Problem with I2C on PA8 and PC9

markusherrmann9 · ‎2014-07-10

Posted on July 10, 2014 at 11:41

Hello,

i tried to use I2C3 on PA8 and PC9. I noticed that i can send data over the i2c bus but the stop condition was not working.

So the bus never came back. After some investigation i found that the SCL line starts oscillating with 16MHz (I use the HSI oscillator).

I checked the RCC MCO settings and noticed that it is set to HSI output. When i change the prescaler i can change the frequency.

Since i use a STM32F407VGT6 i am not able to use alternative pins. Is there any workaround to avoid MCO to overtake the I2C lines?

#stm32f407 #mco #i2c

waclawek.jan · ‎2014-07-10

Posted on July 10, 2014 at 12:28

> After some investigation i found that the SCL line starts oscillating with 16MHz

Under what circumstances does this happen?

What is the content of GPIOA_AFRH in that moment?

JW

markusherrmann9 · ‎2014-07-11

Posted on July 11, 2014 at 13:36

Finally i got it working. The Problem was that i did the usb init after i2c init.

The usb init caused the pin mode change. After changing the init order everything works fine.

waclawek.jan · ‎2014-07-11

Posted on July 11, 2014 at 14:37

> The usb init caused the pin mode change.

Why would USB init set MCO output?

Is the USB init your code or some library?

JW