Decade and BCD Counters
A decade counter is one that goes through 10 unique output combinations and then resets as the clock proceeds further. Since it is an MOD-10 counter, it can be constructed with a minimum of four flip-flops. A four-bit counter would have 16 states. By skipping any of the six states by using some kind of feedback or some kind of additional logic, we can convert a normal four-bit binary counter into a decade counter. A decade counter does not necessarily count from 0000 to 1001. It could even count as 0000, 0001, 0010, 0101, 0110, 1001, 1010, 1100, 1101, 1111, 0000, In this count sequence, we have skipped 0011, 0100, 0111, 1000, 1011 and 1110.
A BCD counter is a special case of a decade counter in which the counter counts from 0000 to 1001 and then resets. The output weights of flip-flops in these counters are in accordance with 8421-code. For instance, at the end of the seventh clock pulse, the counter output will be 0111, which is the binary equivalent of decimal 7. In other words, different counter states in this counter are binary equivalents of the decimal numbers 0 to 9. These are different from other decade counters that provide the same count by using some kind of forced feedback to skip six of the natural binary counts.