Ramp up test failure

[berkecem]

Hi

i have pcb with 4 led driver, a transceiver and a microprocessor based.when ı doing the ramp up test to slowly giving voltages to 0 to 20 my pcb won't work.if  i gave 20 directly and rapidly pcb was working.i try various methods but ı cant succeed. ı wonder if anyone can help with me on this subject?

[moffy]

Do you have a proper voltage reset on the microprocessor? If not try manually resetting the micro after a slow ramp.

[berkecem]

yes i have a proper voltage reset. i tried to connect enable to inhibit just now but it didn' work still when the voltage ramp up.

[Andree Henkel]

If the reset circuit is controlled by a threshold of CPU Voltage Supply:
Then the Supply of CPU may still not be stable and within CPU spec at the time when Reset Pulse is over.

If you are ramping Input to CPU supply Circuit very slowly.

Could try a Reset circuit with higher threshold and/or longer reset pulse.

Or a CPU supply circuit with power good output, best if open drain, pull up resistor to CPU supply, connected to power good and reset supervisor input in parallel

[Obin]

What microprocessor and what ramp up speeds are using?
I recall a stubborn turn-on issue of a Raspberry pi, where the 0t o 5V rise on the 5V pins needs to happen within 20 ms (could be another value, it's been a while) or it would refuse to boot. Turns out the output enable of by bench PSU was too slow and hotplugging worked better

Might be something similar

[PCB.Wiz]

Try faster slew rates to find what rise time is needed.
Also note when testing, some systems need to have a quite low cold voltage (< 0.6V) or they do not start correctly.