Now Build a Digital Stopwatch 0-60sec By using the same circuit of the Digital Stopwatch 0-99sec , we can add
an AND gate, and transform the 0 – 99sec stopwatch to a 0 – 60sec
stopwatch.We must find a way to control the RESET function of the BCD
counter, which is responsible for the counting of the seconds. As we
studied above, the circuit resets when we have 99 to 100, that is 1001
1001 à 0001 0000 0000. To make a transformation successfully we must
force the pulse from 59 to 60 0011 1001 à 0100 0000 on the output of the
BCD counter.
By placing the AND gate, with its inputs on the Q1 and Q2 of the BCD
counter of the decades, we make sure that when the gate closes, the RST
input of the BCD counter will be set to logical “1”, which on its turn,
will force the circuit to start over. The transformed circuit appears in
picture 2.
