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Posts Tagged ‘low-power design’

Reset Expectations with X-Propagation Analysis

Thursday, June 25th, 2015

The propagation of unknown (“X”) states has become a more pressing issue with the move toward billion-gate SoC designs, and especially so with power-managed SoC designs. The SystemVerilog standard defines an X as an “unknown” value used to represent the state in which simulation cannot definitely resolve a signal to a “1,” a “0,” or a “Z.”

Synthesis, on the other hand, defines an X as a “don’t care,” enabling greater flexibility and optimization. Unfortunately, Verilog RTL simulation semantics often mask the propagation of an X value, while gate-level simulations show additional Xs that will not exist in real hardware.

The sheer complexity and common use of power management schemes increase the likelihood of an unknown “X” state in the design translating into a functional bug in the final chip. This possibility has been the subject of two technical presentations at the Design and Verification Conference during the last couple of years: I’m Still in Love With My X! But, Do I Want My X to Be an Optimist, a Pessimist, or Eliminated? and X-Propagation Woes: Masking Bugs at RTL and Unnecessary Debug at the Netlist. Let’s look more closely at this issue and the requirements for a solution.


Reset Optimization Pays Big Dividends Before Simulation

Thursday, June 26th, 2014

This article was originally published on TechDesignForums and is reproduced here by permission.

Reset optimization is another one of those design issues that has leapt in complexity and importance as we have moved to ever more complex system-on-chips. Like clock domain crossing, it is one that we need to resolve to the greatest degree possible before entering simulation.

The traditional approach to resets might have been to route to every flop. Back in the day, you might have been done this even though it has always entailed a large overhead in routing. That would help avoid X ‘unknown’ states arising during simulation for every memory location that was not reinitialized at restart. It was a hedge against optimistic behavior by simulation that could hide bugs.

Our objectives today, though, include not only conserving routing resources but also capturing problems as we bring up RTL for simulation to avoid unfeasible run times there at both RTL and – worse still – the gate level.

There is then one other important factor for reset optimization: its close connection to power optimization.

Matching power and performance increasingly involves the use of retention cells. These retain the state of elements of the design even if appears to be powered off: in fact, to allow for a faster restart bring-up these must continue to consume static power even when the SoC is ‘at rest’. So, controlling the use of retention cells cuts power consumption and extends battery life. (more…)

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