[sdiy] dsPIC33 question..

Seb Francis seb at burnit.co.uk
Wed Nov 12 17:37:00 CET 2008

Tom Wiltshire wrote:
> Running at full speed, the SPI can be clocked at 10MHz, which means 
> 16bit words at 625KHz. That's not so slow, and certainly fine for 
> control interfaces, not least when compared with MIDI.

That's not too slow at all .. I remember looking into using SPI for 
audio before and there were reasons why it wasn't practical - maybe 
because  of the CPU overhead of using the interface?  With DMA this is 
probably not an issue any more, so even audio could be sent between 2 
separate boards over the SPI.

> One possible use would be to connect a multichannel SPI DAC to it and 
> use it to generate control signals based on audio inputs (an envelope 
> follower would be a simple example, and audio-level controlled LFO 
> would be another). It might be possible to use the codec for this (at 
> least for two channels), but SPI gives you other options.

I'm fairly sure the CODEC won't output DC (or very LF), and certainly 
not with the output buffer circuit I'm going to use, so for sure it's 
worth having the option of an SPI DAC.

The dsPIC I'm going to use has 2 SPI channels - I'll probably expose 
both sets of pins directly as there's no shortage of other I/O to use 
for buffered inputs.

> Again, it's potentially very useful. I've got a small PCB that I 
> designed which just has three MIDI sockets (In, Out, Thru) on it, plus 
> the interface hardware. This will connect to the Rx and Tx pins on a 
> PIC/dsPICs UART and give you a full MIDI interface. The ability to add 
> such a board to your module would be useful, even if you can't get the 
> MIDI interface on the main PCB.

Well, MIDI Out is 2 resistors, MIDI In is 3 resistors and an 
opto-isolator, with MIDI Thru being another 2 resistors.  So may as well 
just stick it on the PCB, especially with SMD resistors it's not going 
to take a lot of space.

Reminds me how far things have come when I think about the first MIDI to 
drum trigger unit I made with a 6402 UART and loads of discrete logic 
ICs.  Actually I think they still make the 6402s in 40pin DIP only .. 
who knows what they're still used for!

> They use considerably *less* power than the 5V chips.

That's good to know, the 4xD was quite power hungry and needed 
heatsinking on the regulators, something I'd like to avoid if I'm going SMD.


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