stm32/usart: Immediate Error::Overrun when receiving data with embassy_stm32::usart::BufferedUart or RingBufferedUartRx

[janschiefer]

I'm trying to communicate with a LTE modem and run into a couple of problems with the current UART implementations of embassy_stm32.

• When using BufferedUartRx or the new RingBufferedUartRx I run into an immediate Error::Overrun when testing in loopback mode when I send and receive from the same board in parallel. Even with very big rx_buffer and very little transmitted data. This applies to fill_buf() and read(&mut [u8]) methods.

• When I try receiving from a serial to UART converter in low speed (because I type in the text to send on my PC) I can receive some text, before I also receive Error::Overrun.

• The consume(&self, usize) from BufferedUartRx / RingBufferedUartRx method seems to have no effect in preventing overruns.

• UartRx in DMA mode with read_until_idle() kinda works together with embassy time::with_timeout, but this solution is quite cumbersome and suboptimal.

Tested on an old STM32F103C8T6 "BluePill" board with original STM32.

[abelREK]

I have also replicated this issue on a STM32f405RG.

[mxseev]

Perhaps the problem is in the default RCC configuration. I had the same issue on my STM32F411CEU6 (Black pill), but after manual RCC setup (at least enabling HSE) everything worked fine. My config (black pill has an external quartz at 25mhz.):

    let mut config = Config::default();
    config.rcc.hse = Some(Hse {
        freq: Hertz(25_000_000),
        mode: HseMode::Oscillator,
    });
    config.rcc.pll_src = PllSource::HSE;
    config.rcc.pll = Some(Pll {
        prediv: PllPreDiv::DIV25,
        mul: PllMul::MUL200,
        divr: Some(PllRDiv::DIV2),
        divp: Some(PllPDiv::DIV2),
        divq: None,
    });
    config.rcc.ahb_pre = AHBPrescaler::DIV1;
    config.rcc.apb1_pre = APBPrescaler::DIV2;
    config.rcc.apb2_pre = APBPrescaler::DIV2;
    config.rcc.sys = Sysclk::PLL1_P;

    let p = embassy_stm32::init(config);

[FourLeafTec]

Same issue on STM32F407VET6.