embassy_stm32/can/
util.rs

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//! Utility functions shared between CAN controller types.

use core::num::{NonZeroU16, NonZeroU8};

/// Shared struct to represent bit timings used by calc_can_timings.
#[derive(Clone, Copy, Debug)]
pub struct NominalBitTiming {
    /// Value by which the oscillator frequency is divided for generating the bit time quanta. The bit
    /// time is built up from a multiple of this quanta. Valid values are 1 to 512.
    pub prescaler: NonZeroU16,
    /// Valid values are 1 to 128.
    pub seg1: NonZeroU8,
    /// Valid values are 1 to 255.
    pub seg2: NonZeroU8,
    /// Valid values are 1 to 128.
    pub sync_jump_width: NonZeroU8,
}

/// Calculate nominal CAN bit timing based on CAN bitrate and periphial clock frequency
pub fn calc_can_timings(periph_clock: crate::time::Hertz, can_bitrate: u32) -> Option<NominalBitTiming> {
    const BS1_MAX: u8 = 16;
    const BS2_MAX: u8 = 8;
    const MAX_SAMPLE_POINT_PERMILL: u16 = 900;

    let periph_clock = periph_clock.0;

    if can_bitrate < 1000 {
        return None;
    }

    // Ref. "Automatic Baudrate Detection in CANopen Networks", U. Koppe, MicroControl GmbH & Co. KG
    //      CAN in Automation, 2003
    //
    // According to the source, optimal quanta per bit are:
    //   Bitrate        Optimal Maximum
    //   1000 kbps      8       10
    //   500  kbps      16      17
    //   250  kbps      16      17
    //   125  kbps      16      17
    let max_quanta_per_bit: u8 = if can_bitrate >= 1_000_000 { 10 } else { 17 };

    // Computing (prescaler * BS):
    //   BITRATE = 1 / (PRESCALER * (1 / PCLK) * (1 + BS1 + BS2))       -- See the Reference Manual
    //   BITRATE = PCLK / (PRESCALER * (1 + BS1 + BS2))                 -- Simplified
    // let:
    //   BS = 1 + BS1 + BS2                                             -- Number of time quanta per bit
    //   PRESCALER_BS = PRESCALER * BS
    // ==>
    //   PRESCALER_BS = PCLK / BITRATE
    let prescaler_bs = periph_clock / can_bitrate;

    // Searching for such prescaler value so that the number of quanta per bit is highest.
    let mut bs1_bs2_sum = max_quanta_per_bit - 1;
    while (prescaler_bs % (1 + bs1_bs2_sum) as u32) != 0 {
        if bs1_bs2_sum <= 2 {
            return None; // No solution
        }
        bs1_bs2_sum -= 1;
    }

    let prescaler = prescaler_bs / (1 + bs1_bs2_sum) as u32;
    if (prescaler < 1) || (prescaler > 1024) {
        return None; // No solution
    }

    // Now we have a constraint: (BS1 + BS2) == bs1_bs2_sum.
    // We need to find such values so that the sample point is as close as possible to the optimal value,
    // which is 87.5%, which is 7/8.
    //
    //   Solve[(1 + bs1)/(1 + bs1 + bs2) == 7/8, bs2]  (* Where 7/8 is 0.875, the recommended sample point location *)
    //   {{bs2 -> (1 + bs1)/7}}
    //
    // Hence:
    //   bs2 = (1 + bs1) / 7
    //   bs1 = (7 * bs1_bs2_sum - 1) / 8
    //
    // Sample point location can be computed as follows:
    //   Sample point location = (1 + bs1) / (1 + bs1 + bs2)
    //
    // Since the optimal solution is so close to the maximum, we prepare two solutions, and then pick the best one:
    //   - With rounding to nearest
    //   - With rounding to zero
    let mut bs1 = ((7 * bs1_bs2_sum - 1) + 4) / 8; // Trying rounding to nearest first
    let mut bs2 = bs1_bs2_sum - bs1;
    core::assert!(bs1_bs2_sum > bs1);

    let sample_point_permill = 1000 * ((1 + bs1) / (1 + bs1 + bs2)) as u16;
    if sample_point_permill > MAX_SAMPLE_POINT_PERMILL {
        // Nope, too far; now rounding to zero
        bs1 = (7 * bs1_bs2_sum - 1) / 8;
        bs2 = bs1_bs2_sum - bs1;
    }

    // Check is BS1 and BS2 are in range
    if (bs1 < 1) || (bs1 > BS1_MAX) || (bs2 < 1) || (bs2 > BS2_MAX) {
        return None;
    }

    // Check if final bitrate matches the requested
    if can_bitrate != (periph_clock / (prescaler * (1 + bs1 + bs2) as u32)) {
        return None;
    }

    // One is recommended by DS-015, CANOpen, and DeviceNet
    let sync_jump_width = core::num::NonZeroU8::new(1)?;

    let seg1 = core::num::NonZeroU8::new(bs1)?;
    let seg2 = core::num::NonZeroU8::new(bs2)?;
    let nz_prescaler = core::num::NonZeroU16::new(prescaler as u16)?;

    Some(NominalBitTiming {
        sync_jump_width,
        prescaler: nz_prescaler,
        seg1,
        seg2,
    })
}