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bidirectional counter

Bidirectional Counters

As well as counting “up” from zero and increasing or incrementing to some preset value, it is sometimes necessary to count “down” from a predetermined value to zero allowing us to produce an output that activates when the zero count or some other pre-set value is reached.

This type of counter is normally referred to as a Down Counter, (CTD). In a binary or BCD down counter, the count decreases by one for each external clock pulse from some preset value. Special dual purpose IC’s such as the TTL 74LS193 or CMOS CD4510 are 4-bit binary Up or Down counters which have an additional input pin to select either the up or down count mode.

4-bit Count Down Counter

count down bidirectional counter

 

In the 4-bit counter above the output of each flip-flop changes state on the falling edge (1-to-0 transition) of the CLK input which is triggered by the Q output of the previous flip-flop, rather than by the Q output as in the up counter configuration. As a result, each flip-flop will change state when the previous one changes from 0 to 1 at its output, instead of changing from 1 to 0.

Bidirectional Counter

Both Synchronous and Asynchronous counters are capable of counting “Up” or counting “Down”, but their is another more “Universal” type of counter that can count in both directions either Up or Down depending on the state of their input control pin and these are known as Bidirectional Counters.

Bidirectional counters, also known as Up/Down counters, are capable of counting in either direction through any given count sequence and they can be reversed at any point within their count sequence by using an additional control input as shown below.

Synchronous 3-bit Up/Down Counter

bidirectional up down counter

 

The circuit above is of a simple 3-bit Up/Down synchronous counter using JK flip-flops configured to operate as toggle or T-type flip-flops giving a maximum count of zero (000) to seven (111) and back to zero again. Then the 3-Bit counter advances upward in sequence (0,1,2,3,4,5,6,7) or downwards in reverse sequence (7,6,5,4,3,2,1,0).

Generally most bidirectional counter chips can be made to change their count direction either up or down at any point within their counting sequence. This is achieved by using an additional input pin which determines the direction of the count, either Up or Down and the timing diagram gives an example of the counters operation as this Up/Down input changes state.

Nowadays, both up and down counters are incorporated into single IC that is fully programmable to count in both an “Up” and a “Down” direction from any preset value producing a complete Bidirectional Counter chip. Common chips available are the 74HC190 4-bit BCD decade Up/Down counter, the 74F569 is a fully synchronous Up/Down binary counter and the CMOS 4029 4-bit Synchronous Up/Down counter.

20 Comments

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  • M
    Munnasingh

    What is programmable up/down counter

  • N
    Navjyot

    Use a refreanvce books???

  • O
    Obii

    I need hep on how to implement a hex up counter using d flip flop

  • P
    Pavan

    Can Anybody explain me in the timing diagram when the UP/Down signal is made Logic low for the second time, why is it still performing up count? After 001 it is incrementing to 010, 011 so on. But it should start decrementing when the UP/Down signal is asserted low.

  • B
    Boyan

    Do you know how to modify the schematic to make it 22 to 0

  • k
    karthika

    I need full information about 4 bit up down counter circuit diagram, truth table, explanation

  • S
    Simon Sadler

    Simon in Spain here.Anyone have a very simple up down counter for 7 of 5mm leds .poss with zero reset.N/O switches up down and reset.Thanks.

  • d
    deutsch1329

    I don’t really understand how the an up-down counter could be made using a fourbit sequence of D-flip flops. I’ve figured the switch for ‘clear’ or ‘reset’, however a toggle switch connected from the clock output isn’t doing anything. Does anyone know how I would created this up-down switch? I’ve tried using 04s (NOT gates). But they don’t seem to work either.

  • o
    olive

    Couldn’t you use T-Flip-Flops?

  • Unknown

    Well to be honest the explanation is very good and it is easy for slow learners as well.
    I would like to appreciate this work and encourage to do more such good work to hepl people in studying kepp this good work up!

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