BE Lab Manual Experiment 13
Experiment 13
Aim: - To study the principle of MOD - N counter.
Apparatus: - MOD - N counters kit, patch chords.
Theory: - A
circuit used for counting the pulses is known is as a counter. The number of
states in an N – stage ring counter is N, whereas it is 2N in the case of modulus
counter. These counters are referred to as module N (or divide by N) and module
2N (or divide by 2N) counters respectively, where module indicates the number
of states in the counter. When the pulses to be counted are applied to a
counter, it goes from state to state and the output of the flip flops in the
counter is decoded to read the count. The circuit comes back to its start in
state after countering N pulses in the case of module N counter.
The ring counter and the twisted-ring counter do not make efficient use of flip-flops. A flip–flop has two states. Therefore groups of N flip–flop will have a 2N state. This means it is possible to make a module 2N counter using N flip–flops. Basically there are two types of such counters.
- Asynchronous counter ( ripple counter )
- Synchronous counter
In
the case of an Asynchronous counter, all the flip–flops are not clocked
simultaneously, whereas in a Synchronous counter all the flip–flops are
clocked simultaneously. The ring and the twisted ring counters are examples of
synchronous counters.
MOD – N 16
Ripple Counter
For the ripple counter, the flip–flop is connected in series i.e. the output of the previous flip–flop is connected to the clock in out of the next flip–flop. For
n – bit ripple counter (maybe up or down) n flip–flop are required. The
clock input is given to the LSB flip–flop. Consider 4 – bit ripple counter.
Where
figure Q0 is connected to the clock input of FF1, the output of FF1 i.e. Q1
is connected to clock input of FF2 & so on. Note that clock pulse is
applied at LSB flip–flop. This 4–bit ripple counter counts from 0000 to
1111 i.e. It has 16 different output states. Hence this mod–16 (modules–16)
counter.
The
modules of the counter are the number of different output states through which the counter goes. The input J & K are connected to logic ‘1’ & hence JK
flip–flop is in ‘toggle’ mode. Each flip–flop toggles on the arrival of the negative-going edge of the clock pulse. By using the counting sequence for a
mod – 10 counter is from 0000 to 1001 (0 to 9 in decimal). This is down to the
heavy line in the figure. This mod – 10 counter, then has four place values 8s, 4s,
2s and 1s. This takes four flip–flops connected as a ripple counter in the figure.
We must add a NAND gate to the ripple counter to clear all the flip–flops
back to zero immediately after the 1001 (9) count. The trick is to look at the
figure and determine what the next count will be after 1001. You will find it is
1010 (decimal 10). You must feed the two 1sin 1010 into the NAND gate as shown
in the figure. The NAND gate then clears the flip–flop back to 0000. The counter
then starts its count from 0000 up to 1001 gain. We say we are using the NAND gate
to reset the counter to 0000. By using a NAND gate in this manner we can make
several other module counters.
Where
figure Illustrates a mod – 10 ripple counter. This type of counter might also
be called a decade (meaning 10) counter.
Counter Sequence
Table:-
Procedure:-
Study the circuit provided on the front panel of the kit.
Decide the type of counter to be constructed say MOD–10 & it will count from
This counter should produce 0000 after 1001. Hence used the NAND gate to clear flip–flop. We have to change Q3: 1–0 & Q0: 1–0 so connect Q3 & Q0 to NAND gate input and its output to CLR (clear) of flip–flops. Apply clock pulses & verify the truth table for the count up to 1001.
Repeat the above step for the different count sequence says MOD–9.
Observation table:-
Conclusion:-By resetting the flip–flop using the NAND gate, any type of MOD counter can be constructed & verified
its truth table.
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