[sdiy] SHARC DSP programming

[rsdio at audiobanshee.com]

On Sep 24, 2016, at 10:05 AM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> That's why I thought I must have missed it.  But I'm thinking that it is maybe omitted with the assumption that everyone uses the floating-point support anyway.  As you said, the accumulator scaling and saturation on the dsPIC was a really nice feature if you just wanted to make up some gain on a signal that sounded a little quiet, and didn't care if the occasional peak got clipped.
> 
> I can't even get what shift instructions I've found on the SHARC to saturate instead of overflow!!! ...and this seems decidedly wrong for a DSP.  I must be doing something wrong, so I'll wait patiently and hopefully a SHARC expert can put me straight.

I'm more of a TMS320 expert, where there is a bit-shift option on the MAC instruction as well as many other instructions that operate on the full 40-bit accumulator width. This option is rather limited, though, allowing only 1 bit shift, not any more. It's called FRCT mode.

I hope Analog Devices doesn't assume that people are using float instead of fixed. That would be a lot of silicon to dedicate to an unused feature. What might be the case is that fixed-point developers are expected to scale all of the processing stages so that the final output is in range for being quantized to the smaller, general-purpose registers. Not quite sure how to accomplish that with their instruction set, but perhaps it's a matter of scaling the coefficients before applying the MAC or other instructions.

Brian