Bidirectional counters are capable of counting up or down through any given counting sequence.
In addition to counting "up" from zero and increasing or increasing to a preset value, it can sometimes count "down" from a predetermined value to zero, which allows us to produce an output that is activated when there is a zero count or another preset value.
This type of counter is normally called the Down Counter (CTD). In a binary or BCD down counter, counting is reduced by one from a preset value for each external clock pulse. Custom dual-purpose IC's, such as TTL 74LS193 or CMOS CD4510, are 4-bit binary up or down counters with an additional input pin to select up or down counting mode.
In the 4-bit counter above the output of each flip-flop, the situation changes on the falling edge of the CLK input (switching from 1 to 0) triggered by the Q output of the previous flip-flop. As a result, each flip-flop will change the situation when it changes from 0 to 1 instead of changing from 1 to 0 in the output of the previous one.
Both synchronous and asynchronous counters are capable of counting "Up" or counting "Down", but depending on the state of the input control pins, up or down are another type of "Universal" counter that can count in both directions, and they are known as Duplex counters.
Bidirectional counters, also known as up/down counters, are capable of counting in both directions throughout any given counting sequence and can be reversed at any point in the counting sequences using an additional control input, as shown below.
The circuit above is a simple 3-bit Up/Down sync counter that uses JK flip-flops configured to work as transition or T-type flip-flop that provides maximum zero (000) to seven (111) and zero countdown. The 3-Bit counter then proceeds upwards (0.1,2,3,4,5,6,7) or in reverse order (7,6,5,4,3,2,1,0).
Typically, most bidirectional counter integrations can be configured to change the counting direction up or down at any point in the counting sequences. This is achieved by using an additional input pin that determines the direction of the Up or Down count, and the timing diagram gives an example of counters running when this Up/Down entry changes the status.
Today, both up and down counters are included in the only fully programmable integration to count both "Up" and "Down" from any preset value that produces a complete Duplex Counter integration. The available common integrations are the 74HC190 4-bit BCD tenteen Up/Down counter, the 74F569 is a fully synchronous Up/Down binary counter, and the CMOS 4029 4-bit Sync Up/Down counter.