SAM D21 SysTick Overview
The SAM D21 ARM® Cortex®-M0+ Processor Core includes an optional System Timer Module (SysTick) that provides a simple, 24-bit clear-on-write, decrementing, wrap-on-zero counter with a flexible control mechanism.
This hardware may be configured to periodically generate an interrupt and trigger the following handler routine:
void SysTick_Handler(void){
...
}
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The SysTick operation is specified in Section B3.3 of the "ARM®v6-M Architecture Reference Manual."