* Any ideas why this should "freeze" up - note that the blue led on my board flashes a couple times then stays on. The USB device does not appear in `/dev/.cu*`.  However, this usb-serial example in the HAL crate works,  but the LED blinks for a long time before I can see the USB device https://github.com/stm32-rs/stm32h7xx-hal/blob/master/examples/usb_serial.rs#L103-L129

```rust
    loop {
        led.set_high();
        delay.delay_ms(100_u16);

        led.set_low();
        delay.delay_ms(100_u16);

        if !usb_dev.poll(&mut [&mut serial]) {
            continue;
        }

        serial.write(b"Hello, human...!").unwrap();
        serial.flush().unwrap();
    }
```