Introduction 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.
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