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Reason for Cortex A53 delays

urian

Hello,

I want to write a bit-banging driver for a Raspberry Pi 3 (Cortex A53 with 4 cores).
For testing I developed a simple Linux kernel mode driver which toggles a GPIO pin with approx. 1Mhz.
In order to get no interference by the Linux kernel I disabled IRQ and FIQ before entering the toggling loop.
I have also set CPU speed fix to 1.2Ghz.
Then I monitor the GPIO pin with a scope.
If I set a pulse-width > 2 us trigger,  I see gaps of different length, which sometimes last longer than 5 us (depends on load for the other cores)   

On a Rasperry Pi Zero ( ARM1176JZF-S with 1 Core) no such effect occurs. The signal is total clean at 1 Mhz
So on the Pi 3 I conclude the delays must be some hardware interference from the other cores.
Can anyone explain me how the hardware is causing such long delays?  

Thanks,
Rainer 

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