Re: Re: [f-cpu] No latches, please !

[Juergen Goeritz <goeritz@oekomm.de>]

On Wed, 13 Feb 2002 whygee@club-internet.fr wrote:

> The latches nightmare continues here ;-P
> Don't read if you're easily technically offended
> 
> 
> >On Wed, 13 Feb 2002, Ben Franchuk wrote:
> >> nicolas.boulay@ifrance.com wrote:
> >> > 
> >> > I have miss that : please NO LATCH in our design !!!!
> >> > 
> >> > It's almost useless compare to classique flip-flop. I
> >> > have said nothing for the register set because of the
> >> > speed. But latch are generaly a very very bad idea.
> >> > It's almost impossible to test correctly and a lot of
> >> > tools have problem with them (static timing analysis,
> >> > compiler ...).
> >> > 
> >> Time for better tools? Latches have their place in design, but require
> >> more care with design and clock generation. 
> >
> >Usually you only use latches to synchonize input signals
> >to keep input setup times of other circuitry.
> 
> <start delirious block>
> 
> If we were SURE that the number of pipeline stages was EVEN,
> we could do something really... CRAZY !
> 
> Remember : a FlipFlop is a pair of transparent latches
> that are activated with an opposite signal. Under the following
> conditions :
>  - setup/hold times respected (more difficult because of the distance
>     and phase shift/delay)
>  - even number of stages
> it would be possible to use only latches in a pipeline...
> 
> <end delirious block>
> 
> <standard_disclaimer>
> The above is only pure speculation. I know some people will
> like while others will love it. It's funny but is not part of the F-CPU project.
> <standard_disclaimer>

It may not work because if you invert the clock inside the
pipeline stages using latches the 'filling logic' must fit
the delay of less than half a clock (clk/2 - prev-latch-delay
- latch-setup) in all (!) stages. And you have to stop the
clock to freeze the contence of the pipeline stages. But
how to keep from floating in the transparent stages because
of changing pipeline input signals then? The whole cores
clock would have to be stopped.

JG

*************************************************************
To unsubscribe, send an e-mail to majordomo@seul.org with
unsubscribe f-cpu       in the body. http://f-cpu.seul.org/