Difference between HSE and HSE Bypass

[widemeadows]

Posted on February 21, 2015 at 17:51

After some hardware struggles with my new STM32F3 disco board I finally got it running correctly and at high speed. I was wondering, however, that there seems to be no difference between HSE and HSE bypass mode, as both settings result in clocking the system with 72 MHz.

Sadly, I am unable to find some clear explanation in the reference manual. What is the difference between the two settings, technically? Is it just that in bypass mode, an external clock is provided to OSC_IN (with OSC_OUT being free for GPIO adventures) while in regular HSE mode a real oscillator needs to be present? Is it correct to assume that bypass mode would then be the ''best'' setting for the discovery board (a regular version with one oscillator for the F103 and none on X2)?Regards,Markus #stm32f3 #hse #stm32f3-discovery

[Tesla DeLorean]

Posted on February 22, 2015 at 00:53

The bypass mode disables the oscillator (inverter) function of the output pin. I'm not sure this actually frees up the pin for GPIO, I seem to remember either the data sheet or reference manual saying it doesn't, but you'd need to check.

Most designs use a crystal rather than an external oscillator (TCXO, VCXO, etc), the DISCO's do it to save a couple of parts, as the ST-LINK requires one, and can feed it out.