[sdiy] dsPIC33 question..

Seb Francis seb at burnit.co.uk
Thu Nov 13 19:15:01 CET 2008


Eric Brombaugh wrote:
>
> Someone please correct me if I'm wrong, but I don't believe that SPI 
> DMA is particularly useful. The problem is that (at least as I have 
> found) the dsPIC SPI port doesn't automatically control the Chip 
> Select line in master mode - you need to toggle that bit with firmware 
> instructions. That rules out using DMA on the SPI port to handle 
> anything except long sequential SPI transfers where the CS line is 
> static.
>
> Since typical audio delay applications are going to require multiple 
> interleaved read/write RAM accesses, you'll have to manage SPI RAM at 
> a per-transaction basis. DMA would work, but setup/teardown overhead 
> isn't going to be worth it for two-byte transfers.

Yes, this is right actually.  In the 4xD code I am writing 2 bytes, then 
reading 2 bytes alternately.  So DMA is not going to help.  I suppose 
one could have the code use buffers and transfer blocks at a time, but 
for a delay application where the delay time needs to be changed 
dynamically without glitching, that's going to be headache to program.

And hey, I don't mind using an 80 pin package .. all the more room for 
CV inputs :)

Seb






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