How to use SysTick timer to count ticks from a rotary encoder? about stm32f0xx-hal HOT 3 OPEN

exception has a special meaning for Cortex-M CPUs. Some exceptions are used for error status', but more generally they are just built-in interrupts.

This is the exception number definition for ARMv6-M: https://developer.arm.com/documentation/ddi0419/c/System-Level-Architecture/System-Level-Programmers--Model/ARMv6-M-exception-model/Exception-number-definition?lang=en

2 is HardFault, that occurs only when there's an error condition, but 15 is SysTick, and that occurs as part of normal operation. All the interrupts added by ST then get allocation to number 16+

from stm32f0xx-hal.

I gave it a quick skim, it looks like the SysTick is simply used to calculate the speed of the encoder: stm32f1xx_it.c#L197

The actual position comes from a different mechanism, HAL_TIM_IC_CaptureCallback (if I am reading that right, I did skim this fairly quickly 😅 )

Assuming you have that position the SysTick handler could be written in rust along the lines of:

// probably missing some use statements, I didn't actually compile this.
use cortex_m_rt::{entry, exception};

static OLDPOS: i16 = 0;
static INDX: i32 = 0;

#[exception]
fn SysTick() {

    unsafe {
        INDX += 1;
        if (INDX == 500) {
            SPEED = ((POSITION - OLDPOS) * 2);
            OLDPOS = POSITION;
            INDX = 0;
        }
    }

    // whatever else here
}

#[entry]
fn main() -> ! {
    let p = cortex_m::Peripherals::take().unwrap();
    let mut syst = p.SYST;

    // configures the system timer to trigger a SysTick exception every second
    syst.set_clock_source(SystClkSource::Core);
    // CPU clock of 8 MHz
    syst.set_reload(8_000_000);
    syst.clear_current();
    syst.enable_counter();
    syst.enable_interrupt();

    loop {
        compiler_fence(SeqCst);
    }
}

Actually getting the position is a bit more effort that I don't know about off the top of my head.

That C example you linked isn't too big, you could probably recreate that using the PAC to get started.

from stm32f0xx-hal.

@newAM Thanks very much for this!

I saw in the examples that the #[exception] macro was used, but based on the comments it seems like it 's only called when an error occurs in the interrupt. But based on your example, it's the interrupt routine itself?

from stm32f0xx-hal.