Synchronous counters are digital circuits where all flip-flops (FFs) are clocked by a common clock signal at the same time. This eliminates the "ripple" effect seen in asynchronous counters, leading to more precise and faster operation.
Up Counter
A synchronous up counter increments its count by 1 with each clock pulse. Here's how it works:
1. Flip-Flop Connections:
Each flip-flop's clock input is connected to the same clock signal. The outputs of some flip-flops (usually even-numbered ones) are fed to the logic gates controlling the clock or data inputs of the next flip-flop (usually odd-numbered ones).
2. Logic Gates:
Combinational logic gates (like AND, OR, NAND) determine the next state of each flip-flop based on the current state of the counter. They ensure proper counting behavior (incrementing by 1 on each clock pulse).
3. Counting Sequence:
The counter starts at 0 (all flip-flops reset) and increments through binary values (0001, 0010, 0011, ...) until it reaches its maximum value, which depends on the number of flip-flops (n-bits).
Example: 3-Bit Synchronous Up Counter
solution - Consider a 3-bit synchronous up counter using D flip-flops:
Components: Three D flip-flops (DFF_A, DFF_B, DFF_C) and logic gates (typically AND gates).
Clock: Common clock signal applied to all DFFs.
Logic: The output (Q) of DFF_A is connected to the D input of DFF_B. The AND gate's output (based on Q and QB') controls the clock input of DFF_B (ensuring it only clocks on specific transitions). Similarly, the combined outputs of DFF_A and DFF_B control DFF_C's clocking data input.
Counting Sequence: 000 -> 001 -> 010 -> 011 -> 100 -> 101 -> 110 -> 111. After reaching 111, it overflows and resets to 000 on the next clock pulse.
Synchronous Down Counter
A synchronous down counter decrements its count by 1 with each clock pulse. This can be achieved in two ways:
Complementing Outputs: Invert the outputs of each flip-flop before feeding them to the logic gates controlling the next stage. This effectively reverses the counting direction.