STM32F4 external oscillator

tg.developer · ‎2014-01-28

Posted on January 28, 2014 at 13:16

Hello,

I have connected external clock source of 25 MHz from local oscillator to the

OSC_IN pin of STM32F4. The problem is, that sometimes happens, that

CPU does not start from external HSE and bit RCC_CR_HSERDY is never set.

I checked clock with oscilloscope and it looks fine. I also put CPU in reset and

wait until oscillator stable, but even this does not guarantee valid start-up.

In manual I did not found any detailed explanation about HSERDY bit when

CPU is clocked by the external clock source.

Is there any limitation when connection external oscillator instead of crystal?

chen · ‎2014-01-28

Posted on January 28, 2014 at 13:25

What do you mean by ''I have connected external clock source of 25 MHz from local oscillator''

What exactly is this clock source?

Tesla DeLorean · ‎2014-01-28

Posted on January 28, 2014 at 14:03

Is there any limitation when connection external oscillator instead of crystal?

Yes, you need to use BYPASS mode.

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tg.developer · ‎2014-01-28

Posted on January 28, 2014 at 20:21

Thank you. I have missed this when reading manual.