However, Muller now says ARM has been working on the technology since then.
We did a couple of subthreshold designs and then a bunch of near-threshold designs. Near-threshold is easier, because you can be closer to the SPICE models. We did some library development to go with that.
It turns out there are some real implementation choices to be made when you design slow processors; where and how do you do power gating, where do you do clock gating. We've come up with some techniques for power gating in the [clock] cycle. And then there are things like drowsy logic.
Drowsy logic, an alternative to putting the processor into sleep mode, is applicable where leakage currents are managed. Muller said that it is possible to turn off the power and let the power rail voltage decay. But it is not necessary to provide explicit state retention as long as the voltages on flipflops do not go too low. Essentially, it is a method of intermittently providing voltage sufficient to maintain state and operations.
There are multiple techniques, including clock gating, and how these techniques are best applied depends on the clock frequency being targeted. "With the clock gating 20kHz is very different to 2MHz or 2GHz."
The advantage of selecting a near-threshold voltage is scalability from conventional CMOS operations, making it more likely that a foundry could provide support, Muller said. Companies have designed circuits to operate at subthreshold voltages, but they have to take all the risk in terms of characterizing and testing their silicon. "There are very few companies who are prepared to do that. Near-threshold is more compatible with the foundries."
When asked if ARM was working with particular foundries to offer near-threshold manufacturing processes, Muller said: "That's all part of market development." The technology is compatible with the Cortex-M0, an ARM 32-bit core used in low-power microcontrollers, Muller said, adding: "It's all about the implementation" When asked about the status of the research, Muller said it had moved into development with partner companies and would then go to implementation. He did not provide a timetable.