> I notice that the Basic Stamp 2 has a BASIC command (DTMFOUT) to
> generate DTMF via a single digital output (aka 1 bit DAC), which
> it says can be turned into proper waveforms with filtering.  (It
> also has a command to output one or two arbitrary tones).
>
> My question is: How is this done?
>
> I can see that a single tone can be done via a square wave (and
> attenuating the 3, 5'th etc harmonics would approximate a sine wave).
> But directly summing two square waves of equal amplitude and
> different frequency would take a 3 level output.  I don't see how to
> reduce it to 2 levels and keep the desired dual tones even
> approximately.

There are a number of methods for this.  Probably the simplest to understand
is something like the following:

Loop:
        bcf     PORTB,0
        btfsc   Phase1,7
         bsf    PORTB,0
        movf    Freq1,w
        addwf   Phase1
        nop

        movf    Freq2,w
        addwf   Phase2
        bsf     PORTB,0
        btfss   Phase2,7
         bcf    PORTB,0
        decfsz  OverallTime
         goto   Loop

When both waves are high, the output is [almost] constantly high; when both
are low, the output is [almost] constantly low.  When the two waves are
opposite each other, the output is a 50%-duty-cycle very rapid square wave.