I have just found and fixed a nasty bug that was slowing down main program loop when using Arduino Zero.

The problem was in the watchdog system. If you don't have experience with processor watchdog, I'll make a short intro: A watchdog is a countdown timer that once it reaches zero causes a processor reset. That means you have to periodically reset the counter so it never reaches zero. And periodically resetting means calling it in some kind of a loop. If this loop freezes, the watchdog timer will eventually reach zero and reset the processor, which is what it is supposed to do – an automatic reset when your program freezes or enters an infinite loop.

A watchdog can use a dedicated oscillator. In the case of Arduino Zero and my sketch, it is the ultra-low power internal oscillator running at 32.768 kHz. And because it is very slow, any write to a system that uses it – such as the reset of the watchdog timer – needs a synchronization with the main CPU clock, which runs at 48 MHz on Arduino Zero.

I was wrongly assuming this synchronization was fast, whereas it was not, governed by the speed of the slower clock. My code made a blocking wait for this synchronization:

WDT->CLEAR.reg = WDT_CLEAR_CLEAR_KEY; /* reset watchdog timer */

while(WDT->STATUS.bit.SYNCBUSY){} /* wait for sync */

A stupid mistake that was, fortunately, easy to find and fix to a non-blocking call:

if(WDT->STATUS.bit.SYNCBUSY == false) /* synchronization is not busy, meaning it is synchronized */

{

WDT->CLEAR.reg = WDT_CLEAR_CLEAR_KEY; /* reset watchdog timer */

}

This call will only reset timer if the previous reset was already synchronized. There. Fixed.

The performance gain? Tremendous. Previously, I was getting around 100 measurements per second and 400 turnarounds of the main loop, now I am at 237 measurements per second with 12500 (!) main loops per second.

So today's lesson is clear: Don't make blocking calls if you don't have to.