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From which output of the Crystal oscillator is the signal for clocking the microcontroller taken?

MMust.5
Senior II

STM32F407


_legacyfs_online_stmicro_images_0693W00000biXIiQAM.png
_legacyfs_online_stmicro_images_0693W00000biXIdQAM.pngUsually the signal is taken from the PH1-OSC_OUT pin if it is a parallel resonant oscillator circuit.

The signal for clocking the microcontroller is formed on the Crystal oscillator X2, fed to the microcontroller and amplified in the microcontroller.

If so, why is PH0-OSC_IN also connected to the microcontroller?

The MCO(PH0-OSC_IN) signal is needed only to generate a signal on the Crystal oscillator X2, why is this signal fed to the microcontroller?

If you disable PH0-OSC_IN from the microcontroller, then the signal from the Crystal oscillator X2 will continue to be fed to the microcontroller via  PH1-OSC_OUT.

Why is PH0-OSC_IN connected to the microcontroller then?

19 REPLIES 19

>if you remove/de-solder R68 (= disconnect MCO ), X2 + F407 set to external clock are the 8MHz source then.

X2 does not generate a signal after the MCO disappears.

I checked with an oscilloscope, there is no 8 megahertz signal on PH0-OSC_IN.


_legacyfs_online_stmicro_images_0693W00000bkUC2QAM.png
_legacyfs_online_stmicro_images_0693W00000bkUCWQA2.png>also on my F407 board LSE , 32kHz crystal, is fitted, but in diagram is marked as "not fitted" .

On my motherboard X2 is installed.

ok, now set F407 to "crystal..." , ok. Then it will work !

but energy is low, so you cannot go with any probe to crystal - you stop it with the probe !

just probe at 10:1 , low capacity, can (maybe!) see oscillation on osc-out pin. try.

on osc-in you will probably see "nothing" , probe stops oscillation.

to check it right way: enable MCO on F407 and then check with probe (10:1 setting always ! ) on mco pin - there you see it...

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I'm sorry, I forgot about SB17, now I'll try.

>on (old) F407 boards this "switch" SB is missing

SB17 is present on my STM32F407, but I think this is not the jumper you are talking about.

But I still tried to unplug it.

If I remove this jumper, the 8 MHz signal still does not appear on PH0-OSC_IN.

There is 3 volts DC on PH1-OSC_OUT, but no frequency.

There is nothing on PH0-OSC_IN.

Tested in 10:1 mode.

this looks like HSE is set to "bypass" .

shure, you have set it to "crystal.." and load /debug then ? and use MCO setting !

+

see circuit of dev-E board , F407 black board:


_legacyfs_online_stmicro_images_0693W00000bkUHqQAM.pngso add 500k ... 1 meg resistor in par. to crystal !

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MMust.5
Senior II

Somewhere there is a Solder Bridge that shorts out C15.

I'll look for one tomorrow. I didn't find it on the diagram.

Many thanks for the help))

I made this "Solder Bridge" myself when I desoldered the R68.

Everything works, there is a signal of 8 MHz without an MCO connected)

MMust.5
Senior II

I have another question on the same topic.

HSE is formed by a crystal and is equal to 8 megahertz.

HSI is formed inside the STM32F407 microcontroller and is equal to 16 MHz.

Are there settings for the 8 MHz MCO signal that comes from ST Link? (I know that this frequency is unchangeable, maybe some other settings, for example, setting MCO 8 MHz from ST Link as a clock source.)

Or is ST Link's 8 MHz MCO just a fallback for HSE?

There are no separate settings for MCO 8 MHz from ST Link? STM32F407 microcontroller sees MCO 8 MHz from ST Link as HSE?

AScha.3
Chief

> Are there settings for the 8 MHz MCO signal that comes from ST Link?

no. except you set in clock settings an external 8M clock.

>Or is ST Link's 8 MHz MCO just a fallback for HSE?

no. is cheap standard clock on most nucleo boards, because stlink anyway there.

>There are no separate settings for MCO 8 MHz from ST Link? 

no.

> STM32F407 microcontroller sees MCO 8 MHz from ST Link as HSE?

yes. is same as external clock oscillator for F407 ; can use the "bypass" clock setting.

(you have changed this, now F407 on his own crystal clock. )

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