Addendum: PWM

One last note before we move on.

Interrupts are kind of expensive. The processor must finish or abort the currently-running instruction, then save enough state to restart execution, then call an interrupt handler. All this takes a few CPU cycles of precious runtime.

The way the solution of the previous section is written, it will take two interrupts per cycle of speaker output. That's something like 1000 interrupts per second. On a processor like our nRF52833, that works fine.

The nRF52833 does have an on-board peripheral that could cut our siren's interrupt rate way down. The Pulse-Width Modulation (PWM) unit can, among other things, generate cycles on the speaker pin at a rate controlled by a PWM register. This could be used to generate the basic square wave used for our siren. We would still need an interrupt every time we wanted to change the frequency, but this might be more like 10 interrupts per second than 1000.

I did not use the PWM unit in my solution. This was partly because I wanted to focus on interrupts. Another big reason, though, was that the nRF52833 PWM unit is pretty complicated and hard to understand. Getting something working a simple way in the tight bare-metal environment is always attractive.

If you are up for a challenge, I would encourage you to try using the PWM unit for your siren.