Re: Beginner Question: Loop Unrolling & Use of Tuple Elements

Hey Collin, how are things going?

On 6/28/15 12:59 PM, [email protected] wrote:
> Hello,
>
> I was attempting to make a simple FIR filter to compare MyHDL to previous
> work I'd done using Vivado C-based HLS. I'm basing this entirely on
> Christopher Felton's IIR example from
> https://bitbucket.org/cfelton/examples/src/tip/siir/siir.py .
>
> Two things have tripped me up, and I'm not sure if it's some limitation of
> MyHDL or my own failures? The first is as part of the FIR I'd like to have a
> line like this:
>
> @always(clk.posedge)
> def rtl_fir():
>     for i in range(0, len(B-1)):
>        ffd[i+1].next = ffd[i]
>        ffd[i].next = x
>
> But I can't seem to find references to loop unrolling anywhere? There was a
> few threads from ~2008 was all I saw. Instead I end up having to do
> something like this:

The MyHDL converter will not un-roll the loops
(currently).  The loops will be converted as loops
to the target HDL - where they will be un-rolled
by the synthesizer.

This post:
http://www.fpgarelated.com/showarticle/631.php

shows a "naive" (non-target-optimized) FIR
implementation using loops.

<snip>
> The second problem is for the accumulator (which again I'm manually
> unrolling), the most straight-forward definition to me would look like this:
>
> @always_comb
>      def rtl_acc():
>          # Double precision accumulator
>          yacc.next = B[0]*ffd[0] + B[1]*ffd[1] + B[2]*ffd[2] + B[3]*ffd[3] +
> B[4]*ffd[4]

I think the above link will help here as well,
when you create a tuple of ints, it is converted
as a ROM [1], what you need to do is:

     def rtl_acc():
         for ii in range(4):
             btmp = B[ii]
             yacc.next = btmp + fdd[ii]


Hope that helps,
Chris

[1] 
http://docs.myhdl.org/en/latest/manual/conversion_examples.html#rom-inference


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