IC source was set to PLL , but it is not currently enabled

Error message

IC{} source was set to PLL{}, but it is not currently enabled

What it means

On STM32N6, an output-clock (ICx) divider was configured with a source whose `pll_source_ready` check fails, meaning the referenced PLL (or PLLx input) is not enabled. embassy-stm32 panics rather than silently producing a wrong frequency, since the IC output would be invalid.

Solutions

Enable the PLL that the IC source references: add its `PllConfig` (with valid source) to the rcc config.
Or change the ICx `source` to a currently-enabled PLL/oscillator.
If the IC is unused, set that IC entry to None so it is skipped.

Example fix

// before
ics: [Some(Ic { source: IcSource::Pll2, ..Default::default() }), ..],
// with no pll2 configured
// after
// either configure pll2:
pll2: Some(PllConfig { source: PllSource::Hse, ..Default::default() }),
// or drop the IC:
ics: [None, ..],

Defensive patterns

Strategy: validation

Validate before calling

for ic in &config.rcc.ics {
    if let Some(ic) = ic {
        assert!(matches!(ic.source, IcSource::Pll1 if config.rcc.pll1.is_some()), "IC source PLL must be enabled");
    }
}

Type guard

fn ic_source_ready(ic: &Ic, rcc: &RccConfig) -> bool {
    match ic.source {
        IcSource::Pll1 => rcc.pll1.is_some(),
        IcSource::Pll2 => rcc.pll2.is_some(),
        IcSource::Pll3 => rcc.pll3.is_some(),
        _ => true,
    }
}

Prevention

Configure every PLL that any ICx source references before init().
Set unused IC entries to None instead of pointing them at unenabled PLLs.
When copying board configs, check ics and pll sections change together.

When it happens

Trigger: Setting `config.ics[?] = Ic { source: IcSource::Pll1/Pll2/Pll3... , ... }` for a PLL that was not enabled in the same init (its `pll` entry absent/None, or the PLL output feeding the IC not configured).

Common situations: Configuring an ICx routed to PLL2 or PLL3 while only PLL1 is set up; copy-pasting IC configs between boards; partial PLL config after refactoring so a PLL referenced by ICs is dropped.

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/415c115356903ec1. Report an issue: GitHub.

Appendix: source

Thrown at embassy-stm32/src/rcc/n6.rs:314

        config.ic12,
        config.ic13,
        config.ic14,
        config.ic15,
        config.ic16,
        config.ic17,
        config.ic18,
        config.ic19,
        config.ic20,
    ]
    .iter()
    .enumerate()
    {
        // Skip disabled ICs
        let Some(ic) = ic else { continue };

        let ic_source = ic.source.to_bits();
        if !pll_source_ready(ic_source) {
            panic!(
                "IC{} source was set to PLL{}, but it is not currently enabled",
                index + 1,
                ic_source
            )
        }

        RCC.iccfgr(index).write(|w| {
            w.set_icsel(ic.source);
            w.set_icint(ic.divider);
        });
        RCC.divensr().modify(|w| w.0 = 1 << index);
    }

    // handle increasing dividers
    debug!("configuring increasing pclk dividers");
    RCC.cfgr2().modify(|w| {
        if config.apb1 > w.ppre1() {
            debug!("  - APB1");

View on GitHub (pinned to 463a07b963)