A fast-locking all-digital delay-locked loop for phase/delay generation in an FPGA

A fast-locking all-digital delay-locked loop (ADDLL) is proposed for the DDR SDRAM controller interface in a field programmable gate array (FPGA).The ADDLL performs a 90° phase-shift so that the data strobe (DQS) can enlarge the data valid window in order to minimize skew.In order to further reduce the locking time and to prevent the harmonic locking problem,a time-to-digital converter (TDC) is proposed.A duty cycle corrector (DCC) is also designed in the ADDLL to adjust the output duty cycle to 50％.The ADDLL,implemented in a commercial 0.13 μm CMOS process,occupies a total of 0.017 mm2 of active area.Measurement results show that the ADDLL has an operating frequency range of 75 to 350 MHz and a total delay resolution of 15 ps.The time interval error (TIE) of the proposed circuit is 60.7 ps.