the Teensy works detached from my PCB, powered from external source (not through USB).
 
Many thanks! Good thread, surprised I didn't find it myself.
Very unfortunate though; I wish I knew before I got the boards made.
 
Perhaps you could try Teensy 3.5, they seem to be more immune to the problem.
I am also developing device with Teensy 3.6, using all the IO pins and I am wondering if I should include MIC803 IC on the main PCB board:confused:
 
I need to read up on it, but right now I’m thinking to try and find a different regulator with a faster rise time in the same package as my current one. If can’t find one, I’m thinking to use the simple capacitor on reset pin method. I tried already and it works good, but it doesn’t allow programming. My device doesn’t require constant re-programming so I’m thinking to instal a normally closed switch in series with the capacitor: when I want to program, I press the switch first.
If/when I redo the PCB, I’ll deff look into a “proper” solution.
 
Thanks for the link! That's really nice for development or a one-off product.
First, I will check with the current limit on my power supply cranked up. I think last time I touched it I turned it way down for the initial power up of my PCB; and that would deff limit the voltage raise time. My board actually supplied 3.3V to the Teensy, not 5V.
If that doesn't work, I'd like to take advantage of the fact the teensy has a perfectly good power supply on board; don't want to re-invent the wheel... or default to one of the previously mentioned solutions.
I'll do some testing and see what works better for me.

Thank You!
 
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