[Cryptech Tech] [Cryptech-Commits] [core/platform/novena] 21/21: Sick hacks to compensate for sparse MUX within TRNG core.

[Joachim Strömbergson]

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Aloha!

Bernd Paysan wrote:
> The reason for async reset is that you don't need a clock to get
> your device into a default stable state, and it also works, when
> reset goes away before the clock starts. This is not something you
> always need, but when you do, sync is no option. On the other hand,
> async has no such show-stopper disadvantage.
> 
> In both cases, you need a similar logic to make sure that there is
> no conflict between clock and reset. And if you want sync reset,
> this logic needs an async reset.
> 
> I speak with the experience of having converted two Xilinx-specific 
> designs for an ASIC. Any Xilinx-specific approach for me is a 
> mistake.

The big problem with async reset is that the reset tree must be balanced
in relation to the clock tree so that deassertion is properly detected
at the same clock flank. See this paper by Cummings et al for a good
description:

http://www.sunburst-design.com/papers/CummingsSNUG2003Boston_Resets.pdf


As far as my recollection goes, early on in the project when you Bernd
came into the project we had reset strategy discussions and we ended up
going with your suggestion. I specifically updated the few cores we had
then.

Most of our HW design has async active low reset (all hash cores, aes,
cha, trng, entropy providers, parts of eim etc). The big one that does't
do this is modexp6s. Then there are some Xilinx specific stuff. Some
blockrams have no reset at all. And eim arbiter actually has async
active high reset, even though the signal is called "eim_cs0_n".
	
Since we have previously gone for async active low and by far most of
our design uses this strategy, I propose that we use async active low
reset (with synch deassertion). Where platform specific FPGA macros
require something different we solve that in each case, preferably in a
wrapper.

- -- 
Med vänlig hälsning, Yours

Joachim Strömbergson - Alltid i harmonisk svängning.
========================================================================
 Joachim Strömbergson          Secworks AB          joachim at secworks.se
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