Re: [f-cpu] Something new to play with :)

[Michael Riepe <michael@stud.uni-hannover.de>]

On Tue, Sep 25, 2001 at 01:30:31AM +0200, Yann Guidon wrote:
[...]
> > > > Speed: 66MHz (optimized for speed, no special tricks)
> > > wow :-)
> > > what is the fastest thing you have synthesised for this device ?
> > That's nothing -- the multiplier was faster :)
> > But I'm quite satisfied with that result.
> does that mean that you have something better in mind ? :-)

It means: 66 MHz is pretty good (for an FPGA).  Although, know you
mentioned it... I *may* be able to do even better ;)

> > > > Utilization: 30%
> > > wow :-/
> > > How big is it btw ?
> > The multiplier also took something like 30%.  The SHL EU is very
> > space-consuming...
> yep. From the other mail i wrote, i might come from the wires
> (which btw make P&R more difficult in a 2D frame).

Or the number of cells required for the muxes.  There are four of them
for every bit, approximately -- and since left-shifts and right-shifts
are handled separately, that number is doubled once more.  That is,
there are 512 multiplexers with 8...12 inputs.  But that's nothing
compared to the Xbar ;)

> > > > btw. the muxes are quite huge :) FPGA architectures usually have some
> > > > problems with complex multiplexer structures.
> > > this is why i prefered to use 4-in muxes :-)
> > Too big for most FPGAs.  All you can do with 4-input cells is a 2:1 mux.
> read my other mail that answers to Richard. This puts the pressure
> on the decoder, OTOH. With some smart thinking it maybe be possible
> to find a suitable trade of between the cascade of OR and the decoder
> complexity.

I guess my 2nd stage decoder is already pretty simple.  Took me quite a
while to find a solution that does not suffer from extreme elephantiasis ;)

> > > Can you track where the critical datapaths are located ?
> > I guess it's in the control logic for stage 2.
> ggggrrrrrr i'll have to look at the sources :-)

Good idea ;)

-- 
 Michael "Tired" Riepe <Michael.Riepe@stud.uni-hannover.de>
 "All I wanna do is have a little fun before I die"
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