ARM Technical Support Knowledge Articles

Why is the SysTick Calibration TENMS value one less than the number of clock cycles required for 10ms?

Applies to: Cortex-M0, Cortex-M3, Cortex-M4

Answer

The TENMS field is defined to be the reload value for the counter which will yield a 10ms period.

Since the counter is an (n-1) downto 0 counter, the TENMS field must therefore be programmed to be one decrement less than the number of cycles which make up the 10ms period.

In order to scale to a different time interval, the software must read the TENMS value, increment it, scale by the required factor, then subtract one from the result to obtain the required reload register value.

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