How Many Power Islands Is Too Many?

By Ed Sperling

Power domains, also known as power islands, have become to design engineers what multiple cores are to processor architects. They can serve a purpose, namely reducing static current leakage and saving battery life. But they also can add so much complexity that they can make it almost impossible to get a new chip out the door.

Just as there has been talk of hundreds of cores, there has been talk of hundreds of power islands. But trying to verify a chip with that number of power islands is beyond human comprehension at this point, and so far there are no tools to make it simpler. As with multicore programming, there may never be, which is why companies like AMD are now considering dedicating different features for one or more cores rather than trying to split applications into myriad parts.

But power islands bring their own set of unique challenges. When you have 20 power islands, for example, each combination has to be tested. If one is one while another is off, that combination has to be tested when both are on, both are off, both are in sleep mode (or various modes that draw less power). Add another couple dozen power islands and the problem begins approaching epic proportions.

Shireesh Verma, a verification expert in Conexant’s Imaging and PC Media Group, said at this point there are definitely practical limits for the number of power islands.

“The maximum I have seen is 28, but typical is less than 20,” Verma said. “But it is not the complexity in the number of domains. It’s the combination of domains and the sequences you have—how many you have at different power states.”

Power islands must be balanced with the number of cores and the ability to verify the design. At least some of these techniques are needed. Bhanu Kapoor, founder of Mimasic, a consultancy in Richardson, Texas, said that clock gating sufficed as a way of controlling dynamic power until 90nm. But he said from 65nm on, every trick is needed.

“I’ve seen 5 to 9 power islands as the most common number,” Kapoor said. “The largest I’ve seen is from Renesas, which had 23. They had an interesting hierarchical power management scheme. But they’re not all independent [power islands].”

He noted that Nvidia is working on chips with up to 500 cores for graphics processing, which is one of the very few highly parallelizable mainstream applications. He said each power island on an Nvidia chip may control 24 cores.

In addition, there are diminishing returns for power islands. While shutting down power on functions clearly can save battery power by limiting the amount of static leakage, waking up and managing power islands impacts power, as well—both from the state change to the management of those various states.

For most design engineers, though, power islands are a relatively new concept. While the largest semiconductor companies have been working with them since 90nm, most of the work has been experimental.

ARM has had power domain test chips since the 130nm node, but most customers never really began thinking about them until the 90nm node. They’re now starting to hit production in high-volume applications such as smart phones, where turning off functions is essential for preserving battery life.

“A lot of times people will settle for two power domains—here’s the CPU and here’s everything else,” said Rob Aitken, R&D fellow at ARM. “We’ve been interested from the question of how many domains per CPU. We have settled on two. It’s only the more recent cores architected with cores in mind.”

ARM has been demonstrating its 1176 processor cores with state retention, but Aitken said there’s a question of whether design engineers will want to keep everything in the same state. He said that with state retention, there are no more than two power domains per CPU.

“It limits architecturally the things the processor ought to do. There’s also a concept that if you can do it in a nice way that’s transparent to the rest of the methodology, then you can have more. If your RAM had a power switch and it didn’t interfere with anyone’s verification or regulators, then you could put switches on it and buy people something. The added complexity of these domains limits what you can do before you throw up your hands in despair. The limits are in the 20s,” he said.