Posted by Alex Seibulescu on September 30th, 2011
My friend Coverage turned into a revolutionary. We live in tumultuous times so this may not come as a surprise but seeing my friend pacing up and down the room, threatening imaginary adversaries was unnerving so I had to get to the bottom of it. What could possible turn my mild mannered friend into a raging firebrand? Once he settled down a bit, things started to clear up. Turns out that for a while now, people have been turning a blind eye on those last few percent of coverage holes as long as they did not belonged to the elite of cover targets, those that always need to be properly taken care of. “Even if there’s a bug in that area, we can always come up with a workaround or fix it in software” people would say, and who can blame them? Deadline pressures are not for the faint of heart and 2%-3% of obscure coverage targets are not going to stand in the way of tape-out bliss, right? Well, it turns out that with the relentless increase of design sizes and complexity and the worrisome shortening of the verification cycle, the number of second class coverage targets has swollen, their voice has become louder and they now threaten the verification establishment. It is ever more likely that continuing to ignore an increasing number of coverage holes will eventually lead to a silicon bug for which no quick ECO or software fix will be available and disaster will strike.
Since we’re all pragmatists, we know that some un-hit coverage targets will always end up being waved off but wouldn’t it be better to make an upfront decision which coverage goals we’re ready to grudgingly sweep under the carpet if need be, and let sound engineering rather than last minute time pressure be the judge of our compromise? One could mark such cover targets with special attributes as part of the verification plan so that it is properly documented and tracked rather than leave it to a hasty decision in the heat of the tape-out battle.
Treat your cover targets well, even the less important ones and my friend will once again be by your side when the tape-out bell rings.