LSE frequency more than 5% off from 32.768 kHz, cannot use…

Error message

LSE frequency more than 5% off from 32.768 kHz, cannot use for MSI auto-calibration

What it means

Same guard as the U3 variant, in embassy-stm32's STM32U5 RCC init: MSI auto-calibration against LSE is requested, but the configured LSE frequency deviates more than 5% from the nominal 32.768 kHz. The hardware expects a standard 32.768 kHz reference for trimming MSI, so the driver panics to avoid calibrating MSI to a wrong frequency.

Solutions

Correct config.ls.lse.frequency to Hertz(32_768) to match the fitted crystal
Disable MSI auto-calibration on msis/msik if the LSE reference is not ~32.768 kHz
Double-check the board schematic for the actual LSE crystal value and align the config

Example fix

// before
config.ls.lse = Some(LseConfig { frequency: Hertz(30_000), .. }); // >5% off
config.msisk = ...; // auto-cal enabled
// after
config.ls.lse = Some(LseConfig { frequency: Hertz(32_768), .. });
// keep auto-cal enabled only with a valid 32.768 kHz reference

Defensive patterns

Strategy: validation

Validate before calling

if let Some(lse) = &config.ls.lse {
    assert!((31_132..=34_406).contains(&lse.frequency.0), "LSE out of 32.768kHz +/-5% band");
}

Type guard

fn lse_in_band(freq: Hertz) -> bool {
    (31_132..=34_406).contains(&freq.0)
}

Prevention

Use the standard 32.768 kHz crystal value in LseConfig
Only enable MSI auto-cal with a genuine ~32.768 kHz reference
Re-verify LSE config after any hardware revision
Test clock init early in board bring-up

When it happens

Trigger: config.msis or config.msik with auto_calibration enabled and config.ls.lse.frequency outside 32.768 kHz ± 5% (31132–34406 Hz) on an STM32U5 target.

Common situations: Wrong frequency value typed into LseConfig; using a non-32.768 kHz clock into the LSE pins while auto-cal is on; reusing an RCC config from a different board; changing the crystal without updating the frequency field.

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/61da749fe3ad48c8. Report an issue: GitHub.

Appendix: source

Thrown at embassy-stm32/src/rcc/u5.rs:237

            // Check that the calibration is applied to an active clock
            match (
                config.auto_calibration.base_mode(),
                config.msis.is_some(),
                config.msik.is_some(),
            ) {
                (MsiAutoCalibration::MSIS, true, _) => {
                    // MSIS is active and using LSE for auto-calibration
                    Some(lse_config.frequency)
                }
                (MsiAutoCalibration::MSIK, _, true) => {
                    // MSIK is active and using LSE for auto-calibration
                    Some(lse_config.frequency)
                }
                // improper configuration
                _ => panic!("MSIx auto-calibration is enabled for a source that has not been configured."),
            }
        } else {
            panic!("LSE frequency more than 5% off from 32.768 kHz, cannot use for MSI auto-calibration");
        }
    } else {
        None
    };

    let mut msis = config.msis.map(|range| {
        // Check MSI output per RM0456 § 11.4.10
        match config.voltage_range {
            VoltageScale::Range4 => {
                assert!(msirange_to_hertz(range).0 <= 24_000_000);
            }
            _ => {}
        }

        // RM0456 § 11.8.2: spin until MSIS is off or MSIS is ready before setting its range
        loop {
            let cr = RCC.cr().read();
            if cr.msison() == false || cr.msisrdy() == true {

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