```rust
for gpio in [pac::GPIOA, pac::GPIOB, pac::GPIOC] {
    gpio.lckr.write_value(Lckr(0x1FFFF));
    gpio.lckr.write_value(Lckr(0x0FFFF));
    gpio.lckr.write_value(Lckr(0x1FFFF));
    gpio.lckr.read();
    if !gpio.lckr.read().lckk() {
        panic!();
    }
}