Digital Counters
Counter is a sequential circuit. A digital circuit which is used for a counting pulses is known counter. Counter is the widest application of flip-flops. It is a group of flip-flops with a clock signal applied. Counters are of two types.
- Asynchronous or ripple counters.
- Synchronous counters.
Asynchronous or ripple counters
The logic diagram of a 2-bit ripple up counter is shown in figure. The toggle (T) flip-flop are being used. But we can use the JK flip-flop also with J and K connected permanently to logic 1. External clock is applied to the clock input of flip-flop A and QA output is applied to the clock input of the next flip-flop i.e. FF-B.
Logical Diagram
Operation
| S.N. |
Condition |
Operation |
| 1 |
Initially let both the FFs be in the reset state |
QBQA = 00 initially |
| 2 |
After 1st negative clock edge |
As soon as the first negative clock edge is applied, FF-A will toggle and QA will be equal to 1.
QA is connected to clock input of FF-B. Since QA has changed from 0 to 1, it is treated as the positive clock edge by FF-B. There is no change in QB because FF-B is a negative edge triggered FF.
QBQA = 01 after the first clock pulse. |
| 3 |
After 2nd negative clock edge |
On the arrival of second negative clock edge, FF-A toggles again and QA = 0.
The change in QA acts as a negative clock edge for FF-B. So it will also toggle, and QB will be 1.
QBQA = 10 after the second clock pulse. |
| 4 |
After 3rd negative clock edge |
On the arrival of 3rd negative clock edge, FF-A toggles again and QA become 1 from 0.
Since this is a positive going change, FF-B does not respond to it and remains inactive. So QB does not change and continues to be equal to 1.
QBQA = 11 after the third clock pulse. |
| 5 |
After 4th negative clock edge |
On the arrival of 4th negative clock edge, FF-A toggles again and QA becomes 1 from 0.
This negative change in QA acts as clock pulse for FF-B. Hence it toggles to change QB from 1 to 0.
QBQA = 00 after the fourth clock pulse. |
Truth Table
Synchronous counters
If the "clock" pulses are applied to all the flip-flops in a counter simultaneously, then such a counter is called as synchronous counter.
2-bit Synchronous up counter
The JA and KA inputs of FF-A are tied to logic 1. So FF-A will work as a toggle flip-flop. The JB and KB inputs are connected to QA.
Logical Diagram
Operation
| S.N. |
Condition |
Operation |
| 1 |
Initially let both the FFs be in the reset state |
QBQA = 00 initially. |
| 2 |
After 1st negative clock edge |
As soon as the first negative clock edge is applied, FF-A will toggle and QA will change from 0 to 1.
But at the instant of application of negative clock edge, QA , JB = KB = 0. Hence FF-B will not change its state. So QB will remain 0.
QBQA = 01 after the first clock pulse. |
| 3 |
After 2nd negative clock edge |
On the arrival of second negative clock edge, FF-A toggles again and QA changes from 1 to 0.
But at this instant QA was 1. So JB = KB= 1 and FF-B will toggle. Hence QB changes from 0 to 1.
QBQA = 10 after the second clock pulse. |
| 4 |
After 3rd negative clock edge |
On application of the third falling clock edge, FF-A will toggle from 0 to 1 but there is no change of state for FF-B.
QBQA = 11 after the third clock pulse. |
| 5 |
After 4th negative clock edge |
On application of the next clock pulse, QA will change from 1 to 0 as QB will also change from 1 to 0.
QBQA = 00 after the fourth clock pulse. |
Classification of counters
Depending on the way in which the counting progresses, the synchronous or asynchronous counters are classified as follows −
- Up counters
- Down counters
- Up/Down counters
UP/DOWN Counter
Up counter and down counter is combined together to obtain an UP/DOWN counter. A mode control (M) input is also provided to select either up or down mode. A combinational circuit is required to be designed and used between each pair of flip-flop in order to achieve the up/down operation.
- Type of up/down counters
- UP/DOWN ripple counters
- UP/DOWN synchronous counter
UP/DOWN Ripple Counters
In the UP/DOWN ripple counter all the FFs operate in the toggle mode. So either T flip-flops or JK flip-flops are to be used. The LSB flip-flop receives clock directly. But the clock to every other FF is obtained from (Q = Q bar) output of the previous FF.
UP counting mode (M=0) − The Q output of the preceding FF is connected to the clock of the next stage if up counting is to be achieved. For this mode, the mode select input M is at logic 0 (M=0).
DOWN counting mode (M=1) − If M = 1, then the Q bar output of the preceding FF is connected to the next FF. This will operate the counter in the counting mode.
Example
3-bit binary up/down ripple counter.
3-bit − hence three FFs are required.
UP/DOWN − So a mode control input is essential.
For a ripple up counter, the Q output of preceding FF is connected to the clock input of the next one.
For a ripple up counter, the Q output of preceding FF is connected to the clock input of the next one.
For a ripple down counter, the Q bar output of preceding FF is connected to the clock input of the next one.
Let the selection of Q and Q bar output of the preceding FF be controlled by the mode control input M such that, If M = 0, UP counting. So connect Q to CLK. If M = 1, DOWN counting. So connect Q bar to CLK.
Block Diagram
Truth Table
Operation
| S.N. |
Condition |
Operation |
| 1 |
Case 1 − With M = 0 (Up counting mode) |
If M = 0 and M bar = 1, then the AND gates 1 and 3 in fig. will be enabled whereas the AND gates 2 and 4 will be disabled.
Hence QA gets connected to the clock input of FF-B and QB gets connected to the clock input of FF-C.
These connections are same as those for the normal up counter. Thus with M = 0 the circuit work as an up counter.
|
| 2 |
Case 2: With M = 1 (Down counting mode) |
If M = 1, then AND gates 2 and 4 in fig. are enabled whereas the AND gates 1 and 3 are disabled.
Hence QA bar gets connected to the clock input of FF-B and QB bar gets connected to the clock input of FF-C.
These connections will produce a down counter. Thus with M = 1 the circuit works as a down counter.
|
Modulus Counter (MOD-N Counter)
The 2-bit ripple counter is called as MOD-4 counter and 3-bit ripple counter is called as MOD-8 counter. So in general, an n-bit ripple counter is called as modulo-N counter. Where, MOD number = 2n.
Type of modulus
- 2-bit up or down (MOD-4)
- 3-bit up or down (MOD-8)
- 4-bit up or down (MOD-16)
Application of counters
- Frequency counters
- Digital clock
- Time measurement
- A to D converter
- Frequency divider circuits
- Digital triangular wave generator.
