> > Also, for output you could tie one leg of the speaker to a port pin, and
> > the other to a port pin via a cap.  If both port pins switch simultaneously
> > in opposite directions you could get +/- 6 volts on the speaker which should
> > give it a louder drive than merely using a single port pin.
> But why do I have to tie one leg of the speaker to a PIC pin _via a cap_?
> It _is_ the advantage of a bridge speaker driver not to need any cap!

True it's nice to avoid a cap, but you'll almost certainly need either a cap
or a resistor to limit current, and if you use a cap it will allow a higher
peak current initially, and store current fed to it during one cycle for out-
put in the next.  Thus, assuming an initial steady state with the cap's port
pin high and the other one low, we have [all voltages relative to ground]:

  PIN   CAP  CAP [SPKR] PIN
  3.0   3.0  0.0        0.0

If we switch the high/low on the port pins we then have
  0.0   0.0 -3.0        3.0
[six volts one polarity on the speaker--twice Vdd]

After a little while, the cap will equilibrate to
  0.0   0.0  3.0        3.0

which after another click becomes
  3.0   0.0  6.0        0.0
[six volts other polarity on the speaker--twice Vdd]

So the effect of the cap is to double your peak voltage, though at the
expense of horrendously distorting your frequency response (not an issue
for a metronome producing "clicks")