[sdiy] FPGA digital audio

[Magnus Danielson]

From: harry <harrybissell at prodigy.net>
Subject: Re: [sdiy] FPGA digital audio
Date: Sat, 05 Jan 2002 12:57:43 -0500

Harry,

> Hmmm... with the price of the chips I'd go with a bigger one and let
> the router do it. Like PCBs... there is no doubt that a careful hand design
> would outperform it. Computers are persistant, but not clever.
> 
> Another rule is don't fill the chip. I started at 65% with the initial
> design and
> went to 70%... now that it works.

There is a golden rule with FPGA design:

It is easier to scale down later (when you know how much you really
needed) than trying to scale up (since you did not have enougth space
left).

The trick is to choose encapsulation and pinassignments with scaling
up/down issues in mind. Expensive experience have told us that if you
choose a too small package ("It MUST fit") to grow in, then one has to
redo the PCB just in order to get a larger FPGA. If you choose with a
bit of margin, you may later chose a smaller FPGA in the same capsule
but with smaller amount of CLBs (and less price).

The danger is naturally the classical Creeping Featurisms and bad
designs (taking less care of silicon usage). What you however attempts
to counter-balance is lack of early design complexity analysis and
lack of actual problem for which the design must solve.

Experince has shown that not too selldom has the initial views on how
large a design becomes been too optimistic and that the full scale
view of what the actual problems to be solved where much more than
originally perceived.

Cheers,
Magnus