[sdiy] Getting dsPICs talking

[Jay Schwichtenberg]

Tom,

Using DACs/ADCs makes the system more complex than it needs to be.

Look at using one of the uC serial busses to talk between the two processors
(or more). Looks like you have SPI, I2C and just straight serial hardware
available from the dsPIC33F data sheet. Make the Mod uC the master and all
the other 'tone' uCs slaves. Figure out how the data works for the bus
you're using, write some code and you're ready to go. You can run some of
these busses pretty fast so speed won't be an issue.

I worked in electronic instrumentation on my last job. There were probably a
good dozen or so sub-systems that were are tied together on busses from the
PPC host we used. We needed a high speed dedicated bus, general bus and some
of the chips we used had a slow bus so they were on a third bus.

Also you might look at the PIC24H for the Mod uC. Similar to the dsPIC33 but
more of a general purpose processor. I'm dumping all my 16F designs and
starting to use it as my processor of choice.

Good luck.
Jay S.

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Tom Wiltshire
> Sent: Thursday, May 28, 2009 9:38 AM
> To: synth-diy at dropmix.xs4all.nl diy
> Subject: [sdiy] Getting dsPICs talking
>
>
> Hi All,
>
> I'm finally into the construction stage of my "Protowave" monosynth
> project. The synth uses two dsPIC 33FJ128GP802 processors for its
> voice, followed by analog VCF/VCA.
> One of these uPs is dedicated to production of audio and can generate
> two oscillators at 62.5KHz (the "Sources uP"). The second uP is
> dedicated to modulation and deals with all the envelopes, LFOs etc.
> The final sample rate for this hasn't been determined, but is likely
> to be around 10KHz. Both processors are controlled via an SPI link
> from a third processor which deals with MIDI, scans the keyboard and
> reads the panel controls.
>
> The modulation processor has a 2-channel DAC on chip. The output from
> this DAC is multiplexed out to sample-and-hold buffers in the usual
> way. These analogue control voltages control the VCF and VCA.
>
> Now my query:
>
> I have to get modulation information (pitch and waveshape
> modulations) from the Mod uP to the Sources uP: How best to do it?
>
> I had thought to also feed analogue control voltages from the Mod uP
> to the ADC inputs on the Sources uP. The fact that the signals go
> from the digital domain to the analogue domain and then back again
> conveniently gets around the fact that they are running at different
> sample rates (off different clocks, actually), since the anti-alias
> filtering acts as interpolation between sample values.
>
> However, this conversion digital->analogue->digital does seem a bit
> crazy. Especially so when you consider that the chips have a codec
> interface on chip. Having realised this, I wondered whether I could
> send the modulation signals to the Sources uP via the codec interface
> instead. Presumably I'd be able to send multichannel data (I've got 6
> channels to transmit) over the digital connection. But how would I
> handle the different sample rates? Would I need to synchronise the
> two uPs? Do I need to digitally filter the outgoing/incoming
> modulation data to deal with the different sample rates? Am I crazy
> having different rates? Would I be better having one as a multiple of
> the other?
> Does anyone have any experience making two uPs talk to each other
> like this? One is essentially pretending to be an audio codec, and
> the other is processing that signal and outputting it. The difference
> with the usual situation is that the codec rate and the output sample
> rate would be the same.
>
> If I can make this work, it'll be really great. But there's a fair
> mountain of work still. I'd hoped to finish before this year's SDIY
> UK, but it'll have to be next year!
>
> Any suggestions appreciated.
>
> Thanks,
> Tom
>
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>