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Mini57
Apr. 06, 2017
Page 79 of 475
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6.2.10 System Timer (SysTick)
The Cortex
®
-M0 includes an integrated system timer, SysTick, which provides a simple, 24-bit
cleared-on-write, decrementing, wrap-on-zero counter with a flexible control mechanism. The
counter can be used in several different ways, for example:
An RTOS tick timer fires at a programmable rate (for example 100Hz) and invokes a SysTick
routine.
A high-speed alarm timer uses Core clock.
A variable rate alarm or signal timer
– the duration range is dependent on the reference clock
used and the dynamic range of the counter.
A simple counter can be used by software to measure task completion time.
An internal Clock Source control based on missing/meeting durations. The COUNTFLAG bit-field
in the control and status register can be used to determine if an action completed within a set
duration, as part of a dynamic clock management control loop.
When enabled, the timer will count down from the value in the SysTick Current Value Register
(SYST_CVR) to 0, and reload (wrap) to the value in the SysTick Reload Value Register
(SYST_RVR) on the next clock edge, and then decrement on subsequent clocks. When the
counter transitions to 0, the COUNTFLAG status bit is set. The COUNTFLAG bit clears on read.
The SYST_CVR value is UNKNOWN on reset. Software should write to the register to clear it to 0
before enabling the feature. This ensures the timer will count from the SYST_RVR value rather
than an arbitrary value when it is enabled.
If the SYST_RVR is zero, the timer will be maintained with a current value of zero after it is
reloaded with this value. This mechanism can be used to disable the feature independently from
the timer enable bit.
For more detailed information, please refer to the “ARM
®
Cortex
®
-M0 Technical Reference
Manual” and “ARM
®
v6-
M Architecture Reference Manual”.
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