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SYSTICK is really funky - I think

Question asked by Jacobs.Carl.001 on Sep 10, 2012
Latest reply on Sep 11, 2012 by Jacobs.Carl.001
In my application I set up SysTick with:
     SysTick_Config(SystemCoreClock / 1000);
to generate a 1ms interrupt.
I got some weird timing anomalies and eventually used a timer interrupt to generate 1ms interrupts using the PSC (prescale) and ARR (auto-reload) registers.
When I compare the two I get about 1012 to 1032 SysTick interrupts for every 1000 timer interrupts - yup that's right folks, it varies every cycle. When I compare timer interrupts to other timer interrupts, they all report as being perfectly in sync with each other.
To me, it seems that SysTick counts "ticks" and expecting anything more than that might not be correct. 
The difference probably wouldn't matter if I wasn't using a 3ppm oscillator to clock the system. Although to be honest I have no way of telling which time reference is correct.
Any ideas as to what's going on?