SysTick less accurate than expected

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by mfc on Mon Jan 30 03:02:27 MST 2012
I was hoping to use SysTick as a precise real-time clock by setting the SysTick count to interrupt every 100 microseconds and then simply counting (every 10,000 counts bumps a seconds counter, [I]etc[/I].).

On running this overnight, I find that there were 0.3% more interrupts than expected (3000 parts per million, whereas the external 12MHz crystal from which the clock is driven should be within 30 ppm).   This equates to a gain of about 270 seconds (4.5 minutes) per day, which is too large for my application.

Anyone have any ideas what might be causing this?  I've checked the SysTick CALIB register and it has the expected value (0x000F423F) for the 1768 chip that I'm using.

Many thanks -- Mike