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STM32 power-on problem

Question asked by armandas on May 30, 2012
Latest reply on May 30, 2012 by Clive One
I have designed a PCB that has an STM32F205VC microcontroller. It kind of works, but the micro has a weird power-on/startup problem. Basically, when the power is applied the micro will not start on it's own and the crystal will not oscillate. Pressing reset helps, but only if it's done 1.5-2 seconds after the power is applied. Before that, the micro is unresponsive.

I am using a 24Vdc SMPS to provide power to the board. Then an LM2674 brings that down to 3.3V for the micro and other devices. Below is the lost of things that I have tried:

  • Adding capacitor to the reset line. A small cap, like 1uF, does not make any difference. A BIG cap (I soldered a 1000uF just for the laughs) adds a long enough delay to help.
  • Soft starting the regulator. When I plug in the 24V PSU, I see a 6V spike on the 3.3V line. I was not sure if this was real, so I tried adding a 9ohm resistor to ground - it did not affect the spike. I thought that this spike might be upsetting the micro, so I used the ON/#OFF pin on the LM2674 to enable the regulator after applying the power. This solved the spike problem, the voltage now rises linearly in about 2ms. The microcontroller still does not start.
  • Setting different BOR (brown-out reset) levels. The STM32 has programmable BOR thresholds, which defaults to 1.8-2.1V. I tried setting level 2 (2.4-2.7V) and level 3 (2.7-3.6V), but neither helped to resolve the problem.
  • I also tried using a bench PSU to apply power directly to 3.3V line. The result is the same. However, I noticed that if I wind up the voltage slowly, the micro does start.

I have tried everything I can think of, but I was not able to resolve the problem. I hope that some of you will be able to give me a good advice based on the above clues.