HSE clock bypass means?

ctc.ctc · ‎2016-11-27

Posted on November 28, 2016 at 03:20

Recently, I read reference manual about stm32f4 rcc,

but I have no idea about what is HSE clock bypass,

I only know that there is a bit in RCC->CR for HSE BYPASS.

I study the driver function RCC_HSEConfig()

there are three parameter RCC_HSE_OFF, RCC_HSE_ON, RCC_HSE_Bypass

But I still don't know why there is Bypass option?

Can anyone introduce bypass more detail? thanks =)

#rcc #bypass #stm32

Radosław · ‎2016-11-28

Posted on November 28, 2016 at 09:00

When You use cristal generator or any other source of clock, not cristal oscilator. then this option will off XTAL loop output.

Tesla DeLorean · ‎2016-11-28

Posted on November 28, 2016 at 17:33

For situations where you have an external clock source (TCXO, Ethernet PHY, ST-LINK 8MHz MCO, etc), and you want to disable the inverter/output on the STM

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shingadaddy · ‎2016-11-28

Posted on November 28, 2016 at 20:10

Apparently you don't HAVE to set that to use an external clock input on the OSC IN pin either.

It DOES free up the OSC *OUT* pin for other uses though.

Walid FTITI_O · ‎2016-11-29

Posted on November 29, 2016 at 10:59

Hi ctc.ctc,

As mentionned in application note

http://uglyduck.ath.cx/PDF/STMicro/ARM/STM32F4/STM32F4xx_Hardware_Dev.pdf

''Getting started with STM32F4xxxx MCU hardware development'':

4.1.1 External source (HSE bypass)

In this mode, an external clock source must be provided. It can have a frequency from 1 to 50 MHz (refer to STM32F4xxxx datasheets for actual max value). The external clock signal (square, sine or triangle) with a duty cycle of about 50%, has to drive the OSC_IN pin while the OSC_OUT pin must be left in the high impedance state.

-Hannibal-

shingadaddy · ‎2016-11-29

Posted on November 29, 2016 at 15:30

Interesting link you have there Hannibal. ''uglyduck.ath.cx........''

STM32L4XX_Hardware_Dev.PDF?

Blocked here at work. And AN4488 Rev 5 doesn't seem to even have a paragraph 4.1.1

But nevertheless, anyone winding up here by an HSE bypass search and reading these - beware. Check YOUR part manuals that are for YOUR part specifically. STM is inconsistent between part relatives about what can go on with the OSC OUT pin in the BYPASS mode. This is for the L4 for instance.

http://www.st.com/content/ccc/resource/technical/document/application_note/group0/93/81/9b/8d/2f/09/4a/e8/DM00125306/files/DM00125306.pdf/jcr:content/translations/en.DM00125306.pdf