Hi,
This is also really a [PIC]: question, but as this is PICLIST, I reckon =
it's ok ;)  Anyway, while I wait for my programmer kit to arrive =
(acutally, while I wait for paypal to validate me..) I'm trying to =
design a tone generator circuit (basically a PIC that can generate =
musical notes).  I've been considering the possiblities, just thought =
I'd share my thoughts for people to criticise etc.

Methods
1. Using a frequency synthisizer IC - perhaps controlling a 555 =
*somehow* (minimal interrupt use - i.e. set some pins to change =
frequency.  I was thinking using 8 pins for A/D conversion..)
2. Using hardware PWM - not sure on this one, as there's "only" 1 =
channel (16F877).. maybe to be combined with...
3. Using software PWM (interrupts).  I want 3 or 4 channels, and I feel =
this is relatively easy using this method:
  20Mhz crystal, clock divider =3D 5MIPS, TMR0 overflows 19531.25 times =
a second - 19.5KHz is way higher than the range I need (4.9Khz being =
D#8, quite a high note :D)
  Some example code (pseudocode):

---------

// play_note plays a *note* on channel *chan*
// basically sets the timeout value for each pulse (measured in =
interrupts)
// data[] is a table of values, telling the number of interrupts to keep =
the pulse high/low for.
// example: C5 @ 523.25 Hz.. 19531.25/523.25 =3D 37.2 (say 37)

play_note(chan,note)
  chandata[chan] =3D data[note]
  chantimer[chan] =3D chandata[chan]

// on_tmr0_overflow occurs at ~19.5Khz
// decrements timeouts, sees if it's time to switch pulse state
on_tmr0_overflow
  for (each in chan)=20
   dec chantimer[chan]
   if (chantimer[chan] =3D=3D 0)
   {
     chantimer[chan] =3D chandata[chan]
     invert PWM_output_pin[chan]
   }
  next chan

(all PWM output pins are tied together as +ve for a speaker)


Anyway, just checking if they're anything obvious I've missed out here =
(probably is).  Another bonus mystery question:  What about other =
waveforms?  Is it possible to attach caps in a "dodgy" fashion to =
"smooth out" the square wave, creating a sine-ish wave?  And just for =
fun, I want some bass/treble controls (I know it won't be that =
effective, but it's fun :P).. just some rotary potentiometers combined =
with caps?

Thanks,
Kieren


P.S. On a slightly unrelated theme, I've designed a circuit for checking =
the status of 36 pushbuttons with only 7 I/O pins.  If anyone can beat =
this (there's probably a magical IC for it), I'd be superhappy..

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