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asynchronous counter

Asynchronous Counter

In the previous tutorial we saw that an Asynchronous counter can have 2n-1 possible counting states e.g. MOD-16 for a 4-bit counter, (0-15) making it ideal for use in Frequency Division applications.

But it is also possible to use the basic asynchronous counter configuration to construct special counters with counting states less than their maximum output number. For example, modulo or MOD counters.

This is achieved by forcing the counter to reset itself to zero at a pre-determined value producing a type of asynchronous counter that has truncated sequences. Then an n-bit counter that counts up to its maximum modulus ( 2n ) is called a full sequence counter and a n-bit counter whose modulus is less than the maximum possible is called a truncated counter.

But why would we want to create an asynchronous truncated counter that is not a MOD-4, MOD-8, or some other modulus that is equal to the power of two. The answer is that we can by using combinational logic to take advantage of the asynchronous inputs on the flip-flop. If we take the modulo-16 asynchronous counter and modified it with additional logic gates it can be made to give a decade (divide-by-10) counter output for use in standard decimal counting and arithmetic circuits.

Such counters are generally referred to as Decade Counters. A decade counter requires resetting to zero when the output count reaches the decimal value of 10, ie. when DCBA = 1010 and to do this we need to feed this condition back to the reset input. A counter with a count sequence from binary “0000” (BCD = “0”) through to “1001” (BCD = “9”) is generally referred to as a BCD binary-coded-decimal counter because its ten state sequence is that of a BCD code but binary decade counters are more common.

Asynchronous Decade Counter

asynchronous decade counter

This type of asynchronous counter counts upwards on each trailing edge of the input clock signal starting from 0000 until it reaches an output 1001 (decimal 9). Both outputs QA and QD are now equal to logic “1”. One the application of the next clock pulse, the output from the 74LS10 NAND gate changes state from logic “1” to a logic “0” level.

As the output of the NAND gate is connected to the CLEAR ( CLR ) inputs of all the 74LS73 J-K Flip-flops, this signal causes all of the Q outputs to be reset back to binary 0000 on the count of 10. As outputs QA and QD are now both equal to logic “0” as the flip-flop’s have just been reset, the output of the NAND gate returns back to a logic level “1” and the counter restarts again from 0000. We now have a decade or Modulo-10 up-counter.

Decade Counter Truth Table

Clock Count Output bit Pattern Decimal Value
QD QC QB QA
1 0 0 0 0 0
2 0 0 0 1 1
3 0 0 1 0 2
4 0 0 1 1 3
5 0 1 0 0 4
6 0 1 0 1 5
7 0 1 1 0 6
8 0 1 1 1 7
9 1 0 0 0 8
10 1 0 0 1 9
11 Counter Resets its Outputs back to Zero

Decade Counter Timing Diagram

asynchronous counter timing

By using the same idea of truncating counter output sequences, the above circuit could easily be adapted to other counting cycles be simply changing the connections to the inputs of the NAND gate or by using other logic gate combinations.

Related Products: Clock Generator and Synthesizer | Logic and Timing Misc

So for example, a scale-of-twelve (modulo-12) can easily be made by simply taking the inputs to the NAND gate from the outputs at “QC” and “QD“, noting that the binary equivalent of 12 is 1100 and that output “QA” is the least significant bit (LSB).

Since the maximum modulus that can be implemented with n flip-flops is 2n, this means that when you are designing truncated asynchronous counters you should determine the lowest power of two that is greater than or equal to your desired modulus.

Lets say we wish to count from 0 to 39, or mod-40 and repeat. Then the highest number of flip-flops required would be six, n = 6 giving a maximum MOD of 64 as five flip-flops would not be enough as this only gives us a MOD-32.

Now suppose we wanted to build a “divide-by-128” counter for frequency division we would need to cascade seven flip-flops since 128 = 27. Using dual flip-flops such as the 74LS74 we would still need four IC’s to complete the circuit.

asynchronous counter

One easy alternative method would be to use two TTL 7493’s as 4-bit ripple counter/dividers. Since 128 = 16 x 8, one 7493 could be configured as a “divide-by-16” counter and the other as a “divide-by-8” counter. The two IC’s would be cascaded together to form a “divide-by-128” frequency divider as shown.

Related Products: Clock Buffer and Driver

Of course standard IC asynchronous counters are available such as the TTL 74LS90 programmable ripple counter/divider which can be configured as a divide-by-2, divide-by-5 or any combination of both. The 74LS390 is a very flexible dual decade driver IC with a large number of “divide-by” combinations available ranging form divide-by-2, 4, 5, 10, 20, 25, 50, and 100.

Frequency Dividers

This ability of the ripple counter to truncate sequences to produce a “divide-by-n” output means that counters and especially ripple counters, can be used as frequency dividers to reduce a high clock frequency down to a more usable value for use in digital clocks and timing applications. For example, assume we require an accurate 1Hz timing signal to operate a digital clock.

We could quite easily produce a 1Hz square wave signal using a standard 555 timer chip configured as an Astable Multivibrator, but the manufacturers data sheet tells us that the 555 timer has a typical 1-2% timing error depending upon the manufacturer, and at low frequencies of 1Hz, this 2% timing error is not good.

However, the data sheet also tells us that the maximum operating frequency of the 555 timer is about 300kHz and a 2% error at this high frequency, while still large at about 6kHz maximum, would be acceptable. So by choosing a higher timing frequency of say 262.144kHz and an 18-bit ripple (Modulo-18) counter we can easily make a precision 1Hz timing signal as shown below.

1Hz timing signal from a 18-bit asynchronous ripple counter.

asynchronous timing counter

This is of course a very simplistic example of how to produce accurate timing frequencies, but by using high frequency crystal oscillators and multi-bit frequency dividers, precision frequency generators can be produced for a full range of applications ranging from clocks or watches to event timing and even electronic piano/synthesizer or music type applications.

Unfortunately one of the main disadvantages with asynchronous counters is that there is a small delay between the arrival of the clock pulse at its input and it being present at its output due to the internal circuitry of the gate.

In asynchronous circuits this delay is called the Propagation Delay giving the asynchronous ripple counter the nickname of “propagation counter” and in some high frequency cases this delay can produce false output counts.

In large bit ripple counter circuits, if the delay of the separate stages are all added together to give a summed delay at the end of the counter chain the difference in time between the input signal and the counted output signal can be very large. This is why the Asynchronous Counter is generally not used in high frequency counting circuits were large numbers of bits are involved.

Also, the outputs from the counter do not have a fixed time relationship with each other and do not occur at the same instant in time due to their clocking sequence. In other words the output frequencies become available one by one, a sort of domino effect. Then, the more flip-flops that are added to an asynchronous counter chain the lower the maximum operating frequency becomes to ensure accurate counting. To overcome the problem of propagation delay Synchronous Counters were developed.

Then to summarise some of the 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.

Related Products: Delay Line | PLL 

In the next tutorial about Counters, we will look at the Synchronous Counter and see that the main characteristic of an synchronous counter is that the clock input of each flip-flop in the chain is connected to all of the flip-flops so that they are clocked simultaneously.

20 Comments

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  • o
    ongazy

    thank you

  • s
    sumit

    Hello,
    Could anyone please clearly justify the use of various analogue and digital components/devices used in this circuit?
    Thanks

  • J
    Jesse

    It seems in this Asynchronous Decade Counter the transition from 9 to 0 and from 0 to 1 only takes half clock cycle, while other transitions take a full cycle?
    After the falling edge that sets the counter to 9, the next raising edge will reset to 0, and the next falling edge will get 1.
    And is there any way to overcome this to make every transition take the same amount of time?
    Or am I missing something?

    • Wayne Storr

      Quite possibly, this basic Asynchronous Divider circuit can result in unwanted “glitches” as at 9 (1001), two of the NAND gate inputs are already high and the positive going transition of the clock pulse from low to high switches the NAND gates output low resulting in 1010, reseting the flip-flops outputs to zero (0000). The next negative going clock pulse half a cycle later switches the output from 0 to 1 (0001). It can be improved by adding some extra decoding or by connecting the flip-flops as a synchronous decade counter.

  • T
    TUNUGUNTLA VENKATA JAYADEEP

    WHAT IS THE DIFF B/W DECADE COUNTER AND DIVIDE BY 10 COUNTER

    • Wayne Storr

      A Decade Counter such as the 74LS90 or 4017 only counts from 0 to 9 and then recycles back to 0 again. A Divide-by-10 counter can be any type of counter, such as a standard Binary Counter which counts from 0 to 15, that has been configured using external logic gates to count to 10 or any predetermined number for that matter.

  • Rachel

    Hello how come the counter never counts to 1111 then resets?

    • Wayne Storr

      Because in this example, its configured as a MOD-10 Asynchronous Decade Counter producing a binary-coded-decimal sequence from 0000 to 1001.

  • W
    William Adams WD8OSJ

    I have been reading your tutorials and have been trying to work out a circuit to display a VFO frequency. The VFO in question is from a Kenwood TS-530S and at 0 on the dial it reads 5.5 MHz and at 500 on the dial it reads 6.0 MHz at 200mvolts output. I would like the circuit to display 0 at the 5.5 MHz and 500 at the 6.0 MHz on a 3 digit display. Since my stroke in 1995, I have lost a lot of the things I use to know well. Reading your articles has helped bring back some of it, but not enough to design a circuit as I use to do.
    Bill WD8OSJ

  • V
    Volvagia

    Sorry, I don’t understand, why the circuit doesn’t count downward.
    Assuming all JK-Flipflops (JKa – JKd) and the clock start with a logical 0, the output is “0000”.
    When the clock raise, JKa’s state become 1, so his output and JKb’s clock input become 1. JKb’s state become also 1, and raise JKc’s clock input, and so on. The next stable state after the clock raise of the counter is “1111”.
    When the clock raise the next time, JKa’s state become 0 and change nothing on JKb. This state would be “1110”, wouldn’t it? (JKa is the least significant bit)

    • Wayne Storr

      Hello Volvagia, The example given is of a Decade (divide-by-10) Counter as explained in the tutorial text. When the up count reaches 1001 the logic gate resets ALL the counters back to 0000 on the next clock pulse. It never reaches 1111 as you have described.

      • V
        Volvagia

        Thank you 🙂

        Yes, I forgot this part. But this JK-Flipflops hasn’t inverted/low-enabled clock inputs. Hency, they are changing their state when the previous Flipflop become a 1. But in Reality (sorry, I couldn’t say this so good, because I speak not very well english), the more significant bits change, when the less sign. bit falling to 0 (=the next more sign. bit get added by the carry which get 1)

        • Wayne Storr

          Ok, to remove any more confusion I have modified it from a generalised decade counter circuit to a tested decade counter using 74LS73 JK flip-flops. The circuit counts UP from 0000 to 1001 and resets.

  • A
    Andries

    I might be wrong but the logic diagram is incorrect, that counter will count down not up. To count up NOT Q must be connected to each successive CLK.

  • V
    Vishnu

    Thank you

  • r
    rohit dwivedy

    this site is very useful for me,thanks:)

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