Time Management for Low-Power Design of Digital Systems

The implementation of complex functionality in low-power (LP) nano-CMOS technologies must be carried out in the presence of enhanced susceptibility to PVT (Process, power supply Voltage and Temperature) variations. VT variations are environmental or operation-dependent parametric disturbances. Power constraints (in normal and test mode) are critical, especially for high-performance digital systems. Both dynamic and leakage power induce variable (in time and space) thermal maps across the chip. PVT variations lead to timing variations. These should be accommodated without losing performance. Dynamic, on-line time management becomes necessary. The purpose of this paper is to present a VT-aware time management methodology which leads to improved PVT tolerance, without compromising performance or testability. First, the methodology is presented, highlighting its characteristics and limitations. Its underlying principle is to introduce additional tolerance to VT variations, by time borrowing, dynamically controlling the time of the clock edge trigger driving specific memory cells (referred to as critical memory cells, CME). VT variations are locally sensed, and dynamic delay insertion in the clock signal driving CME is performed, using Dynamic Delay Buffer (DDB) cells. Then, methodology automation, using the proprietary DyDA tool, is explained. The methodology is proved to be efficient, even in the presence of process variations. Finally, it is shown that VT tolerance insertion does not necessarily reduce delay fault detection, as multi-VDD or multi-frequency self-test can be used to recover detection capability.