[sdiy] Microchip DSP kit $60

[Eric Brombaugh]

Scott Gravenhorst wrote:

> What about the 16K RAM?  Is that DSP code RAM?  Data RAM such as for delay line use?

I looked a little more closely at the specs. This is using one of their 
GP (General Purpose as opposed to MC for Motor Control) variants and has 
256KB of program flash with 16KB of SRAM. The program is (not 
surprisingly) stored in the program flash while RAM is available for 
variables, buffers, etc.

Note that program instructions are 24-bits wide on the dsPIC, so 256KB 
is really ~85K instructions. That's still a lot though, especially 
considering that the DSP features allow multiple things to happen per 
instruction (Multiply, Add, prefetch data, increment/decrement pointers, 
etc).

Data space is 16 bits, so 16KB is really 8K words. You could certainly 
use it for delay lines, but it's really not enough for audibly useful 
delays at the 48kHz sample rate they quote. Less than 0.17 second 
depending on how much variable overhead you need. For useful delay you'd 
need some sort of external RAM, and that takes up GPIO for a parallel 
interface since I'm not aware of any large SPI RAMs. This board doesn't 
appear to have a large RAM on it, so you'd be stuck scabbing it on - not 
a pleasant prospect.

This particular part does have the DCI codec interface, and the 
description on their website indicates the board includes a codec. That 
implies you can do better than the 12-bit / PWM that was discussed earlier.

It seems they're targeting this development board to audio annunciator 
and answering machine applications (note the ginormous 4Mb SPI flash 
they include). If so, providing both a codec and a PWM audio output 
addresses the low-cost/low-fidelity market as well as the more upscale 
segments.

Eric