BCD Counter A BCD counter counts in binary-coded decimal from 0000 to 1001 and back to 0000. Because of the return to 0 after a count of 9, a BCD counter does not have a regular pattern, unlike a straight binary count. To derive the circuit of a BCD synchronous counter, it is necessary to go through a sequential circuit design procedure. The state table of a BCD counter is listed in Table 6.5. The input conditions for the T flip-flops are obtained from the present- and next-state conditions. Also shown in the table is an output y, which is equal to 1 when the present state is 1001. In this way, y can enable the count of the next-higher significant decade while the same pulse switches the present decade from 1001 to 0000. The flip-flop input equations can be simplified by means of maps. The unused states for minterms 10 to 15 are taken as don't-care terms. The simplified functions are Tọ1 = 1 To2 = Q&Q1 To4 = Q2Q1 Tos = QsQ1 + Q.Q2lı y = Q&Q1 %3D The circuit can easily be drawn with four T flip-flops, five AND gates, and one OR gate. Synchronous BCD counters can be cascaded to form a counter for decimal numbers of any length. The cascading is done as in Fig. 6.11, except that output y must be con- nected to the count input of the next-higher significant decade.