Timer project: sound generation

[kpreid]

One of the requirements for the timer project is that the alarm sound be not too unpleasant.

So: What can I do with the current prototype hardware, a piezo element taken from a watch connected directly to a (digital) output pin on the PIC?

Trivially, fixed-volume square waves, by either toggling the output pin in software or setting up the PIC's PWM feature to drive it automatically. (Note that the PWM produces a minimum frequency of 1/16384 of the main oscillator.)

(I've moved the piezo output on the prototype to the RC2/CCP1 pin. This change is not yet reflected in the published circuit and program.)

However, I want software volume control, so I tried something more elaborate. I set the PWM to a very high frequency, and produce the audio-frequency signal by varying the PWM's duty cycle parameter. The characteristics of the piezo crystal filter out the high frequency, producing an effectively analog output.

After much twiddling getting this to work (CCPR1L is not the least-significant bits of the duty cycle), I found that square waves work well, sine waves are very quiet, and changing the amplitude has the expected effect. So while it won't sound great, I can at least program it to increase the volume gradually.

I haven't yet integrated this into the main timer program, so I don't know whether I'll be able to produce decent tones despite (or making use of) the timer program's counting interrupt.