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NRST low + HSE source gone = system freeze instead of reset

Question asked by kostka.tim on Mar 11, 2016
Latest reply on Mar 12, 2016 by kostka.tim
I have two chips, a master and a slave, both STM32F429.  The master is using the internal HSI as a PLL clock source.  The master is outputting a 16 MHz signal that the slave is using as a PLL clock source through HSE bypass.

The NRST lines of both chips are tied together and when I pull NRST low for 100ms, I expect a system reset on both chips, with both of them booting up nicely afterwards.  What I'm actually seeing is the master chip boot up just fine, but the slave chip gets stuck and fails to boot up.  By this I mean that it stops doing what it was doign (blinking LEDs among other things) and also fails to boot up (which I notice by the LEDs not blinking), even when the HSE clock source is present again.


I assume that taking away the HSE clock source at the same time I'm pulling NRST low is causing the hangout here.  I've noticed that holding NRST low for ~2 seconds causes things to boot up just fine.


So, some questions:


Is there anywhere in the RM or ANs that discuss this, specifically if stopping the HSE source is okay or not?  And is the improvised solution (holding NRST low for longer) guaranteed to work?  Or can I fix the circuit without additional circuitry somehow?

Or Is there anything I can do in software to have a failsafe if the HSE source disappears?

Obviously I could have an external clock source that isn't interrupted by the NRST process.  Or I could tie NRST to the power supply such that NRST low causes the power to be cut to the chip as well, presumably triggering a reset.


Thanks.

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