4 Bit Down Counter

A 4-bit down counter is a digital counter circuit, which provides a binary countdown from binary 1111 to 0000. This circuit uses four D-type flip-flops, which are positive edge triggered. At each stage, the flip-flop feeds its inverted output (/Q) back into its own data input (D). However, it feeds its non-inverted output (Q) to the clock input (CK) of the following stage. This type of circuit operates in an asynchronous (ripple) manner because the flip-flop stages do not rely on a common clock pulse for timing. The operational speed of the counter depends upon the signal propagation through successive stages, rather than a common clock pulse as in synchronous circuits.

When RESET is applied on all the flip-flops simultaneously, their Q outputs become logic 0 state, and /Q outputs becomes logic 1 state. The /Q outputs are fed back into their own data inputs (D) and therefore all that they require is the rising edge of a clock pulse to transfer the data at input (D) to output Q.

Truth Table

This type of circuit is useful in timer applications. A count down from decimal 15 to decimal 11 becomes simply a matter of looking for the 1-0-1-1 output, because at the decimal count of 11, the logic state of QA=1, QB=1, QC=0, and QD=1. In TTL digital circuits, logic 1 state, usually represents +5 V electrically, and we can use these outputs in many circuits for timer applications.