Disable SAMD21 SysTick interrupt during sleep

Disabling the systick timer interrupt during sleep has previously been discussed on the Microchip/Atmel community forum: https://community.atmel.com/comment/2625116#comment-2625116.

In summary, due to a hardware bug on the SAMD21, the SysTick interrupts become active before the flash has powered up from sleep, causing a hard fault after a random time. To prevent this the SysTick interrupts are disabled before entering sleep mode and enabled oncemore after the processor has woken up:

SysTick->CTRL &= ~SysTick_CTRL_TICKINT_Msk;  // Disable SysTick interrupts
__DSB();   // Data sync to ensure outgoing memory accesses complete
__WFI();   // Wait for interrupt (places device in sleep mode)
SysTick->CTRL |= SysTick_CTRL_TICKINT_Msk;   // Enable SysTick interrupts

Author: robdmob

src/RTCZero.cpp

@@ -145,11 +145,15 @@ void RTCZero::detachInterrupt()
 
 void RTCZero::standbyMode()
 {
+  // Disable systick interrupt
+  SysTick->CTRL &= ~SysTick_CTRL_TICKINT_Msk;
   // Entering standby mode when connected
   // via the native USB port causes issues.
   SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk;
   __DSB();
   __WFI();
+  // Enable systick interrupt
+  SysTick->CTRL |= SysTick_CTRL_TICKINT_Msk;
 }
 
 /*