Systick Time-Driver Demo

Although we do not recommend using it and there are some shortcomings, it does work (after all, some chips lack sufficient multi-channel timers).

Currently, the systick-time-driver implementation in py32-hal uses a retry loop. The reload value register (RVR) of the Systick timer is set to a fixed value, causing high-frequency interrupts. During these interrupts, it updates the tick count and checks whether any alarm has expired (since Systick lacks compare-capture functionality).

The interrupt frequency of Systick matches the tick frequency configured for embassy-time. It is not recommended to use excessively high frequencies because time-driver-systick generates frequent interrupts, unlike time-driver-timx. Additionally, time-driver-systick updates the tick count and wakes up alarms in software, which takes longer.

Usage

Enable the time-driver-systick feature. Then, before initializing py32-hal, add the following:

use cortex_m::Peripherals;
let cp = Peripherals::take().unwrap();
let systick = cp.SYST;

Pass the systick instance during initialization:

let p = py32_hal::init(Default::default(), systick);

Complete code demo can be found in the same directory as this documentation.

Feature: `td-systick-multi-alarms`

By default, only one alarm is provided (similar to a 2-channel timer). Enabling this feature provides three alarms (similar to a 4-channel timer).

Of course, this will also increase the execution time of the interrupt handler.

More

SysTick time driver · Issue #786 · embassy-rs/embassy

Here are the key disadvantages of using SysTick as a time-driver:

Clock Source Uncertainty: SysTick supports two clock sources, but their speeds are not known at compile time, requiring vendor-specific runtime translation.
Interrupt Limitation: SysTick can only trigger an interrupt at zero, complicating setting alarms for specific points in the cycle.
Dual-Core Challenges: On dual-core devices like the RP2040, each core has an independent SysTick timer with no cross-access.
Perturbation Issues: Modifying the counter register while counting can perturb the overall tick count, affecting timing accuracy.

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