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Unexplained MCO behavior (?) on STM32F372CBT6

Question asked by Megadan on Dec 26, 2012
Latest reply on Dec 27, 2012 by Megadan
I'm running an external 26MHz xtal (10ppm) in HSE mode with the right multipliers/dividers to clock my STM32 core at 71.5MHz. Which works great. In addition, I have PA8 configured in MCO mode to drive said 26MHz clock out to a Bluetooth radio chip. There is definitely the expected frequency of square wave coming out. However, sometimes the radio link works and sometimes it does not. Here is what I've figured out.

If I run my mainline code full blast, talking over I2C, running the DMA engine into the USART Tx, etc. the radio link is unreliable.

However if I preempt my main while loop with a while(1); and just sit in one place looping on myself the radio works flawlessly.

Does anyone (Clive?!) know of any reason why the arbitrary execution of code could alter the quality of the MCO clock output? There is a lot going on on GPIOA in my design but nothing ever messes with that pin, of course.

(I did test the radio chip with a separately dedicated 26MHz xtal and turn off MCO - results were a rock solid radio link.)

Thank you all,