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Getting Low Power With USB When Not Suspended (STM32F415RG)

Question asked by horkin.charles on Mar 31, 2016

We're trying to reduce power consumption when not suspended and using USB.  During the time when we are not handing USB interrupts, we would like to run at a slower core clock speed.  I'm wondering if this is possible or not?

Looking at CubeMx and the Reference Manual, I see that the AHB clock must be 30MHz or higher for the USB HS PHY.  However, I also see that this clock is used to access the USB registers.  So, my question is if I can switch the SYSCLK to use the HSI using the System Clock Mux as long as I am not accessing the USB peripheral?  This would bring the AHB down to 16MHz.  I would then disable interrupts and do a WFI and then use the PLL clock again after I wake.

If I can do this, are there steps I need to take when I switch back to the PLLCLK?  For example, do I need to set the HRST bit in the USB PHY?  

If I can't do this, is there a suggestion?  We don't really want to run at full tilt when we aren't doing anything.