TS Cache: A Fast Cache With Timing-Speculation Mechanism Under Low Supply Voltages

Abstract

To mitigate the ever-worsening “power wall” problem, more and more applications need to expand their working voltage to the wide-voltage range including the nearthreshold region. However, the read delay distribution of the static random access memory (SRAM) cells under the nearthreshold voltage shows a more serious long-tail characteristic than that under the nominal voltage due to the process fluctuation. Such degradation of SRAM delay makes the SRAM-based cache a performance bottleneck of systems as well. To avoid unreliable data reading, circuit-level studies use larger/more transistors in a bitcell by sacrificing chip area and the static power of cache arrays. Architectural studies propose the auxiliary error correction or block disabling/remapping methods in fault-tolerant caches, which worsen both the hit latency and energy efficiency due to the complex accessing logic. This article proposes a timing-speculation (TS) cache to boost the cache frequency and improve energy efficiency under low supply voltages. In the TS cache, the voltage differences of bitlines (BLs) are continuously evaluated twice by a sense amplifier (SA), and the access timing error can be detected much earlier than that in prior methods. According to the measurement results from the fabricated chips, the TS L1 cache aggressively increases its frequency to 1.62× and 1.92× compared with the conventional scheme at 0.5- and 0.6-V supply voltages, respectively.