A decade counter is a type of digital circuit that counts in binary-coded decimal (BCD) format. It's designed to count from 0 to 9 and then reset back to 0, similar to the digits in a decimal number system. The counter advances its count by one for each clock pulse it receives. The key feature of a decade counter is that it follows a specific counting sequence, which is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and then it wraps back to 0.
Let's break down how a basic decade counter works:
Flip-Flops: The basic building block of a decade counter is the flip-flop. In this context, typically D-type flip-flops are used. A D-type flip-flop has a data input (D), a clock input (CLK), a set input (S), a reset input (R), and a complementary output (Q). The state (0 or 1) of the flip-flop is determined by the data input (D) at the rising edge of the clock signal.
Cascading Flip-Flops: To create a decade counter, several D-type flip-flops are connected in a chain, with the output of each flip-flop connected to the clock input of the next one. In a decade counter, four flip-flops are typically used to represent the four BCD digits (ones, tens, hundreds, and thousands).
Clock Input: The common clock signal is connected to the clock input (CLK) of the first flip-flop in the chain. When a clock pulse is received, it causes the flip-flops to transition to their next states simultaneously.
Counting Sequence: The key to the decade counting behavior is how the flip-flops are connected. Each flip-flop represents one digit of the BCD number. The flip-flop representing the ones digit (rightmost digit) counts from 0 to 9. However, when it transitions from 9 to 10, it triggers a carry output (usually labeled as "CARRY OUT") which then acts as a clock input for the next flip-flop (representing the tens digit). This carry process propagates through the flip-flops, creating the desired counting sequence.
Reset: When all the flip-flops reach the count of 9 (BCD code for 9), the next clock pulse triggers a carry output that resets the ones digit flip-flop back to 0, and the counting sequence starts again.
By properly connecting and configuring the flip-flops, the decade counter circuit is able to count from 0 to 9 and then reset, effectively emulating the behavior of a decimal counter. Decade counters find applications in various digital systems, including digital clocks, timers, and frequency dividers.