I've got the following code and USB is working now:
```
                match serial.read(&mut buf) {
                        Ok(count) => {
                                pwm_debug += 1;
                                if pwm_debug == pwm_max {
                                        pwm_debug = 0;
                                }
                                isInit = true;
                        }
                        _ => {
                                if !isInit {
                                        continue;
                                }
                        }
                }
```
But this read is blocking and only advances pwm_debug when I send something over the serial console. How do I make it non-blocking?