A fully digital DLLs integrated in FPGAs

This paper presents fully digital dedicated on-chip DLLs, allowing for synchronization of external and internal clocks in FPGAs. DLL clock delay compensation circuit, digital clock phase shifter, digital duty-cycle-correction circuit and clock divider. In a Smic 0.18um CMOS process, its operation frequency range is 25MHz~300MHz at 1.8V. The peak-to-peak jitter is 35ps. Dueing to the digital architecture of the DLL, it only need a single synchronization step when the frequency of the input clock signal is stable. DLL’s locking time is 13 clock cycles. In addition to providing zero delay with respect to a user source clock, the DLL can provide three phase-shifted version of the source clock. The DLL can also divide the user source clock by up to 16. The values allowed for this property are 1.5, 2, 2.5, 3, 4, 5, 8, or 16; the default value is 2.