Check out this page

http://puggy.symonds.net/~catalyticideas/rat_ring/index.php?page=guide&chapter=3#1

This is Ken's implementation of a DTMF dialer, but you could use it as a
launching pad for greatness.

Scott


At 06:20 AM 5/25/04, you wrote:
>No PIC (that I'm aware of) has real analog outputs - you'll need to produce
>a PWM output (on/off at varying duty cycles) at a significantly higher
>frequency than the highest you need to generate, and filter it externally.
>
>3 separate PWM modules are generally not found except in extremely high
>pin-count PICs, so you'd probably want to combine the 3 sinewaves
>internally and output them on a single PWM channel.  Fortunately, this is
>no problem - the standard PWM module has 10-bit resolution, so you could
>simply add up to 4 8-bit signals.
>
>An 18Fxxx PIC would be a good choice.  Its hardware multiplier is exactly
>what you need to adjust the amplitude of the individual sinewaves.
>
>The sinewave itself comes from a table of precalculated values.  Normally,
>a table covering a quarter cycle is used - you play it forwards, then
>backwards, then forwards with the value negated, then backwards negated.
>However, a table covering a full sine cycle might be easier to work with,
>and any 18F PIC should have plenty of room to hold that at reasonable
>resolution.
>
>You'd have three pointers into that table, one for each sinewave.  At each
>step, you'd grab the values from each pointer, multiply each by its
>corresponding amplitude, add them together, and send to the PWM module.
>You'd then advance each pointer by an amount depending on the corresponding
>frequency, wrapping around at the end of the table.  That's all there is to
>it.
>         Jason Harper
>
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