Improved State Integrity of Flip-Flops for Voltage Scaled Retention Under PVT Variation

Through measurements from 82 test chips, each with a state retention block of 8192 flip-flops, implemented using 65-nm design library, we demonstrate that state integrity of a flip-flop is sensitive to process, voltage, and temperature (PVT) variation. It has been found at 25°C that First Failure Voltage (FFV) of flip-flops varies from die to die, ranging from 245 mV to 315 mV, with 79% of total dies exhibiting single bit failure at FFV, while the rest show multi-bit failure. In terms of temperature variation, it has been found that FFV increases by up to 30 mV with increase in temperature from 25°C to 79°C, demonstrating its sensitivity to temperature variation. This work proposes a PVT-aware state-protection technique to ensure state integrity of flip-flops, while achieving maximum leakage savings. The proposed technique consists of characterization algorithm to determine minimum state retention voltage (MRV) of each die, and employs horizontal and vertical parity for error detection and single bit error correction. In case of error detection, it dynamically adjusts MRV per die to avoid subsequent errors. Silicon results show that at characterized MRV, flip-flop state integrity is preserved, while achieving up to 17.6% reduction in retention voltage across 82 dies.