embassy-rs/embassy · error

attempted to use peripheral

Error message

attempted to use peripheral '{}' but its clock mux is not set to a valid clock. Change 'config.rcc.mux' to another clock.

What it means

embassy-stm32 generates a match on the RCC clock-mux register field for each peripheral requiring a selected clock source; the wildcard arm panics with the peripheral name when the mux value is neither a known clock nor a recognized encoding. This is a runtime panic meaning the RCC mux register holds an invalid value, or the mux was never set to a valid source before the peripheral was used.

Solutions

1. Set the peripheral's clock mux explicitly via embassy's config: `config.rcc.mux.<xxxsel> = val` before using the peripheral.
2. Don't hand-write to RCC mux registers outside embassy's RCC setup.
3. If a bootloader changed clocking, reset the mux or re-run embassy's RCC init before peripheral use.

Example fix

// before
let mut config = Config::default(); // mux never set
let mut adc = Adc::new(p.ADC1);
// after
let mut config = Config::default();
config.rcc.mux.adc12sel = embassy_stm32::rcc::mux::Adc12sel::SYS;
let mut adc = Adc::new(p.ADC1);

// set the mux in Config before creating the peripheral:
let mut config = Config::default();
config.rcc.mux.adc12sel = embassy_stm32::rcc::mux::Adc12sel::SYS;

When it happens

Trigger: Using a peripheral whose RCC clock-mux register field holds an invalid/reserved value at runtime — the generated `match RCC.#fieldset().read().#field()` falls through to the wildcard `_ =>` panic arm.

Common situations: Modifying RCC mux registers manually (unsafe writes) before using the peripheral; clock configuration applied by a bootloader that leaves an unrecognized mux encoding; missing `config.rcc.mux` entries.

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

• Bank 1 not present
• Bank 2 not present
• OTP not present
• The LCD driver needs the RTC/LCD clock to be running
• if PLL source is HSI, PLL prediv must be 2.

AI-assisted analysis of embassy-rs/embassy@463a07b963 (2026-09-10). Data as JSON: /api/errors/75dd8104bdd70e50. Report an issue: GitHub.

Appendix: source

Thrown at embassy-stm32/build.rs:875

            for v in enumm.variants.iter().filter(|v| v.name != "Disable") {
                let variant_name = format_ident!("{}", v.name);
                let upper_snake = pascal_to_upper_snake(v.name);
                let expr = if let Some(mux) = self.chained_muxes.get(upper_snake.as_str()) {
                    self.gen_mux(peripheral, mux)
                } else {
                    self.gen_clock(peripheral, &upper_snake)
                };
                match_arms.extend(quote! {
                    crate::pac::rcc::vals::#enum_name::#variant_name => #expr,
                });
            }

            quote! {
                match crate::pac::RCC.#fieldset_name().read().#field_name() {
                    #match_arms
                    #[allow(unreachable_patterns)]
                    _ => panic!(
                        "attempted to use peripheral '{}' but its clock mux is not set to a valid \
                         clock. Change 'config.rcc.mux' to another clock.",
                        #peripheral
                    )
                }
            }
        }
    }

    let mut refcount_idxs = BTreeSet::new();

    for p in METADATA.peripherals {
        if !singletons.contains(&p.name.to_string()) {
            continue;
        }

        if let Some(rcc) = &p.rcc {
            let rst_reg = rcc.reset.as_ref();

View on GitHub (pinned to 463a07b963)