[libre-riscv-dev] Needed standard cell development for libre SOC

[Luke Kenneth Casson Leighton]

On Saturday, January 11, 2020, Jean-Paul Chaput <Jean-Paul.Chaput at lip6.fr>
wrote:

>
> Hello All,
>
> On Sat, 2020-01-11 at 12:42 +0100, Staf Verhaegen wrote:
> > All,
> >
> > As said in other mail I will be doing standard cell development on my
> NLNet
> > project. In discussions I had before with Luke it was mentioned that
> there
> > may be need for specific standard cells for certain look-up tables maybe
> > coupled with a custom  place-and-route procedure. With current synthesis
> and
> > place-and-route tools this is typically not how one works. There one
> normally
> > just writes the RTL code and leaves everything to the synthesis and
> place-
> > and-route tool.
>
>   I would make some refinement here. The direct synthesis then P&R is good
>   if there is no regularity whatsoever in your design. On the opposite you
>   have RAM
>

hi JP, what Staf is referring to is the Dependency Matrices.

these are typically 128 x 30, 30 x 30 and in the 6600 and Mitch Alsup's
designs use an SR NOR Latch which is no longer part of standard cells
because it is "unverifiable" by proprietary tools.

i have had to substitute an asymmetrical SR NOR concept which synthesises
under nmigen to a DFF with an awful lot of extra logic.

this turns those Dependency Matrices from 50,000 gates into 250,000 or
above.

it is such a massive jump that it makes sense to try to do a dedicated cell.

SR NOR is literally 2 transistors.

DFF plus other logic is something like 10 possibly even 20.

however Staf tells me that SR NOR is CMOS or nonstandard.

therefore we need to look at redesigning the Dependency Matrix system to
see if SR NAND can be used instead.

the "problems" associated with SR NOR were all avoided in the 6600 through
careful design.  no reset latches were needed.  no instability occurred.
per-row inherent and implicit resets were provided that ensured that
unknown startup states for which SR NOR is well known never hit the
pipelines.

l.



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