What causes "must not select RESERVED_1 source as DISABLE" in embassy-rs/embassy?

Set pll.source to PllSource::Hse or PllSource::Hsi.

must not select RESERVED_1 source as DISABLE

Error message

must not select RESERVED_1 source as DISABLE

What it means

init_pll panics when the PLL source is PllSource::_RESERVED_1, a reserved value in the RCC register enum. Reserved register encodings have no defined behavior per the reference manual, so the driver refuses to program them.

Solutions

Set pll.source to PllSource::Hse or PllSource::Hsi.
Do not construct the reserved variant directly; only obtain source values from the documented PllSource variants.
Regenerate or fix any code-generation that emits _RESERVED_* variants into user configs.

Example fix

// before
source: PllSource::_RESERVED_1,
// after
source: PllSource::Hsi,

Defensive patterns

Strategy: validation

Validate before calling

assert!(!matches!(pll.source, PllSource::_RESERVED_1), "PLL source must not be reserved");

Type guard

fn pll_source_defined(s: PllSource) -> bool { matches!(s, PllSource::Hse | PllSource::Hsi | PllSource::Disable) }

Prevention

Only construct PllSource from its documented variants.
Never transmute raw register values into enum config types.
Review code-generation output for reserved variants leaking into configs.

When it happens

Trigger: Constructing a PllConfig with pll.source set to PllSource::_RESERVED_1 (either manually or via an unsafe/transmuted value) and passing it through rcc init.

Common situations: Fiddling with register-level enums copied from reference manuals; type-punned or bit-converted configs; corrupted/incorrect generated code.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of embassy-rs/embassy@463a07b963 (2026-09-10). Data as JSON: /api/errors/776c5c977085459e. Report an issue: GitHub.

Appendix: source

Thrown at embassy-stm32/src/rcc/wba.rs:497

    pub r: Option<Hertz>,
}

fn pll_enable(enabled: bool) {
    RCC.cr().modify(|w| w.set_pllon(enabled));
    while RCC.cr().read().pllrdy() != enabled {}
}

fn init_pll(config: Option<Pll>, input: &PllInput, voltage_range: VoltageScale) -> PllOutput {
    // Disable PLL
    pll_enable(false);

    let Some(pll) = config else { return PllOutput::default() };

    let pre_src_freq = match pll.source {
        PllSource::Disable => panic!("must not select PLL source as DISABLE"),
        PllSource::Hse => unwrap!(input.hse),
        PllSource::Hsi => unwrap!(input.hsi),
        PllSource::_RESERVED_1 => panic!("must not select RESERVED_1 source as DISABLE"),
    };

    // Only divide by the HSE prescaler when the PLL source is HSE
    let src_freq = match pll.source {
        PllSource::Hse => {
            // read the prescaler bits and divide
            let hsepre = RCC.cr().read().hsepre();
            pre_src_freq / hsepre
        }
        _ => pre_src_freq,
    };

    // Calculate the reference clock, which is the source divided by m
    let ref_freq = src_freq / pll.prediv;
    // Check limits per RM0515 § 12.4.3
    assert!(Hertz::mhz(4) <= ref_freq && ref_freq <= Hertz::mhz(16));

    // Check PLL clocks per RM0515 § 12.4.5

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