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7 Asynchronous Counter: Realization of 4-bit counter
Aim:To design and setup asynchronous Up Counter/Down
Counter.
Components required:Digital IC trainer kit,IC 7473 dual JK
master slave flipflop with reset, IC 7400 quad 2 input NAND,IC7486
Quad 2 input EXOR.
Toggle Flip-Flop:
Another type of digital device that can be used for frequency
division is the T-type or Toggleflip-flop. With a slight
modification to a standard JK flip-flop, we can construct a new
type of flip-flopcalled a Toggle flip-flop. Toggle flip flops can
be made from D-type flip-flops as shown above, or fromstandard JK
flip-flops such as the 74LS73. The result is a device with only two
inputs, the Toggle inputitself and the negative controlling Clock
input as shown.
A Toggle flip-flop gets its name from the fact that the
flip-flop has the ability to toggle or switchbetween its two
different states, the toggle state and the memory state. Since
there are only two states,a T-type flip-flop is ideal for use in
frequency division and binary counter design.
Binary ripple counters can be built using Toggle or T-type
flip-flops by connecting the output of oneto the clock input of the
next. Toggle flip-flops are ideal for building ripple counters as
it toggles fromone state to the next, (HIGH to LOW or LOW to HIGH)
at every clock cycle so simple frequencydivider and ripple counter
circuits can easily be constructed using standard T-type flip-flop
circuits.Ifwe connect together in series, two T-type flip-flops the
initial input frequency will be divided-by-two bythe first
flip-flop ( f 2 ) and then divided-by-two again by the second
flip-flop ( f 2 ) 2, giving anoutput frequency which has
effectively been divided four times, then its output frequency
becomes onequarter value (25%) of the original clock frequency, ( f
4 ).Each time we add another toggle or T-typeflip-flop to the
chain, the output clock frequency is halved or divided-by-2 again
and so on, giving anoutput frequency of 2n where n is the number of
flip-flops used in the sequence.
Then the Toggle or T-type flip-flop is an edge triggered
divide-by-2 device based upon the standardJK-type flip flop and
which is triggered on the rising edge of the clock signal. The
result is that each bitmoves right by one flip-flop. All the
flip-flops can be asynchronously reset and can be triggered
toswitch on either the leading or trailing edge of the input clock
signal making it ideal for FrequencyDivision.This type of counter
circuit used for frequency division is commonly known as
anAsynchronous 3-bit Binary Counter as the output on QA to QC,
which is 3 bits wide, is a binary countfrom 0 to 7 for each clock
pulse. In an asynchronous counter, the clock is applied only to the
first stagewith the output of one flip-flop stage providing the
clocking signal for the next flip-flop stage andsubsequent stages
derive the clock from the previous stage with the clock pulse being
halved by eachstage.
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Fig. 7.1: A two-bit asynchronouscounter is shown on the right.
Theexternal clock is connected to theclock input of the first
flip-flop (FF0)only. So, FF0 changes state atthe falling edge of
each clock pulse,but FF1 changes only when triggeredby the falling
edge of the Q outputof FF0. Because of the inherentpropagation
delay through a flip-flop,the transition of the input clock
pulseand a transition of the Q output of FF0can never occur at
exactly the sametime. Therefore, the flip-flops cannotbe triggered
simultaneously, producingan asynchronous operation.
Fig. 7.2: Frequency division using 4bit asynchronous counter
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This arrangement is commonly known as Asynchronous as each
clocking event occurs independentlyas all the bits in the counter
do not all change at the same time. As the counter counts
sequentially inan upwards direction from 0 to 7. This type of
counter is also known as an up or forward counter(CTU) or a 3-bit
Asynchronous Up Counter. The three-bit asynchronous counter shown
is typical anduses flip-flops in the toggle mode. Asynchronous Down
counters (CTD) are also available.
Binary Counters:
Then we can see that a counter is nothing more than a
specialized register or pattern generator thatproduces a specified
output pattern or sequence of binary values (or states) upon the
application of aninput pulse signal called the Clock.The clock is
actually used for data transfer in these applications.Typically,
counters are logic circuits that can increment or decrement a count
by one but when used asasynchronous divide-by-n counters they are
able to divide these input pulses producing a clock
divisionsignal.
Counters are formed by connecting flip-flops together and any
number of flip-flops can be connectedor cascaded together to form a
divide-by-n binary counter where n is the number of counter stages
usedand which is called the Modulus. The modulus or simply MOD of a
counter is the number of outputstates the counter goes through
before returning itself back to zero, ie, one complete cycle.Then
acounter with three flip-flops like the circuit above will count
from 0 to 7 ie, 23-1. It has eight differentoutput states
representing the decimal numbers 0 to 7 and is called a Modulo-8 or
MOD-8 counter. Acounter with four flip-flops will count from 0 to
15 and is therefore called a Modulo-16counter and soon.
An example of this is given as.
3-bit Binary Counter = 23 = 8 (modulo-8 or MOD-8)
4-bit Binary Counter = 24 = 16 (modulo-16 or MOD-16)
8-bit Binary Counter = 28 = 256 (modulo-256 or MOD-256)
The Modulo number can be increased by adding more flip-flops to
the counter and cascading is amethod of achieving higher modulus
counters. Then the modulo or MOD number can simply be writtenas:
MOD number = 2n.
4-bit Modulo-16 Counter.
Multi-bit asynchronous counters connected in this manner are
also called Ripple Counters or rippledividers because the change of
state at each stage appears to ripple itself through the counter
from theLSB output to its MSB output connection. Ripple counters
are available in standard IC form, from the74LS393 Dual 4-bit
counter to the 74HC4060, which is a 14-bit ripple counter with its
own built inclock oscillator and produce excellent frequency
division of the fundamental frequency.
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Fig. 7.3:
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7.1 4bit asynchronous up counter.
Fig. 7.4: Consider a 3 bit up countsequence after 7 the counter
is reset to0.D represents the clock pulse. Notethat A(LSB) Toggles
for every clockpulse.SO we can keep it in toggle mode.Now B toggles
whenever A changes from1 to 0.Since we have -ve edge
triggeredflipfops we can connect the output offirst flipflop(A) to
clock input of nextflipflop(B).
Procedure:
Assemble the circuit neatly on the breadboard.
Connect the switch SW1 to ground (logic 0). Since SW1 is
connected to R of all flip-flops all theflip-flop outputs gets
reset(Q=0).
Now to resume normal operation we need to make R high. So flip
the switch to high (logic1).
Now apply the clock pulse and verify the output patterns.
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Fig. 7.5:
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PWRRAILS.DAT
ROOT.CDB
ROOT.DSN
SCRIPTS/PWRRAILS.DAT
*RAILS*BINDINGS
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7.2 4bit asynchronous down counter.
Fig. 7.6: Consider a 3 bit up countsequence after 7 the counter
is reset to0.D represents the clock pulse. Notethat A(LSB) Toggles
for every clockpulse.SO we can keep it in toggle mode.Now B toggles
whenever A changes from1 to 0.Since we have -ve edge
triggeredflipfops we can connect the output offirst flipflop(A) to
clock input of nextflipflop(B).
Procedure:
Assemble the circuit neatly on the breadboard.
Connect the switch SW1 to ground (logic 0). Since SW1 is
connected to R of all flip-flops all theflip-flop outputs gets
reset(Q=0).
Now to resume normal operation we need to make R high. So flip
the switch to high (logic1).
Now apply the clock pulse and verify the output patterns.
Result:
Designed and setup asynchronous up/down counter and obtained the
result.
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A Answers to Additional Questions:
A.1 Asynchronous counters:
1. Advantages of Asynchronous Counters:
Asynchronous Counters can easily be made from Toggle or D-type
flip-flops. They are called Asynchronous Counters because the clock
input of the flip-flops are not all
driven by the same clock signal.
Each output in the chain depends on a change in state from the
previous flip-flops output. Asynchronous counters are sometimes
called ripple counters because the data appears to ripple
from the output of one flip-flop to the input of the next.
They can be implemented using divide-by-n counter circuits.
Truncated counters can produce any modulus number count.
Disadvantages of Asynchronous Counters:
An extra re-synchronizing output flip-flop may be required. To
count a truncated sequence not equal to 2n, extra feedback logic is
required. Counting a large number of bits, propagation delay by
successive stages may become
undesirably large.
This delay gives them the nickname of Propagation Counters.
Counting errors occur at high clocking frequencies. Synchronous
Counters are faster and more reliable as they use the same clock
signal for all
flip-flops.
Counting errors occur at high clocking frequencies.
Following diagram shows how errors can occur at high
frequencies. We have already seen how anasynchronous counter worked
perfect at low frequencies. But as frequencies approached MHz
(thetime period of clock signals approached that of propagation
delay of flipflops the counter becomesunusable). Shows below the
case of 10MHz and as it reaches 20MHz the counter pretty
muchbecomes unusable.
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Fig. A.1: Asynchronous counter with delay:
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2. Design a MOD-10 asynchronous counter using D flipflops.
We can use a D Flip flop instead of JK flipflop as long as each
flip flop works in toggle mode. Nowto connect a D flip flop in
toggle mode connect the Q input to D input of flip flop. This
willconvert D flipflop to toggle mode. Now proceed as in a mod-10
decade counter using JK flipflop.
Fig. A.2: Asynchronous counter using D flip flops:
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B LTspice Simulations.
B.1 Asynchronous Counter: Realization of 4-bit up and down
counter:
Fig. B.1: 4bit asynchronous up counter.
Fig. B.2: Timing diagram for 4bit asynchronous up counter:
LTspice Simulation File:
Logic Circuit Design Lab www.electronicsforktu.com Page 11
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4bit up counter illustrating the delay problem.As the frequency
approaches very high around MHzrange.
Fig. B.3: 4bit asynchronous up counter.
Fig. B.4: Timing diagram for 4bit asynchronous up counter:
LTspice Simulation File:
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voltage -464 32 R0SYMATTR InstName V2SYMATTR Value 5VSYMBOL res
-288 -128 R90WINDOW 0 0 56 VBottom 2WINDOW 3 32 56 VTop 2SYMATTR
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Fig. B.5: 4bit asynchronous down counter.
Fig. B.6: Timing diagram for 4bit asynchronous down counter:
LTspice Simulation File:
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C Everycircuit Simulations.
C.1 4bit Asynchronous UP/DOWN Counter:
(a) 4bit asynchronous Up Counter (b) 4bit asynchronous Down
Counter
Fig. C.1: 4bit asynchronous Up/Down Counter
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http://everycircuit.com/circuit/4892083472564224http://everycircuit.com/circuit/4892083472564224
Asynchronous Counter: Realization of 4-bit counter4bit
asynchronous up counter.4bit asynchronous down counter.
Appendix Answers to Additional Questions:Asynchronous
counters:
Appendix LTspice Simulations.Asynchronous Counter: Realization
of 4-bit up and down counter:
Appendix Everycircuit Simulations.4bit Asynchronous UP/DOWN
Counter: