I am currently using PWM with an active third or fourth order butterworth i=
n another project... a little board with a 16f876a used as a replacement to=
 the original pcb inside a 25 key laptop-size usb keyboard to convert it in=
 a cv-gate keyboard.. I haven't finished it yet because i have to go buying=
 a couple of other ICs first, in that case i would run at 18MHz, pwm at abo=
ut 15kHz to have high speed and all the 10 bits.. and filter at about 200-3=
00 Hz. With a simulator I obtained good results, in the next weeks i'll mak=
e and test the board. Anyway, I think 10 or 12 bits should be fine.. I thin=
k I I have a couple of mpc4822 but I didn't want to use them now... My idea=
 is to continously acquire alle the knobs values, sum or subtract them via =
software, store them in a memory (so i can quick recall patches, configurat=
ions, ...) and use the dac to feed the result in the oscillator circuitery,=
 so, basically they would be constant values

> Date: Wed, 15 Feb 2012 07:59:57 -0500
> To: piclist@mit.edu
> From: pic@dtweed.com
> Subject: Re: [PIC] or others... Are there any DIP microcontroller with an=
alog	outputs?
>=20
> Jacopo Monegato wrote:
> > yes, i think so... I need possibly more than 9 bit precision. What i am
> > trying to do is digital control of an analog audio vco. I need veeeeery=
 low
> > noise ;) I know that some brands, like Dave smith instruments use 8 bit
> > precision for vcf cutoff and even pitch wheel (in older instruments). I=
 want
> > more. I wanted to avoid PWM. In that case I would have used a third ord=
er
> > butterworth vcvs to have about 0,5-1 mVpp noise, but i wanted to try
> > something else. But, at this point, i guess i'll stay on pwm :/
>=20
> What kind of bandwidth (sample rate) do you need?
>=20
> If not more than 100 Hz or so, it sounds like you should consider doing
> delta-sigma in firmware, which is much cleaner (easier to filter) than PW=
M.
> Linearity is excellent, and resolution is really just a question of the
> width of the variables you use in the firmware.
>=20
> For example, here's some C-ish pseudocode that implements N_CHAN channels=
 of
> 15-bit DAC. The dac_isr() should run as frequently as possible, at least =
10x
> to 20x the bandwidth that you desire. If you do this in assembler, then y=
ou
> can squeeze out another bit of resolution, because then you can use the c=
arry
> bit out of the addition operation instead of the MSB of the accumulator. =
You
> might also want to unroll the loop, which makes mapping channels to outpu=
t
> pins trivial. ("u16" and "u8" denote unsigned 16- and 8-bit integer types=
..)
>=20
> struct {
>   u16 value; /* sets the output level of this channel, 0 - 0x7FFF */
>   u16 acc;   /* internal accumulator (integrator) */
> } ch[N_CHAN];
>=20
> /* Timer interrupt handler */
> void dac_isr (void)
> {
>   u8 i;
>   for (i=3D0; i<N_CHAN; ++i) {
>     ch[i].acc +=3D ch[i].value;
>     if (ch[i].acc & 0x8000) {
>       ch[i].acc &=3D ~0x8000;
>       /* set the output bit associated with channel i */
>     } else {
>       /* clear the output bit associated with channel i */
>     }
>   }
> }
>=20
> -- Dave Tweed
> --=20
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
 		 	   		 =20
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.