2. Chapter 14 - IC Counters
Lesson Objectives
Upon completion of this topic, you should be able to:
Students should be able to design simple
synchronous/asynchronous counters using MSI chips.
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3. Chapter 14 - IC Counters
Specific Objectives
Students should be able to :
State some common MSI synchronous/asynchronous
counter chips.
Write down the control pins to select various functions
on an MSI chip.
State the functions of the various control pins.
Design simple synchronous up/down counters based
on given MSI technical reference.
IT2001PA Engineering Essentials (2/2)
4. Chapter 14 - IC Counters
IC Asynchronous Counters
An asynchronous binary 4-bit counter IC 74293:
74293
74193 CP1
Binary 4-bit
Counter CP0
MR1 MR2 Q3 Q2 Q1 Q0
(MSB) (LSB)
Simplified symbol for 74293
Logic diagram for 74293 asynchronous counter IC
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5. Chapter 14 - IC Counters
Description of 74293
It has four J-K FFs with outputs Q0, Q1, Q2, Q3 where Q0 is the LBS
and Q3 is the MSB.
Each FF has a CP (clock pulse) input which is activated by
negative-edge transition (NGT).
Each FF has an asynchronous CLEAR input, C0.
These are connected together to the output of a two-input NAND
gate with inputs MR1 and MR2, where MR means master reset.
Both MR inputs must be HIGH to clear the counter to 0000.
FFs Q1, Q2 and Q3 are already connected as a 3-bit ripple counter.
FF Q0 is not connected to anything internally.
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6. Chapter 14 - IC Counters
Example
Show how the 74293 should be connected to operate
as a MOD-16 counter with a 10 kHz clock input.
A MOD-16 counter requires four FFs. The output Q0 is
connected to CP1 and the clock input, 10kHz pulses is applied
to CP0. The output is taken at Q3.
74293 wired as a
MOD-16 counter
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7. Chapter 14 - IC Counters
Example
Show how to wire the 74293(s) as a
a) MOD-10 counter
b) MOD-14 counter
c) MOD-60 counter
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8. Chapter 14 - IC Counters
Example – 74293 as MOD-10 Counter
A MOD-10 counter requires four FFs.
Connect Q0 to CP1.
We want the counter to recycle back to 0000 when it tries to go to
the count of 1010 (ten).
Thus, Q3 and Q1 outputs have to be connected to the master reset
inputs; when they both go HIGH at the count of 1010, the NAND
output will immediately reset the counter to 0000.
74293 wired as a
MOD-10 counter
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9. Chapter 14 - IC Counters
Example – 74293 as MOD-14 Counter
When the counter reaches the count of 1110 (14), the Q3, Q2, and Q1
outputs are all HIGH.
Unfortunately, the 74293’s built-in reset NAND gate has only two
inputs. Thus, some extra logic is added to ensure that the counter
will reset back to 0000 when Q3 = Q2 = Q1 = 1.
An external AND gate is
needed to wire the 74293
as a MOD-14 counter
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10. Chapter 14 - IC Counters
Example – 74293 as MOD-60 Counter
The circuit divides the input frequency by 60 in 2 steps. The 74293
counter on the right is wired as a MOD-10 counter so that its output
Q3 has a frequency = fin/10. The signal is connected to the CP1 input
of the second 74293 counter, which is wired as a MOD-6 counter.
Thus, the Q3 output of the second counter will have a frequency:
fin/10
fout =
6
f
= in
60
Two 74293s can be
combined to provide a
frequency division of 60
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11. Chapter 14 - IC Counters
Next Lesson
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