Heart rate variability studies made on determining the instantaneous p-p interval of the ECG gives more closer results than that determined over r-r interval. This paper proposes the design and implementation of PWLL (P-Wave Locked Loop), for identifying the p-wave in the ECG and determining the time interval between successive p-waves. With the ECG input given, the PWLL gets locked to the p-wave automatically and produces the p-p time interval in analog and digital form. The p-p interval data obtained from the PWLL could be read into PC for making required analysis.. The system features of PWLL are analyzed and reported.