embassy-rs/embassy · error

baudrate is not permitted in this mode

Error message

{} baudrate is not permitted in this mode

What it means

`configure_clock` in the LPC55 USART driver only supports a fixed set of baudrates per clock mode; the selected mode's clock source maps to specific baudrate divisors, and any other requested baudrate falls into a catch-all arm that panics. The message includes the offending `config.baudrate`.

Solutions

  1. Change `config.baudrate` to one of the values supported by the selected mode (inspect the match arms above line 525 for the allowed set)
  2. Select a different clock mode whose supported baudrate set includes your target rate
  3. Verify with the LPC55Sxx user manual which FLEXCOMM clock source (FCCLKSEL) supports the desired baud rate
  4. If you need an arbitrary baudrate, configure the fractional divider explicitly or extend the driver's match arms

Example fix

// before
let mut usart = Usart::new(usart, tx, rx, &config{ mode, baudrate: 115_200, .. });
// after
let mut usart = Usart::new(usart, tx, rx, &Config { mode, baudrate: 1_000_000, .. }); // a supported rate for this mode
Defensive patterns

Strategy: validation

Validate before calling

const SUPPORTED_BAUDRATES: &[u32] = &[/* rates supported by the selected mode */];
fn baudrate_supported(mode: Mode, baudrate: u32) -> bool {
    SUPPORTED_BAUDRATES.contains(&baudrate) // restrict further by mode if needed
}

Prevention

When it happens

Trigger: Calling `Usart::new`/`configure` with a `config.baudrate` that is not one of the supported values for the chosen clock mode (e.g. requesting 115200 while the mode's FCC clock selection only provides specific rates like 1 MHz source).

Common situations: Copying a baudrate from a different MCU's example (115200 is common elsewhere but unsupported in the mode used here); changing the clock mode enum without re-checking which baudrates remain valid; typo'd baudrate values.

Understand the failure class

Background: "Invalid value" and "allowed values are" config errors: what your library rejected and how to fix it — this error's family across 41 libraries.

Related errors

AI-assisted analysis of embassy-rs/embassy@463a07b963 (2026-09-10). Data as JSON: /api/errors/b35bda64c48e8442. Report an issue: GitHub.

Appendix: source

Thrown at embassy-nxp/src/usart/lpc55.rs:525

                SYSCON
                    .fcclksel(T::instance_number())
                    .modify(|w| w.set_sel(syscon::vals::FcclkselSel::Enum0x3)); // 96 MHz
                96_000_000
            }
            1501..=750_000 => {
                SYSCON
                    .fcclksel(T::instance_number())
                    .modify(|w| w.set_sel(syscon::vals::FcclkselSel::Enum0x2)); // 12 MHz
                12_000_000
            }
            121..=1500 => {
                SYSCON
                    .fcclksel(T::instance_number())
                    .modify(|w| w.set_sel(syscon::vals::FcclkselSel::Enum0x4)); // 1 MHz
                1_000_000
            }
            _ => {
                panic!("{} baudrate is not permitted in this mode", config.baudrate);
            }
        };
        // Calculate MULT and BRG values based on baudrate

        // There are two types of dividers: integer divider (baud rate generator register and oversample selection value)
        // and fractional divider (fractional rate divider).
        // For oversampling, the default is industry standard 16x oversampling, i.e. OSRVAL = 15

        // The formulas are:

        // FLCK = (clock selected via FCCLKSEL) / (1 + MULT / DIV)
        // DIV is always 256, then:
        // FLCK = (clock selected via FCCLKSEL) / (1 + MULT / 256)

        // Baud rate = [FCLK / (OSRVAL+1)] / (BRGVAL + 1) =>
        // Baud rate = [FCLK / 16] / (BRGVAL + 1)

        // There are 2 unknowns: MULT and BRGVAL.

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