Digital frequency comparator

A digital frequency comparator comprising:

• a first double-edge triggered flip-flop for receiving a first reference clock pulse and a data signal, wherein positive and negative triggers of the data signal are employed so as to output first and second state signals of the first reference clock pulse;
• a second double-edge triggered flip-flop for receiving a second reference clock pulse that lags behind the first reference clock pulse and the data signal, wherein the positive and negative triggers of the data signal are employed to output first and second state signals of the second reference clock pulse;
• and
• a combination logic for receiving the first and second state signals of the first double-edge triggered flip-flop and the second double-edge triggered flip-flop, and enabling an UP signal when the transmission frequency of the data signal is higher than the frequency of the first reference clock pulse, as well as enabling a DOWN signal when the transmission frequency of the data signal is lower than the frequency of the first reference clock pulse,
• wherein the second double-edge triggered flip-flop includes:
• a first D-type flip-flop having a D-input end receiving the second reference clock pulse, and the data signal being used as the positive trigger;
• a second D-type flip-flop having a D-input end receiving the second reference clock pulse, and the reverse signal of the data signal being used as the positive trigger;
• a first multiplexer connecting to the Q output ends of the first D-type flip-flop and the second D-type flip-flop, and using the reverse of the data signal as a selecting signal, and outputting the output of the first D-type flip-flop when the data signal is 1, and outputting the output of the second D-type flip-flop when the data signal is 0;
• and
• a second multiplexer connecting to the Q output ends of the first D-type flip-flop and the second D-type flip-flop, and using the data signal as a selecting signal, and outputting the output of the first D-type flip-flop when the data signal is 0, and outputting the output of the second D-type flip-flop when the data signal is 1.