Ring Counters

Ring Counters

In the previous Shift Register tutorial we saw that if we apply a serial data signal to the input of a Serial-in to Serial-out Shift Register, the same sequence of data will exit from the last flip-flip in the register chain after a preset number of clock cycles thereby acting as a time delay to the original signal. But what if we were to connect the output of the Shift Register back to its input, we then have a closed loop circuit that "Recirculates" the DATA around a loop, and this is the principal operation of Ring Counters or Walking Ring Counter. Consider the circuit below.

4-bit Ring Counter

The synchronous Ring Counter example above, will re-circulate the same DATA pattern between the 4 Flip-flops over and over again every 4th clock cycle, as long as the clock pulses are applied to it. But in order to cycle the DATA we must first "Load" the counter with a suitable DATA pattern for it to work correctly as all logic "0"'s or all logic "1"'s outputted at each clock cycle would make the ring counter invalid.

For Ring Counters to operate correctly they must start with the first flip-flop (FFA) in the logic "1" state and all the others at logic "0". To achieve this, a "CLEAR" signal is firstly applied to all the Flip-flops in order to "RESET" their outputs to a logic "0" level and then a "PRESET" pulse is applied to the input of the first Flip-flop (FFA) before the clock pulses are applied. This then places a single logic "1" value into the circuit of the Ring Counters.

The ring counter example shown above is also known as a "MODULO-4" or "MOD-4" counter since it has 4 distinct stages and each Flip-flop output has a frequency equal to one-fourth or a quarter (1/4) that of the main clock frequency. The "MODULO" or "MODULUS" of a counter is the number of states the counter counts or sequences through before repeating itself and a ring counter can be made to output any MODULO number and a "MOD-N" Ring Counter will require "N" number of Flip-flops connected together. For example, a MOD-8 Ring Counter requires 8 Flip-flops and a MOD-16 Ring Counter would require 16 Flip-flops.

Johnson Ring Counters

Johnson Ring Counters or "Twisted Ring Counters", are exactly the same idea as the Walking Ring Counter above, except that the inverted output Q of the last Flip-flop is connected back to the input D of the first Flip-flop as shown below. The main advantage of this type of ring counter is that it only needs half the number of Flip-flops compared to the standard walking ring counter then its Modulo number is halved.

4-bit Johnson Ring Counter

This inversion of Q before it is fed back to input D causes the counter to "count" in a different way. Instead of counting through a fixed set of patterns like the walking ring counter such as for a 4-bit counter, "1000"(1), "0100"(2), "0010"(4), "0001"(8) etc, the Johnson counter counts up and then down as the initial logic "1" passes through it to the right replacing the preceding logic "0". A 4-bit Johnson ring counter passes blocks of four logic "0" and then four logic "1" thereby producing an 8-bit pattern. As the inverted output Q is connected to the input D this 8-bit pattern continually repeats. For example, "1000", "1100", "1110", "1111", "0111", "0011", "0001", "0000" and this is demonstrated in the table below.

As well as counting, Ring Counters can be used to detect or recognise various patterns or number values. By connecting simple logic gates such as AND or OR gates to the outputs of the Flip-flops the circuit can be made to detect a set number or value. Standard 2, 3 or 4-stage Johnson Ring Counters can also be used to divide the frequency of the clock signal by varying their feedback connections and divide-by-3 or divide-by-5 outputs are also available.

A 3-stage Johnson Ring Counter can also be used as a 3-phase, 120 degree phase shift square wave generator by connecting to the outputs from A, B and NOT-B. The standard 5-stage Johnson counter such as the commonly available CD4017 is generally used as a Synchronous Decade Counter/Divider circuit. The smaller 2-stage circuit is also called a "Quadrature" (sine/cosine) Oscillator/Generator and is used to produce 4 outputs that are each "phase shifted" by 90 degrees with respect to each other, and this is shown below.

2-bit Quadrature Generator

As the 4 outputs, A to D are phase shifted by 90 degrees with regards to each other, they can be used with additional circuitry, to drive a 2-phase Full-Step Stepper Motor as shown below.

Stepper Motor Control

2-phase (unipolar) Full-Step Stepper Motor Circuit

The speed of rotation of the Stepper Motor will depend mainly upon the Clock Frequency and additional circuitry would be require to drive the "power" requirements of the motor. As this section is only intended to give the reader a basic understanding of Johnson Ring Counters other good websites explain in detail the types and drive requirements of stepper motors.

Johnson Ring Counters are available in standard TTL or CMOS IC form, such as the CD4017 5-Stage, Decade Johnson Ring Counters with 10 active HIGH decoded outputs or the CD4022 4-stage, Divide-by-8 Johnson counter with 8 active HIGH decoded outputs.