docs: content related to systick

README.md

@@ -66,7 +66,7 @@ For a full list of chip capabilities and peripherals, check the [py32-data](http
 - TODO: I haven't got a dev board yet, help-wanted
 - N/A: Not available
 
-### TODOs
+## TODOs
 
 Too many...
 
@@ -88,7 +88,9 @@ Too many...
 
 This crate provides an implementation of the Embassy `time-driver`.
 
- Embassy requires that any TIM used as a time-driver has at least two channels, so only TIM1 and TIM3 are available for the PY32F030, 003, and 002A series. You can select either `time-driver-tim3` or `time-driver-tim1` to specify the TIM to use.
+Embassy requires that any TIM used as a time-driver has at least two channels, so only TIM1 and TIM3 are available for the PY32F030, 003, and 002A series. You can select either `time-driver-tim3` or `time-driver-tim1` to specify the TIM to use.
+
+`time-driver-systick`: Although we do not recommend using it and there are some shortcomings, it does work. For details, please see [systick-demo](examples/systick-time-driver-f030/README.md)
 
 ## Minimum supported Rust version(MSRV)
 

examples/systick-time-driver-f030/README.md

@@ -0,0 +1,42 @@
+## 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:
+
+```rust
+use cortex_m::Peripherals;
+let cp = Peripherals::take().unwrap();
+let systick = cp.SYST;
+```
+
+Pass the `systick` instance during initialization:
+
+```rust
+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](https://github.com/embassy-rs/embassy/issues/786)
+
+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.