问在STM32F103 (Bluepill)上通过串行发送时钟信号时的抖动问题

EN

#![no_std]
#![no_main]

use cortex_m_rt::entry;
use stm32f1xx_hal::{
  pac, 
  pac::{interrupt, Interrupt, TIM4},
  prelude::*,
  gpio,
  afio,
  serial::{Serial, Config},
  timer::{Event, Timer, CountDownTimer},
};

use core::mem::MaybeUninit;

use stm32f1xx_hal::pac::{USART2};

pub use embedded_hal::digital::v2::{OutputPin, InputPin};

pub type Usart2Serial = Serial<
    USART2, (gpio::gpioa::PA2<gpio::Alternate<gpio::PushPull>>, 
    gpio::gpioa::PA3<gpio::Input<gpio::Floating>>)>;

// When a panic occurs, stop the microcontroller
#[allow(unused_imports)]
use panic_halt;

static mut G_TIM2: MaybeUninit<CountDownTimer<TIM4>> = MaybeUninit::uninit();
static mut G_SERIAL: MaybeUninit<Usart2Serial> = MaybeUninit::uninit();

#[entry]
fn main() -> ! {

  let dp = pac::Peripherals::take().unwrap();

    let rcc = dp.RCC.constrain();
    let mut flash = dp.FLASH.constrain();
    let clocks = rcc.cfgr
      .use_hse(8.mhz()) // set clock frequency to external 8mhz oscillator
      .sysclk(72.mhz()) // set sysclock 
      .pclk1(36.mhz()) // clock for apb1 prescaler -> TIM1
      .pclk2(36.mhz()) // clock for apb2 prescaler -> TIM2,3,4
      .adcclk(12.mhz()) // clock for analog digital converters
      .freeze(&mut flash.acr);

    let mut apb1 = rcc.apb1;
    let mut apb2 = rcc.apb2;
    let mut gpioa = dp.GPIOA.split(&mut apb2);
    let mut afio = dp.AFIO.constrain(&mut apb2);

    // init serial
    let mut serial = init_usart2(dp.USART2, gpioa.pa2, gpioa.pa3, &mut gpioa.crl, &mut afio, &clocks, &mut apb1);

    unsafe { G_SERIAL.write(serial) };

    // init timer
    let bpm = 320;
    let frequency_in_hertz : u32 = (bpm as u32) * 24 / 60;
    let mut timer = Timer::tim4(dp.TIM4, &clocks, &mut apb1).start_count_down((frequency_in_hertz).hz());
    timer.listen(Event::Update);

    // write to global static var
    unsafe { G_TIM2.write(timer); }

    cortex_m::peripheral::NVIC::unpend(Interrupt::TIM4);
    unsafe {
      cortex_m::peripheral::NVIC::unmask(Interrupt::TIM4);
    }

  loop {
    // do nothing
  }
}

fn init_usart2(
  usart2: USART2, 
  pa2: gpio::gpioa::PA2<gpio::Input<gpio::Floating>>,
  pa3: gpio::gpioa::PA3<gpio::Input<gpio::Floating>>,
  crl: &mut gpio::gpioa::CRL,
  afio: &mut afio::Parts, 
  clocks: &stm32f1xx_hal::rcc::Clocks, 
  apb1: &mut stm32f1xx_hal::rcc::APB1
) -> Usart2Serial {
  let tx = pa2.into_alternate_push_pull(crl);
  let rx = pa3;

  return Serial::usart2(
    usart2,
    (tx, rx),
    &mut afio.mapr,
    Config::default().baudrate(31250.bps()),
    *clocks,
    apb1,
  );
}

#[interrupt]
fn TIM4() {
  let serial = unsafe { &mut *G_SERIAL.as_mut_ptr() };
  serial.write(0xF8).ok();

  let tim2 = unsafe { &mut *G_TIM2.as_mut_ptr() };
  tim2.clear_update_interrupt_flag();
}