embassy-rs/embassy · error

MSI is not ready to be selected as system clock source

Error message

MSI is not ready to be selected as system clock source

What it means

On STM32N6, before switching the system clock to MSI, the code checks `RCC.sr().msirdy()`. If MSI is not running and ready, selecting it as the system bus clock source is invalid and the library panics.

Solutions

1. Add an MSI config (`msi: Some(MsiConfig...)`) to the rcc config so MSI is enabled and ready.
2. Or pick a ready sysclk source (Hsi/Hse/Ic2) in `config.sys`.
3. Ensure neither the cpu-clock stage nor other code disables MSI before the sysclk switch.

Example fix

// before
rcc: RccConfig { sys: SysClk::Msi, ..Default::default() }, // msi absent
// after
rcc: RccConfig { msi: Some(MsiConfig::default()), sys: SysClk::Msi, ..Default::default() },

if config.rcc.sys == SysClk::Msi && config.rcc.msi.is_none() {
    panic!("SysClk::Msi requires an msi config in rcc");
}

fn msi_ready_for_sys(rcc: &RccConfig) -> bool {
    rcc.sys != SysClk::Msi || rcc.msi.is_some()
}

When it happens

Trigger: Setting `config.sys = SysClk::Msi` while `msirdy()` is false — MSI not configured/enabled, or MSI disabled earlier in the init sequence before the sysclk switch.

Common situations: Choosing MSI as sysclk without an msi config entry; mixed cpu/sys configs where one stage turns MSI off; low-power configs relying on MSI but not enabling it.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• MSI is not ready to be selected as CPU clock source
• HSI is not ready to be selected as system clock source
• HSE is not ready to be selected as system clock source
• IC source was set to PLL , but it is not currently enabled
• HSI is not ready to be selected as CPU clock source

AI-assisted analysis of embassy-rs/embassy@463a07b963 (2026-09-10). Data as JSON: /api/errors/267978dfffb86c53. Report an issue: GitHub.

Appendix: source

Thrown at embassy-stm32/src/rcc/n6.rs:371

    debug!("configuring cpuclk");
    match config.cpu {
        CpuClk::Hsi if !RCC.sr().read().hsirdy() => panic!("HSI is not ready to be selected as CPU clock source"),
        CpuClk::Msi if !RCC.sr().read().msirdy() => panic!("MSI is not ready to be selected as CPU clock source"),
        CpuClk::Hse if !RCC.sr().read().hserdy() => panic!("HSE is not ready to be selected as CPU clock source"),
        CpuClk::Ic1 if !ic_enabled(1) => panic!("IC1 is not ready to be selected as CPU clock source"),
        _ => {}
    }
    // set source
    let cpusw = Cpusw::from_bits(config.cpu.to_bits());
    RCC.cfgr().modify(|w| w.set_cpusw(cpusw));
    // wait for changes to take effect
    while RCC.cfgr().read().cpusws() != Cpusws::from_bits(config.cpu.to_bits()) {}

    // sysclk
    debug!("configuring sysclk");
    match config.sys {
        SysClk::Hsi if !RCC.sr().read().hsirdy() => panic!("HSI is not ready to be selected as system clock source"),
        SysClk::Msi if !RCC.sr().read().msirdy() => panic!("MSI is not ready to be selected as system clock source"),
        SysClk::Hse if !RCC.sr().read().hserdy() => panic!("HSE is not ready to be selected as system clock source"),
        SysClk::Ic2 if !ic_enabled(2) || !ic_enabled(6) || !ic_enabled(11) => panic!(
            "IC2 is not ready to be selected as system clock source (make sure that IC6 and IC11 were configured as well)"
        ),
        _ => {}
    }
    // switch the system bus clock
    let syssw = Syssw::from_bits(config.sys.to_bits());
    RCC.cfgr().modify(|w| w.set_syssw(syssw));
    // wait for changes to be applied
    while RCC.cfgr().read().syssws() != Syssws::from_bits(config.sys.to_bits()) {}

    // decreasing dividers
    debug!("configuring decreasing pclk dividers");
    RCC.cfgr2().modify(|w| {
        if config.ahb < w.hpre() {
            debug!("  - AHB");
            w.set_hpre(config.ahb);

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