2.7.3.5 SysTick Design Hints and Tips
The SysTick counter runs on the processor clock. If this clock signal is stopped for Low-power mode, the SysTick counter stops.
Ensure software uses aligned word accesses to access the SysTick registers.
The SysTick counter reload and current value are undefined at reset, the correct initialization sequence for the SysTick counter is:
Program reload value.
Clear current value.
Program Control and Status register.