STM32F407 discovery HSE crystal is always oscillating

eli_az · ‎2014-12-23

Posted on December 23, 2014 at 15:36

Hi,

I've been using the STM32F4 discovery kit (the one that contains the STM32F407 MCU) and I've noticed something strange.

When I stopped the code at the SystemInit fuction and probed the HSE I saw that it was already oscillating although it hasn't been activated by software yet (I tried all the code lines before the SetSysClock function and I tried manually disabling the HSE).

Could someone explain this behavior?

Thanks in advance

Tesla DeLorean · ‎2014-12-23

Posted on December 23, 2014 at 18:53

Where exactly are you probing it? Because there is an 8 MHz signal delivered by the ST-LINK's F103 part to a point very close to the XTAL. A lot of other DISCO boards don't have an HSE crystal and have solder bridges, or zero ohm, connectivity to the MCO signal.

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eli_az · ‎2014-12-23

Posted on December 24, 2014 at 08:40

I'm probing C14 and C15 (next to X2), they are all mounted (including R24 & R25).

I've made sure that SB13 & SB14 are not connected so I'm pretty sure I'm measuring the X2 crystal and not the external oscillator.