Design high precision time keeping clock system based on Time Digital Convert

The high-precision crystal is mostly used as the clock source of the high-precision timekeeping systems. However, errors of the crystal output frequency are presented and accumulated due to the environmental temperature, load, and change of the excitation voltage, nature aging and other reasons. It would affect the accuracy of the timekeeping system, so that it cannot be applied to the occasions of the high-precision timekeeping clock. To overcome the errors above, this paper presents the design of a high-precision time keeping clock system. The system mainly consists of high-precision crystal (HPC), the accumulator (ACC), and Time Digital Convert (TDC) module. The HPC generates a pulse with the fixed period, and the pulse is taken as the input clock of ACC to make ACC self added according to the pre-set step. Then the overflowed signal of ACC acts as the output signal of the system's time keeping clock. When working, the system accurately measures the period of the HPC by use of the TDC module, and then the ACC stepping values are adjusted according to the measured results. It makes the overflowed signal of ACC remain the period as a high-accuracy clock signal. The system effectively eliminates the time keeping error which comes from the error and aging of the crystal. Experiments show that the system greatly reduces the cumulative time error, and the average cumulative time keeping error is 26.5ns in one week with the time keeping mean square error of 63ns.