Re: [f-cpu] Bit Shuffler (Take 2)

[Kim Enkovaara <kenkovaa@cc.hut.fi>]

On Sat, 29 Sep 2001, Yann Guidon wrote:

> > Synplify usually likes as high level descriptions as possible. But I think
> > that optimizing for speed at this stage is not wise thing to do.
> how do you tell Synplify to pipeline the multiplier ? and how do you instruct it
> to provide partial results ?

Easiest way to do that is to write a*b in the VHDL and add as many
fliflops after the operation as you need pipeline stages. Then just ask
the Synplify to balance the registers. In that way it creates very well
optimized pipelined multiplier. Also if the FPGA architecture includes HW
multipliers it uses them during the synthesis. In VirtexII architecture
synplify can't use the internal flipflops of the 18x18 multiplier, but
those flipflops are not documented or characterized at the monent, but
they are there.

> > First you need a working chip and testbenches around it. After that it is easy to
> > recode some blocks for ASIC and FPGA for example.
> it won't be easy if we did not foresee the necessary requirements.

Good rule of the thumb is that best ASIC processes are 10 times faster
than the fastest FPGAs for normal logic. For example one vendor promises
about 30 levels of logic at 600MHz with 0.13u process. And the fastest
FPGAs are quite expensive, you can buy a small car with few of them :)

> Today, now that i see that i can code something, i wonder : what is the best
> way to write stuff so it runs as fast as possible. I know that if i start to
> write VHDL "the usual way", the result might be disapointing. If a large part
> of the CPU is designed this way, it won't hold its promise. People will work
> on underefficient designs and it will be too late. This is particularly
> worrying in the control path (scheduler). I trust Michael for the rest.

I think the only way to see the results is to synthesize the results
yourself and analyze the results from the shematics. It is very difficult
to analyze the results if the code is not written by yourself.

=============================================================================
Mr. Kim Enkovaara   | kim.enkovaara@iki.fi | Microelectronic Riemannian
Vasamatie 1 C 16    | IRC: embo            | curved-space fault in
02630 Espoo         |                      | write-only file system

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