Embassy is configured as single-bank, but the hardware is…

Error message

Embassy is configured as single-bank, but the hardware is running in dual-bank mode. Change the hardware by changing the dbank value in the user option bytes or configure embassy to use dual-bank config

What it means

On STM32G4 (and configurable G0) parts, the DBANK option bit selects dual-bank mode. Embassy built with 'single-bank' but with hardware DBANK=1 (dual) would compute wrong flash addresses, so check_bank_setup() panics at initialization. The panic names the exact option byte (dbank) to fix.

Solutions

Rebuild with the 'dual-bank' feature, or clear the DBANK option bit (STM32CubeProgrammer, then power cycle) to put the chip in single-bank mode.
Match the feature flag to the bank count your flash layout (partitions/bootloader) assumes.
Verify after any option-byte erase/reset that firmware features still match.

Example fix

// before (Cargo.toml, G4 hardware dual-bank)
features = ["stm32g474re", "single-bank"]
// after
features = ["stm32g474re", "dual-bank"]
// or clear DBANK option byte and power cycle

Defensive patterns

Strategy: validation

Validate before calling

fn g4_bank_ok(single_bank_feature: bool, dbank: bool) -> bool {
    single_bank_feature == !dbank // DBANK=1 means dual bank
}

Prevention

Check DBANK option bit on G4 boards before choosing flash features
Avoid resetting option bytes to defaults if firmware expects single-bank
Match DBANK with your partition/bootloader layout requirements

When it happens

Trigger: Flash init on G4 (c2/c3/c4 variants) built with feature = "single-bank" while FLASH.optr.DBANK == 1.

Common situations: G4 boards shipped with dual-bank option bytes while firmware built single-bank for simpler sector sizes; after factory reset of option bytes; feature flag toggled for a different board and reused.

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/c4bc501db180046c. Report an issue: GitHub.

Appendix: source

Thrown at embassy-stm32/src/flash/g.rs:256

    if DATA_CACHE_WAS_ENABLED.load(Ordering::Relaxed) {
        // Reset data cache before re-enabling
        pac::FLASH.acr().modify(|w| w.set_dcrst(true));
        pac::FLASH.acr().modify(|w| w.set_dcrst(false));
        pac::FLASH.acr().modify(|w| w.set_dcen(true));
    }
}

// G0 doesn't have data cache, use no-op functions
#[cfg(any(flash_g0x0, flash_g0x1))]
fn save_data_cache_state() {}

#[cfg(any(flash_g0x0, flash_g0x1))]
fn restore_data_cache_state() {}

#[cfg(all(bank_setup_configurable, any(flash_g4c2, flash_g4c3, flash_g4c4)))]
pub(crate) fn check_bank_setup() {
    if cfg!(feature = "single-bank") && pac::FLASH.optr().read().dbank() {
        panic!(
            "Embassy is configured as single-bank, but the hardware is running in dual-bank mode. Change the hardware by changing the dbank value in the user option bytes or configure embassy to use dual-bank config"
        );
    }
    if cfg!(feature = "dual-bank") && !pac::FLASH.optr().read().dbank() {
        panic!(
            "Embassy is configured as dual-bank, but the hardware is running in single-bank mode. Change the hardware by changing the dbank value in the user option bytes or configure embassy to use single-bank config"
        );
    }
}

#[cfg(all(bank_setup_configurable, flash_g0x1))]
pub(crate) fn check_bank_setup() {
    if cfg!(feature = "single-bank") && pac::FLASH.optr().read().dual_bank() {
        panic!(
            "Embassy is configured as single-bank, but the hardware is running in dual-bank mode. Change the hardware by changing the dual_bank value in the user option bytes or configure embassy to use dual-bank config"
        );
    }
    if cfg!(feature = "dual-bank") && !pac::FLASH.optr().read().dual_bank() {

View on GitHub (pinned to 463a07b963)