#[idle]
    fn idle(_: idle::Context) -> ! {
        loop {
            // re-initialize PLL
            dp.RCC.cr.modify(|_, w| w.hseon().bit(true));
            while dp.RCC.cr.read().hserdy().bit_is_clear() {}
            dp.RCC.cfgr
                .modify(|_, w| unsafe { w.sw().bits(input_src.bits()) });

            dp.RCC.cr.modify(|_, w| w.pllon().set_bit());
            while dp.RCC.cr.read().pllrdy().bit_is_clear() {}

            // Do other idle tasks

            // Select the low power mode to enter. For example, here is
            // stop 2
            cp.set_sleepdeep();
            dp.PWR.cr1.modify(|_, w| w.lpms().bits(0b010);
        }
    }