[sdiy] Getting dsPICs talking

[Neil Johnson]

Hi Tom,

> 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.

Couple thoughts:

1/ QFN package ... bit tricky to hand solder, and no use for rework.   
What about TQFP package?

2/ You have two SPI peripherals.  You say one is already assigned to  
the controller comms.  So how about using the second as a dedicated  
link?  2 data values sampled at 10kHz, even at 16 bits resolution you  
would only need 320kb/s (40kB/s), and by the looks of it those SPI  
peripherals can handle 16-bit transfers directly.  So in the Sources  
uP you have an interrupt handler hanging off the SPI transfer  
complete interrupt, and update the respective modulation value with  
the new sample.  For simplicity, use 15-bit values and use the top  
bit to indicate the destination (e.g., 0 = pitch, 1 = waveshape).  If  
you have more channels then sacrifice a few more bits for address.

Cheers,
Neil
--
http://www.njohnson.co.uk