If your travel budget is like ours, the VLSI Symposium is taking place in Honolulu without you again this year. But the stream of interesting papers goes on, and we do our best to get a glimpse.

One of the interesting spots is not a paper about mega-speed or nano-power, but about a chip management technique from NEC Corporate Labs. The company reported today on an embeddable thermal sensor for use in dynamically controlling the operating temperature within functional blocks on an SoC.

The problem NEC is addressing is one of local heating. In modern SoCs, an individual processor, accelerator, or memory instance may find itself operating at a much higher data rate than the surrounding circuitry. At today's frequencies, the dynamic power dissipated by this activity can cause intense local heating. The rising temperature, in turn, substantially increases leakage currents in the transistors, further heating the area. The worst-case result is physical failure of the device, and even in the best case the overall device power is unnecessarily high due to the increased leakage.

So NEC, like other chip design houses, is developing fine-grained ways to measure temperature at critical points on the die—processor pipelines, primary caches, and the like. The company uses the temperature information, interpreted by software, to control an external chip that dynamically shifts the voltage and clock frequencies of blocks on the SoC to manage local heating. This approach—combining local temperature sensing with dynamic voltage-frequency scaling (DVFS) has reportedly been heavily used in Intel processors as well media SoCs from a number of companies.

As an alternative, NEC settled on an elegantly simple circuit: a largish single transistor, Source shorted to Gate. The device generates only leakage current, which as mentioned is quite temperature-sensitive. The Drain is connected through a capacitor to ground, and a shorting switch is connected in parallel with the capacitor to provide a reset.

Once Reset is released, the capacitor integrates the leakage current, under the watchful eye of a simple comparator circuit. If you count up the time required for the comparator to fire, you have an accurate digital measure of the leakage current. But to get from current back to temperature, you have to calibrate the device.

That's where elegantly-simple comes in again. NEC researchers discovered that the slope of the leakage current-vs-temperature curve is closely linked to the actual current at a specific temperature. So by measuring the current at 20C, the researchers could accurately—within 3C—predict the current-temperature relationship over the whole range of interest.