BCD Ripple Counter
A decimal counter follows a sequence of 10 states and returns to 0 after the co unt of9. Such a
counter must have at least four flip-flops to represent each decimal digit. since a decimal digit
is represented by a binar y code with at least four bit s. The sequence of stales in a decimal
counte r is dictated by the binary code used to represent a decimal digit. If BCD is used. the sequence
of slates is as shown in the state diagram of Fig. 6.9.A decimal counter is similar to a
binary counter. except that the state after 100 1 (the code for decimal digit 9) is סס oo (the code
for decim al digit 0).
The logic diag ram of a BCD ripple co unter using 1K flip-flops is shown in Fig. 6. 10. The
four outputs are desig nated by the letter symbol Q. with a numeric subscript equ al to the binary
weight of the corresponding bit in the BCD code. NOEe that the output of QJ is applied 10
the C inputs of both Q2 and Qs and the output of Q2 is applied to the C input of Q-I' The 1 and
K input s are connected either to a perm anent I signal or to outputs of other flip-flops.
A ripple counter is an asynchronous sequential circuit. Signals that affect the flip-tlop transition
depend on the way they change from I to O. The operation of the counter can be explained
by a list of conditions for flip-flop transitio ns. These conditions are deri ved from the
logic diagram and from know ledge of how a l K flip-flop operates. Remembe r thai when the
C input