FYBSC IT Digital Electronics Unit V Chapter I Counters

Counters:
Introduction, Asynchronous counter, Terms related to counters, IC-7493 (4-bit binary counter), Synchronous counter, Bushing, Type T-Design, Type JK Design, Presettable counter, IC-7490, IC 7492, Synchronous counter ICs, Analysis of counter circuits

1. COUNTERS
UNIT V
DIGITAL
ELECTRONICS
PROF. ARTI GAVAS-PARAB
ANNA LEELA COLLEGE OF COMMERCE AND ECONOMICS, SHOBHA
JAYARAM SHETTY COLLEGE FOR BMS
CHAPTER I

2. UNIT V: CONTENTS
Counters:
Introduction, Asynchronous counter, Terms related to counters,
IC 7493 (4-bit binary counter), Synchronous counter,
Bushing, Type T Design, Type JK Design,
Pre-settable counter, IC 7490, IC 7492, Synchronous counter ICs,
Analysis of counter circuits
Shift Register:
Introduction, parallel and shift registers, serial shifting, serial–in serial–out, serial–in parallel–out , parallel–in parallel–out,
Ring counter, Johnson counter,
Applications of shift registers,
Pseudo-random binary sequence generator, IC7495,
Seven Segment displays,
analysis of shift counters.
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3. COUNTERS: INTRODUCTION
Like shift registers and other combinational circuits, there is another important element in digital electronics
which we use most. They are counters.
Counters are used not only for counting but also for measuring frequency and time ; increment memory
addresses.
Counters are specially designed synchronous sequential circuits, in which , the state of the counter is equal to the
count held in the circuit by the flip flops.
Counters calculate or note down the number that how many times an event occurred.
Counters are the crucial hardware components, and are defined as “The digital circuit which is used to count the
number of pulses”.
Counters are well known to us as “Timers”.
Counter circuits are the best example for the flip flop applications.
Counters are designed by grouping of flip flops and applying a single clock signal to them. In simple words, the
counters are those, which have the group of storage elements like flip flops to hold the count.

4. COUNTERS: INTRODUCTION
Counters have mods. The ‘mod’ of the counter represents the number of states of the cycles through it, before
setting the counter to its initial state. For example, a binary mod 8 counter has 8 countable states. They are from
000 to 111. So the mod 8 counter counts from 0 to 7.
A binary mod 4 counter has 4 count states, from 000 to 011. So the mod 4 counter counts from 0 to 4. This
means, in general a mod N counter can contain n number of flip flops, where 2n = N.
Need of Counters
Counting means incrementing or decrementing the values of an operator, with respect to its previous state value. So to
perform the mathematical operation we use no devices other than counters. We cannot perform this action (counting) with
any other logic devices rather than counters.
Types of counters
There are two types of counters available for digital circuits, they are
Synchronous counters
Asynchronous counters

5. SYNCHRONOUS COUNTERS
The counters which use clock signal to change their transition are called “Synchronous counters”. This means the
synchronous counters depends on their clock input to change state values. All flip flops in the synchronous
counters are triggered by same clock signal.
Features:
Their construction is very simple in design. All the flip flops are interconnected and will be driven
by same clock signal.
The state output of the previous flip flop determines the state change of the present flip flop.
As all the flip flops will work synchronously, the synchronous counters don’t require settling.
We require number of logic gates to implement the synchronous counters.
Their operation is fast.

6. ASYNCHRONOUS COUNTERS
The counters in which the change in transition doesn’t depend upon the clock signal input is known as
“Asynchronous counters”. In these counters, the first flip flop is connected to the external clock signal, and the
rest are clocked by the state outputs (Q & Q’) of the previous flip flop.
Features:
Another name for Asynchronous counters is “Ripple counters”.
These are very simple in design.
As its design is simple, they use less number of logic gates to construct an asynchronous counter.
Operation of asynchronous counters is very slow compared to synchronous counters.

7. ASYNCHRONOUS VSSYNCHRONOUS COUNTERS
SYNCHRONOUS COUNTERS ASYNCHRONOUS COUNTERS
The propagation delay is very low. Propagation delay is higher than that of synchronous
counters.
Its operational frequency is very high. The maximum frequency of operation is very low.
These are faster than that of ripple counters.These are slow in operation.
Large number of logic gates are required to designLess number of logic gates required.
High cost. Low cost.
Synchronous circuits are easy to design. Complex to design.
Standard logic packages available for synchronous.For asynchronous counters, Standard logic packages are not
available.

8. APPLICATIONS OF COUNTERS
Counter found their applications in many digital electronic devices. Some of their applications are listed below.
Frequency counters
Digital clocks
Analog to digital convertors.
With some changes in their design, counters can be used as frequency divider circuits. The frequency divider
circuit is that which divides the input frequency exactly by ‘2’.
In time measurement. That means calculating time in timers such as electronic devices like ovens and washing
machines.
We can design digital triangular wave generator by using counters.
There are many other type of counters rather than synchronous and asynchronous counters, such as Decade
counter, Binary counter, Ring counter, Johnson counter, Up / Down counter etc. , which we will discuss about
them in our upcoming sessions.

9. ASYNCHRONOUS COUNTERS
The diagram of a 2-bit asynchronous counter is shown below.
The exterior clock is connected to the clock i/p of the FF0 (first flip-
flop) only. So, this FF changes the state at the decreasing edge of every
clock pulse, but FF1 changes only when activated by the decreasing
edge of the Q o/p of FF0.
Because of the integral propagation delay through a FF, the change of
the i/p clock pulse and a change of the Q o/p of FF0 can never occur at
precisely the same time.
So, the FF’s cannot be activated concurrently, generating an
asynchronous operation.
Generally, all the CLEAR i/psare connected together, so before
counting starts then that a single pulse can clear all the FFs. The clock
pulse fed into FF0 is rippled through the new counters after
propagation delays, such as a ripple on the water, hence the term Ripple
Counter.

10. SYNCHRONOUS COUNTERS
In thistype of counters, the CLK i/psof all the FFs are connected
together and are activated by the i/p pulses.
So, all the FFs change states instantaneously. The circuit diagram
below is a three bit synchronous counter.
The inputs J and K of flip-flop0 are connected to HIGH. Flip-flop 1
has its J &K i/psconnected to the o/p of flip-flop0 (FF0), and the
inputs J & K of flip-flop2 (FF2) are connected to the o/p of an AND
gate that is fed by the o/psof flip-flop0 and flip-flop1. When the both
the outputs of FF0 & FF1 are HIGH. The positive edge of the fourth
CLK pulse will cause FF2 to alter its state because of the AND gate.
The series of the three bit counter table is here.Themajor advantage
of these counters is that there is no increasing time delay due to all
FFs are activated in parallel. Thus, the max operating frequency of this
synchronous counter will be considerably higher than for the
equivalent ripple counter.

11. IC 7493 (4-BIT BINARY COUNTER)
The74LS93is a 4-bit binary counter made of two up-counters. The IC consists of a mode-2 up-counter and a mod-8 up
counter. Can be combined as mod-8 counter or divide by 2 or divide by 8 applications. It is built using four JK Flip Flops.
Where to use 74LS93?
The74LS93is a up-counter built using fourJK flip-flops. The CP1 and Q0 forms a mod-2 counter using a single flip-
flop and the CP0 with Q1, Q2 and Q3 with three flip-flops forms the mod-8 counter. The IC is commonly used by
combining mod-2 and mod-8 to form a mod-16 up-counter. The IC is commonly used in counting applications or in divide
by 2, divide by 8 or divide by 16 designs.
PinNumber PinName Description
1,2,3,6 NC NoConnection
4,5,8,9 Q0,Q1,Q2,Q3Outputpins
7 Ground Connectedtogroundofthesystem
10 CP0 ClockInput–divideby2
11 CP1 ClockInput–divideby8
12,13 MR MasterReset–ClearInput
14 Vcc Supplyvoltage–4.5Vto5.5V

12. WORKING BY SIMULATING THE IC 7493
The complete working can be
understood by simulating the IC.
Here mode-0 (counting mode) is
selected by grounding both MR pins
and for the clock pulse we need to
manually toggle a logic state to
provide pulse.
Every time we make the pin high
and low a single pulse is generated.
Applications
Used for creating long timing
period
Astablefrequency divider or
counter circuit
Timing related applications
Used in project where
Microcontrollers should be avoided
Pulse counter or frequency divider

13. BUSHING AND TYPE T DESIGN
In electric power, a bushing is a hollow electrical insulator that
allows an electrical conductor to pass safely through a
conducting barrier such as the case of a transformer or circuit
breaker without making electrical contact with it. Bushings are
typically made from porcelain; though other insulating materials
are also used.
The Type T condenser bushing is available as both a high
temperature design and as a standard design. The high
temperature design is thermally enhanced to operate properly
when the bushing is applied inside of the high temperature
environment of non-ventilated bus duct. The Type T bushing
meets the requirements of the appropriate IEEE standards. Type
T bushings are available for both cover and side-wall mounting in
current ratings from 400 to 21,500 amperes, and voltage ratings
from 25 to 46 kV.
Bushings of
Burgtec's
Offset Shock
Hardware
M16 Transistor
Plastic Washer
Insulation Bush
Nylon Step T
Type Bushing
Ring

14. PRESETTABLECOUNTER
Counter that can start counting from any preset number, using a counter IC with a preset function. The output of
this counter is displayed by four LEDs.
Figure shows the decimal number for each of the outputs displayed in binary numbers.

15. PRESETTABLECOUNTER
Wire the project and turn power ON.
At this time, the counter is preset to 0 and all LEDs light
off; when you press S1 to send out the pulses, it starts
counting from 0 to 1, 2, 3 and so on. Let it go on counting.
(When it is set to 0, all LEDs OFF).
Now try presetting the counter. You use S5 -S8 to input
the presetting the number. Use binary --see table and
press the key(s) that correspond to 0 in the table.
For example, to input 6 as the preset number, press and
hold S5 and S8 and then press S3. The LED display changes
6, as shown in example 3. With the number 6 is preset this
way, press S1 to send out the pulses. The counter starts
counting from 7 to 8, 9, 10 and so on.

16. IC 7490, IC 7492: DECADE COUNTER
There are some available ICs for decade counters which we can readily use in our
circuit, like 74LS90. It is an asynchronous decade counter.
Decade counter constructed with JK flip flop. The J output and K outputs are
connected to logic 1. The clock input of every flip flop is connected to the output of
next flip flop, except the last one.
The output of the NAND gate is connected in parallel to the clear input ‘CLR’ to all
the flip flops. This ripple counter can count up to 16 i.e. 24.
Decade Counter Operation
When the Decade counter is at REST, the count is equal to 0000. This is first stage of
the counter cycle. When we connect a clock signal input to the counter circuit, then
the circuit will count the binary sequence. The first clock pulse can make the circuit to
count up to 9 (1001). The next clock pulse advances to count 10 (1010).
Then the ports X1 and X3 will be high. As we know that for high inputs, the NAND
gate output will be low. The NAND gate output is connected to clear input, so it
resets all the flip flop stages in decade counter. This means the pulse after count 9 will
again start the count from count 0.

17. STATE DIAGRAM OF DECADE COUNTER
A 4 bit binary counter will act as decade counter by skipping any six outputs out of the 16 (24) outputs.
Observe the decade counter circuit diagram, there are four stages in it, in whicheach stage has single flip flop in
it. So it is capable of counting 16 bits or 16 potential states, in which only 10 are used. The count starts from 0000
(zero) to 1001 (9) and then the NAND gate will reset the circuit.
Multiple counters are connected in series, to count up to any desired number. The number that a counter circuit
can count is called “Mod” or “Modulus”. If a counter resets itself after counting n bits is called “Mod-n counter”
“Modulo-n counter”, where n is an integer.
The Mod n counter can calculate from 0 to 2n-1. There are several types of counters available, like Mod 4 counter,
Mod 8 counter, Mod 16 counter and Mod 5 counters etc.

18. 74LS90 DECADE COUNTER IC DESCRIPTION
It is a simple counter which can count from 0 –9. As it is
a 4 bit binary decade counter, it has 4 output ports QA,
QB, QC and QD. When the count reaches 10, the binary
output is reset to 0 (0000), every time and another pulse
starts at pin number 9. The Mod of the IC 7490 is set by
changing the RESET pins R1, R2, R3, R4.
If any one of R1 & R2 is at high or R3 & R4 are at
ground, the counter will reset all the outputs QA, QB,
QC and QD to 0. If the pins R3 & R4 are high, then the
count on QA, QB, QC and QD is 1001.
Pin configuration

19. ANALYSIS OF COUNTER CIRCUITS
COMPARISON BETWEEN SYNCHRONOUS AND ASYNCHRONOUS COUNTERS
Asynchronouscounters, also known as ripple counters, are not clocked by a common pulse and hence every flip-flop in the counter
changes at different times. The flip-flops in an asynchronous counter is usually clocked by the output pulse of the preceding flip-
flop. The first flip-flop is clocked by an external event.
A synchronous counter however, has an internal clock, and the external event is used to produce a pulse which is synchronized
with this internal clock.
Ripple counter requires less circuitry than a synchronous counter hence asynchronous counter is easier to construct.
Asynchronouscounter is slow.
Firstly, In a synchronous counter, all the flip-flops will change states simultaneously while for an asynchronous counter, the propagation
delays of the flip-flops add together to produce the overall delay. Hence, the more bits or number of flipflopsin an asynchronous counter,
the slower it will be.
Secondly,there are certain "risks" when using an asynchronous counter. In a complex system, many state changes occur on each clock
edge and some ICs respond faster than others. If an external event is allowed to affect a system whenever it occurs (unsynchronized),
there is a small chance that it will occur near a clock transition, after some IC's have responded, but before others have. This
intermingling of transitions often causes erroneous operations. And the worse this is that these problems are difficult to foresee and test
for because of the random time difference between the events.

20. THANK YOU!