Asynchronous 3-bit up down counter
In my previous post on ripple counter we already saw the working principle of up-counter. Now in this post we will see how an up down counter work.
As we know that in the up-counter each flip-flop is triggered by the normal output of the preceding flip-flop (from output Q of first flip-flop to clock of next flip-flop); whereas in a down-counter, each flip-flop is triggered by the complement output of the preceding flip-flop (from output Q^ of first flip-flop to clock of next flip-flop).Here you will see in bellow diagram of 3-bit up down counter.
The operation of such a counter is controlled by the up-down control input. Now question is in which sequence it will count see below the table for the counting sequence of the up down counter in the two modes of counting.
As I discussed earlier that for up down counting operation preceding flip-flop sometime it need input from output from output Q of first flip-flop to clock of next flip-flop for up-counting and sometimes from output Q^ of first flip-flop to clock of next flip-flop for down-counting. So in above circuit diagram it is shown clearly. As we know a flip-flop can hold single bit so for 3 bit operation it need three flip-flops. An inverter has been inserted in between the count-up control line and the count-down control line to ensure that the count-up and count-down cannot be simultaneously in the HIGH state.
When the count-up/down line is held HIGH, the lower AND gates will be disabled and their outputs will be zero. So they will not affect the outputs of the OR gates. At the same time the upper AND gates will be enabled. Hence, QA will pass through the OR gate and into the clock input of the B flip-flop. Similarly, QB will be gated into the clock input of the C flip-flop. Thus, as the input pulses are applied, the counter will count up and follow a natural binary counting sequence from 000 to 111.
Similarly, with count-up/down line being logic 0, the upper AND gates will become disabled and the lower AND gates are enabled, allowing Q′A and Q′B to pass through the clock inputs of the following flip-flops. Hence, in this condition the counter will count in down mode, as the input pulses are applied.