Single Flip-Flop Driving Circuit for Glitch-Free NAND-Based Digitally Controlled Delay-Lines

Abstract

NAND-based digitally controlled delay-lines (DCDLs) are employed in several applications owing to their excellent linearity, good resolution and easy standard cell design. A glitch-free DCDL behavior is often a strict requirement [e.g. spread-spectrum clock generators (SSCG) and digitally controlled oscillators]. Existing glitch-free NAND-based DCDL topologies either require two flip-flops for each DCDL delay-element (DE) or present a very long settling time which limits the maximum working frequency. This paper proposes a novel glitch-free NAND-based DCDL that joins the advantages of previously proposed topologies: uses only a single flip-flop for each DE (reducing area and power) and has relaxed timing requirements (allowing easy integration in applications like SSCG). In the paper, the glitch-free operation of the proposed circuit is firstly demonstrated theoretically and then verified experimentally, with the help of an SSCG built using proposed DCDL and implemented in 28 nm CMOS. Simulation results show that proposed DCDL results in a more that 30 % reduction of the power dissipation and a >20 % reduction in area occupation with respect to double flip-flop DCDL, without any timing constraints penalty.