[sdiy] Microcontrollers for eurorack use... What to use?

Scott Gravenhorst music.maker at gte.net
Sat Jan 3 02:03:22 CET 2015

You mention multi channel audio looper, so I assume you need to input 
audio.  As such, I don't know what sort of quality you're looking for, 
so the information I post here may not be good for your application.

I use the Microchip dsPIC33F, specifically, (for 28 pin DIP DIY) 
dsPIC33FJ128GP802.  This 16 bit device will run up to 40 MIPS and has a 
built in 16 bit delta-sigma stereo DAC capable of sample rates up to 100 
kHz.  The DAC is designed specifically for audio.  The multi-channel 
ADCs can run at either 10 bits or 12 bits and will do much higher sample 
rates.  The dsPIC also has hardware for SPI, UARTs and other interfaces. 
  It's internal SRAM is only 16 kbytes (max).  Low internal memory size 
is the drawback for every microcontroller I've seen so far.  One can use 
SPI to attach more RAMs.  I mention SPI SRAM because your post indicated 
a desire for looping which requires RAM.  Multiple SPI SRAMs can be 
attached and the dsPIC offers 2 internal SPI controllers which operate 
at 10 megabits per second.  The SPI SRAMs I've used are 128 kbyte and 
more than one can be attached to an SPI controller.  The dsPIC33F is 
about $5.50 in singles.  It can be used on stripboard or even on a 
solderless breadboard.

I've done a bit of work with these ICs which includes several MIDI 
polysynths (one of them supports 12 Karplus-Strong voices) so I can 
speak to the fact that they are quite capable.  That synth includes the 
MIDI controller and voice engine in one dsPIC33F.

The tools I use are free (MPLAB).  The C compiler (C30) can be used in 
"free" mode under which optimization is basic unless you pay for a 
license.  Note that the C compiler doesn't utilize the DSP instructions 
(like MAC) even if it could.  I've written one synth in C, but it was 
heavily supported by special assembly language functions I wrote to do 
specific DSP tasks.  To get the most out of this IC, I've found using 
assembly language to be best because then I am the optimizer and I know 
how to use the DSP instructions.

Microchip will ship free samples.  You can request samples on their 
website if you are interested.

Guy Taylor <geekon at gmail.com> wrote:
 >Hi everyone!
 >My first post after observing for many years... :)
 >I have been making some eurorack modules lately, all analog which has
 >been a lot of fun, but now want to try some micro-controller based
 >stuff. And there are two levels of cpu power i'm considering.
 >Something simple, like for a CV recorder and something more complex,
 >like a multi channel audio looper.
 >I realize there are many options but what do you guys think would be a
 >good starting point for projects like the two above?
 >Ideally factoring in:
 >Chip types/families, ram, flash etc
 >Built in features: audio io?
 >Power consumption - the less the better
 >Maturity of existing libraries for audio etc
 >I have decades experience with c/c++/c#, so the code doesn't worry me
 >too much, I really would like to select hardware which is appropriate,
 >rather than just guessing (i tried some test with netduino boards but
 >there seemed slow and not great for realtime stuff)
 >Thanks in advance

-- ScottG
-- Scott Gravenhorst
-- http://scott.joviansynth.com/
-- When the going gets tough, the tough use the command line.
-- Matt 21:22

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