A decade counter is a type of digital counter that counts in decimal (base-10) notation. It is commonly used in digital electronics to produce a sequence of output states or to divide the input clock signal frequency by 10. The name "decade counter" comes from its ability to count in units from 0 to 9, representing all ten possible decimal digits.
There are different types of decade counters, but one of the most common ones is the "Asynchronous Decade Counter" or "Ripple Counter," which uses flip-flops to perform the counting operation.
Here's an explanation of the operation of a 4-bit asynchronous decade counter:
Flip-Flop Arrangement: The decade counter consists of four flip-flops, typically D-type flip-flops, labeled FF0, FF1, FF2, and FF3. Each flip-flop represents one bit of the counter output.
Clock Input: The counter is driven by an external clock signal, which provides the timing for the counting operation. Each flip-flop responds to the rising edge (or falling edge, depending on the type) of the clock signal.
Initial State: At power-up or when the counter is reset, all flip-flops are cleared to 0, representing the initial state of the counter (0000 in binary).
Counting Sequence: When the clock signal goes high, the counter starts counting. The counting sequence follows the binary counting order from 0 to 9, but we are interested in the decimal representation.
Here is the counting sequence in decimal and binary:
Decimal: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
Binary: 0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111, 1000, 1001
Ripple Carry: Asynchronous decade counters use a "ripple carry" mechanism, meaning that when a flip-flop changes from 1 to 0, it triggers the next flip-flop to toggle. For example, when FF0 changes from 1 to 0, it triggers FF1 to toggle, and so on.
Reset: Once the counter reaches 9 (1001 in binary), the next clock pulse causes all flip-flops to reset back to 0 (0000 in binary). This reset action completes the counting cycle, and the counter starts counting from 0 again.
Frequency Division: When using a decade counter as a frequency divider, the input clock frequency is divided by 10 at the output. For example, if the input clock frequency is 10 MHz, the output frequency will be 1 MHz (10 MHz / 10).
Keep in mind that there are other types of decade counters, such as synchronous counters, which operate differently by using a common clock signal for all flip-flops, avoiding the ripple carry delay. Nevertheless, asynchronous decade counters are straightforward and widely used in various digital applications.