STM32F427 Bootloader HSE Signal

[mkuczma]

Hi, is it necesarry to use a crystal oscilator for DFU USB booltloader for STM32F427zit? Can I use a generator instead of crystal?

Marcin

[TDK]

You are required to use a crystal or other clock source with sufficient accuracy. The internal HSI clock is not sufficient.

It's unclear what this "generator" is, but if it has sufficient accuracy, it will work.

Per AN2606, the source must be "An external clock multiple of 1 MHz (between 4 and 26 MHz)"

[mƎALLEm]

Hello,

This is what was stated in the AN2606. It states to use an external crystal.

If you mean by "generator" an external clock source like a crystal oscillator (HSE in bypass mode), I think it may work but it's not guaranteed.

[mkuczma]

Hi, thanks for your replies. I just want to use same clock source for uC and other IC to save PCB space and I am wondering if a standard 3,3V oscilator will work? Is there a difference for a bootloader if clk source is crystal or oscilator? Mayby someone can confirm it.

[TDK · Accepted solution]

The accuracy is what matters, not the source. Required accuracy is about +/- 0.25%. Does your oscillator meet that?

[waclawek.jan]

The bootloader will most probably work OK with external oscillator connected to OSC_IN pin (don't forget that OSC_OUT still needs to remain unconnected), but I don't think ST will rush in to confirm that. Although I know it's not a proof, you can test it on a Nucleo or Disco board, where 8MHz is provided by STLink's MCO (on Disco, you may need to disconnect the crystal, which is there quite strangely connected in parallel with the STLink's MCO).

Also keep in mind that for the DFU bootloader, besides what @TDK  wrote above, it's better to use a lower frequency oscillator.

JW