Dual-DLL-based CMOS all-digital temperature sensor for microprocessor thermal monitoring

Abstract

Today's microprocessors increasingly need on-chip temperature sensors for thermal and power management [1]. Since these sensors do not take part in the main computing activity but rather play the auxiliary, albeit important, role of temperature monitoring, their presence in terms of area, power, and design effort should be minimal, thus, all-digital sensors are desired. Temperature sensing based on temperature-dependent delays of inverters [2] could be suited for microprocessor applications, as it lends itself to digital implementation: by using a time-to-digital converter (TDC), an inverter delay can be compared to an absolute delay reference and converted to a digital temperature output [2] (Fig. 3.7.1). We report on an all-digital CMOS temperature sensor for microprocessor application, which also exploits temperature-dependent inverter delays within the TDC-based framework of Fig. 3.7.1. It, however, has two improvements over prior art of [2]. First, it removes the effect of process variation on inverter delays via calibration at one temperature point (instead of 2-point calibration of [2]), thus, reducing high volume production cost. Second, we use two fine-precision DLLs, one to synthesize a set of temperature-independent delay references in a closed loop, the other as a TDC to compare temperature-dependent inverter delays to the references. The use of DLLs simplifies sensor operation and yields a high measurement bandwidth (5kS/s) at 7b resolution, which could enable fast temperature tracking. This is in contrast to [2], where a counter-based cyclic TDC with an open-loop single delay-reference has a longer measurement time for a similar resolution.