FIGURE 6-11
Biock Diagram of a Three-Decade Decimal BCD Counter
the state diagram of Fig. 6.9. Ql changes state afte r each clock pulse. Q~ comp lements
every time Ql goes from I 10 O. as long as Qs = O. When Qs becomes 1. Q~ remains at O. Q4
complements every time Q2 goes from I to O. Qs rema ins at 0 as long as Q~ or Q-I is O. When
both Q2and Q4 become I. Q8complements when QJ goes from I to O. Qs is cleared on the next
transition ofQj.
The BCD counter of Fig. 6.10 is a decade counter. since it counts from0 to 9. To count in decimal
from0 to 99, we need a two-decade counter.To count from 0 to 999. we need a three-decade
counter. Multiple decade counters can beconstructed by connecting BCD counters in cascade,
one for each decade. A three-decade counter is shown in Fig. 6.11. The inputs to the second and
third decades come from Qs of the previous decade.When Qs in one decade goes from I 10 O. it
trigger s the count for the next higher order decade ",'bile its own decade goes from 9 10 O.