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

Error message

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

What it means

This panic comes from check_bank_setup in embassy-stm32's flash driver for STM32G0x1 devices. The 'dual-bank' cargo feature was enabled, but the OPTR register's DUAL_BANK bit indicates the chip is running in single-bank mode. Embassy panics because a dual-bank driver layout would address flash incorrectly on single-bank hardware.

Solutions

Change your embassy-stm32 feature from "dual-bank" to "single-bank" in Cargo.toml to match the default single-bank hardware
Set the DUAL_BANK option bit to 1 in the user option bytes (STM32CubeProgrammer or probe-rs) if you truly need dual-bank operation
After changing option bytes, power-cycle the chip and verify OPTR via debugger before reflashing

Example fix

// before (Cargo.toml)
embassy-stm32 = { version = "...", features = ["stm32g071xx", "dual-bank"] }
// after
embassy-stm32 = { version = "...", features = ["stm32g071xx", "single-bank"] }

Defensive patterns

Strategy: validation

Validate before calling

// Rust, before initializing the flash driver
let optr = unsafe { pac::FLASH.optr().read() };
assert_eq!(optr.dual_bank(), cfg!(feature = "dual-bank"),
    "embassy flash bank feature must match OPTR.DUAL_BANK");

Prevention

Verify option bytes after factory reset or mass erase, since defaults may be single-bank
Pin per-board cargo features and never copy them between board variants unchecked
Check OPTR via debugger when bringing up a new board revision

When it happens

Trigger: Building with feature = "dual-bank" for an STM32G0x1 target whose user option bytes have DUAL_BANK=0; the panic fires at flash init when check_bank_setup evaluates !pac::FLASH.optr().read().dual_bank().

Common situations: Enabling dual-bank features copied from an L4/G4 project; factory-default G0 option bytes (single-bank) with a dual-bank-configured build; cargo feature set for a different board variant sharing the same codebase.

Understand the failure class

Background: Conflicting config options: "cannot be used together" — configuration validation errors across open-source libraries — this error's family across 162 libraries.

Related errors

AI-assisted analysis of embassy-rs/embassy@463a07b963 (2026-09-10). Data as JSON: /api/errors/7c960beee835dc9a. Report an issue: GitHub.

Appendix: source

Thrown at embassy-stm32/src/flash/g.rs:275

            "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() {
        panic!(
            "Embassy is configured as dual-bank, but the hardware is running in single-bank mode. Change the hardware by changing the dual_bank value in the user option bytes or configure embassy to use single-bank config"
        );
    }
}

View on GitHub (pinned to 463a07b963)