DWT clock cycle count stuck unless trace is enabled

[bschwind]

I tried using a MonoTimer to measure time between two events (in my case, some button pushes). I noticed the time-based code was never firing, and dug into the code until I discovered that DWT::get_cycle_count always returns 1073741824 (2^30)

In order for clock cycles to be successfully reported from DWT, I have to call cp.DCB.enable_trace();

Here is a minimal reproduction of the issue, printing the clock cycle counts over a serial interface. My hardware is a "black pill" stm32f103c8t6, specifically this dev board. I flash and monitor output via a CP2102 serial-usb adapter. I'm not currently using a debugger probe of any sort.

#![deny(unsafe_code)]
#![no_main]
#![no_std]

use panic_halt as _;

use cortex_m_rt::entry;
use stm32f1xx_hal::{
    pac,
    prelude::*,
    serial::{Config, Serial},
};

use core::fmt::Write;

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

    let mut flash = p.FLASH.constrain();
    let mut rcc = p.RCC.constrain();
    let clocks = rcc.cfgr.freeze(&mut flash.acr);

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

    let tx = gpioa.pa9.into_alternate_push_pull(&mut gpioa.crh);
    let rx = gpioa.pa10;

    let serial = Serial::usart1(
        p.USART1,
        (tx, rx),
        &mut afio.mapr,
        Config::default().baudrate(115200.bps()),
        clocks,
        &mut rcc.apb2,
    );

    let (mut tx, _rx) = serial.split();

    // Needed in order for MonoTimer to work properly
    cp.DCB.enable_trace();

    let mut delay = stm32f1xx_hal::delay::Delay::new(cp.SYST, clocks);
    let mono_timer = stm32f1xx_hal::time::MonoTimer::new(cp.DWT, clocks);

    let start = mono_timer.now();

    loop {
        let elapsed = start.elapsed();
        writeln!(tx, "Elapsed clock cycles {}\r", elapsed).unwrap();
        delay.delay_ms(100_u32);
    }
}

I found a few older issues in earlier repositories about this:

• MonoTimer does not work without debugger connected japaric/stm32f103xx-hal#76 (comment)
• Fix MonoTimer when no debugger is connected japaric/stm32f103xx-hal#94

[TheZoq2]

Huh, I didn't know you could enable the DWT without a debugger attached, we probably should do that then... Though it still feels a bit odd to use the debug peripheral for timekeeping

[fuchsnj]

I just tested this on a bluepill, and without cp.DCB.enable_trace(), DWT::get_cycle_count() always returns 1073741824 (same as @bschwind ). However, when I enable trace, it always returns 0.
I haven't been able to get it to actually count cycles yet.

[fuchsnj]

Ah, spoke too soon. If I enable trace before I create the MonoTimer everything is working fine on my bluepill. Enabling it after makes it always return 0.