SoC temperature-sensing IP from Analog Bits aids thermal management

Analog Bits has announced a hard-macro temperature sensor for use in SoC designs. The block, currently available for G-class 40nm CMOS processes, is intended to sit inside a high-activity digital block and monitor temperature on the active surface of the die. A system controller would use the temperature data to reduce the voltage, frequency, or duty cycle on a block that was growing too hot. In a typical application, one sensor would be included in each processor core of a large multicore Soc.

Local temperature sensing has become mandatory in advanced high-performance designs, according to Analog Bits president Alan Rogers, for the simple reason that blocks implemented in these processes can locally heat themselves out of their reliable operating range, and even into thermal runaway. Without direct temperature measurement and feedback control, the alternative is to derate the design until no possible workload can cause excessive local heating.

The block is essentially a diode-based digital thermometer, Rogers explained. Functionally, the block behaves like a precision thermal diode, a sample-and-hold, and a ten-bit A/D converter. Thus the block has clock and enable inputs, and data-valid and a ten-bit data-out outputs. It requires 1.8V analog and 0.9V core supply connections and core ground.

On the rising edge of Enable, the block performs a measurement, outputs the result, and asserts Valid. When Enable falls, the block shuts down its analog biases and enters a leakage-only state. It draws about 100 microA during its roughly 20 millisecond measurement cycle.

The size is a rather noticeable 200 microns on a side. This area, Rogers explained, is necessary in order to achieve the block’s analog accuracy of 1C in relative measurements, or, with the use of an optional 4-bit trimming input to compensate for process variations, 1C absolute. Rogers allowed that this accuracy might not be necessary in all thermal management applications, especially those that simply compare the temperature against a set point and throttle a block when that point is reached. And Analog Bits has a smaller, less accurate block in development. But some thermal management algorithms—for example those using model-based predictive control—might indeed benefit from 1C accuracy.

The CLN40G temperature sensor block is available now for 40 nm G-class processes. Deliverables include Verilog and SPICE models, a LEF footprint file, and a full datasheet.