Reducing Power Through RetimingFebruary 23, 2009
Here is an interesting and almost trivial technique for (potential) power reduction, which I never used myself, nor seen used in others’ designs. Well… maybe I am doing the wrong designs… but I thought it is well worth mentioning. So, if any of my readers use this, please do post a short comment on how exactly did you implement it and if it really resulted in some significant savings.
We usually have many high activity nets in the design. They are in many cases toggling during calculation more than once per cycle. Even worse, they often drive long and high capacitive nets. Since, in a usual synchronous design (which 99% of us do), we only need the stable result once per cycle – when the calculation is done – we can just put a register to drive the high capacitive net. The register effectively blocks all toggling on that net (where it hurts) and allows it to change maximum one time per cycle.
The image below tells the whole story. (a) is before the insertion of the flop, (b) right after.
This is all nice, but just remember that in real life it can be quite hard to identify those special nets and those special high toggling logic clouds. Moreover, most of the time we cannot afford the flop for latency reasons. But if you happen to be in the early design phase and you know more or less your floor plan, think about moving some of those flops so they will reduce the toggling on those high capacitive nets.