A Fault-Tolerant Current-Starved Ring Oscillator with Signal-Flip Protection Delay Cells against Radiation Effects

This paper proposes a fault-tolerant current-starved ring oscillator to provide accurate clock signals against radiation effects. The ring oscillator adopts multiple signal-flip protection delay cells (SPDC) that can prevent unwanted signal flips due to external radiation. The SPDC utilizes input-dependent voltage limiters with delayed switching, maintaining the output voltage level against radiation effects, while minimally affecting normal operations of the delay cell. The proposed ring oscillator with SPDC was fabricated in 180-nm CMOS process and fully verified with chip tests in actual radiation environments. When generating 15 MHz clock signal at 1.8 V supply, the proposed ring oscillator ensures smaller frequency deviation from -5% to 11% with radiation charges of 0.25 pC compared to the conventional ring oscillator which suffers from large deviation from -35% to 79%. The minimum amount of radiation charge that causes >30% frequency deviation was 7.6x improved than conventional clock generators, verifying fault tolerance against large radiation effect.