Busy waiting

The timer should now be properly initialized. All that’s left is to implement the delay functionusing the timer.

First thing we have to do is set the autoreload register (ARR) to make the timer go off in msmilliseconds. Because the counter operates at 1 KHz, the autoreload value will be the same as ms.

  1.     // Set the timer to go off in `ms` ticks
  2.     // 1 tick = 1 ms
  3.     tim6.arr.write(|w| w.arr().bits(ms));

Next, we need to enable the counter. It will immediately start counting.

  1.     // CEN: Enable the counter
  2.     tim6.cr1.modify(|_, w| w.cen().set_bit());

Now we need to wait until the counter reaches the value of the autoreload register, ms, then we’llknow that ms milliseconds have passed. That condition is known as an update event and itsindicated by the UIF bit of the status register (SR).

  1.     // Wait until the alarm goes off (until the update event occurs)
  2.     while !tim6.sr.read().uif().bit_is_set() {}

This pattern of just waiting until some condition is met, in this case that UIF becomes 1, isknown as busy waiting and you’ll see it a few more times in this text :-).

Finally, we must clear (set to 0) this UIF bit. If we don’t, next time we enter the delayfunction we’ll think the update event has already happened and skip over the busy waiting part.

  1.     // Clear the update event flag
  2.     tim6.sr.modify(|_, w| w.uif().clear_bit());

Now, put this all together and check if it works as expected.