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# Pretty Scary, Huh Kids?

Comments(1)Filed under: Low power , Logic Design, CPF, power design, Power Shut-Off, Incisive, halloween, clp, conformal, idts

In honor of Halloween, here are some horror stories about low power bugs.  These are real bugs at real customers that would have led to real dead chips.

Horror story #1:  It was a dark and stormy night…  Ok, it was around lunch time.  But a customer had just spent three weeks coding a UPF file to describe isolation between two power down domains.  The customer’s synthesis tool required that the UPF only implement either from isolation (from a power domain) or to isolation (to a power domain), not both.  So in order to prevent redundant isolation between separate domains enabled by the same power net, he had to specify specific nets between the domains, and specify them as no_isolation.  Only, and here’s the scary bit, he had made a mistake, and had incorrectly identified a couple of pins that really went to always on logic.  So, when the block was powered off, the always on logic would get nuked.  *Shudder*  Fortunately, one of my guys got static checks operational in an hour or so (shameless plug – Conformal Low Power).  The static checks structurally proved that isolation was missing between the power off block and the always on logic.  The dead chip was saved.

Horror story #2:  The door creaked slowly open, revealing…  Ok, he was in a cubicle.  But a customer was doing power shutoff on a processor based design.  The power controller was essentially a register bank.  The real time OS would get an interrupt from the power down timer, and would execute the power down routine.  The routine would hit the bit enabling isolation, and would then hit the bit disabling power to the power down block.  All well and good, and pretty routine.  The design was operational, and had passed all the static checks – everything was perfectly isolated and looked fine.  But there was terror lurking – the processor’s secondary cache was in the power down block.  The processor could execute the code to turn the power off, but would never get another instruction – ever!  *Shiver*  Fortunately, one of my guys got low power simulations operational with a high performance low power simulator (shameless plug – Incisive Design Team Simulator).  Since there wasn’t a significant overhead to the simulation, they ran a lot more test cases.  As a result, they caught the issue and moved the cache to an always on block.  The processor lived to execute another day.

Horror story #3:  There was a sound, and he turned around slowly to see the maniac raise his…  Ok, it was his boss (hey, that is scary!)  But a customer had defined the power intent for a big chip, and had proven the netlist and power intent was correct.  This was a big device, requiring feedthroughs across a power off domain.  His implementation tool, however, didn’t understand the power intent.  So it didn’t understand the always on cells, and so didn’t understand that they needed to be powered by special routes connected to an always on supply.  As a result, the feedthrough buffers in a power down domain were powered like all the other cells in that domain – by a switchable supply.  *Eeeek*  Fortunately, one of my guys got physical netlist checks operational.  These checks check that the always on power pins specified in the LEF are actually powered by always on power rails (shameless plug – Conformal Low Power).  The always on nets remained always on, and the chip made it.

So are you terrified yet?  As Count Floyd said on SCTV, “pretty scary, huh kids?”  Again, these are real stories – nothing made up here.  My point with these stories is not to scare you, but instead to point out the need for a complete verification strategy that includes both static and dynamic checks, and to continue to run these checks throughout the flow.  There are some nasty things that can happen when you do low power design, and these can quickly lead to some very dead chips.  I don’t know about you, but I find the thought of that much scarier than goblins and monsters and kids in Sarah Palin masks.

Happy Halloween, everyone.

Oh, and… BOO!