cortex-m-microclock

This crate provides a software clock which relies on the CYCCNT counter present in most Cortex-M chips to allow user to measure time and produce delays. The precision of the clock depends on your microcontroller core frequency. If you have a core running at at least 1MHZ, you will have a microsecond precision.

Note 0: Some Cortex-M cores like M0, M0+ and M23 cores does not have a CYCCNT counter. Therefore, the present crate cannot be used on these chips Note 1 The present crate does not work on multicore systems.

Underlaying hardware

The clock is based on the CYCCNT counter from the Cortex-M DWT peripheral, which increments with each processor clock cycle. However as the CYCCNT upcounter is only 32 bits wide, it may overflow quite rapidly depending on your SYSCLK frequency. The CYCCNTClock keeps track of multiple CYCCNT cycles using an internal counter so it can be used to evaluate very large durations of time.

Crate structure

The CYCCNTClock is the structure representing the software clock. This structure is a singleton exposing all the methods of the crate available to user. All these methods are static and can be called from any thread without concurrency issues.

How to use this crate

In order to use the clock you should first call the [CYCCNTClock::init()] method which takes ownership of the DWT peripheral. From this point you can use [CYCCNTClock::now()] and [CYCCNTClock::delay()] methods. The [CYCCNTClock::update()] method should be called periodically to avoid missing the CYCCNT wrapping around. Use the following equation to find the minimum update frequency:

min_update_freq = SYS_CLK_FREQ/(2³²)

Note that the [CYCCNTClock::now()] and [CYCCNTClock::delay()] methods implicitely call the [CYCCNTClock::update()] method.

Example

A complete example of the use of this crate on a STMF103 blue pill is featured in the examples directory of this project.

Credits

Many thanks to the authors of fugit and rtic::dwt_systick_monotonic crates