Synchronous Counters
Questions and Answers

Lee Chin Wei
Andrew Long


Questions :

Note : there may be more than one right answer.

  1. What advantages do synchronous counters have over asynchronous counters?
    A. Less likely to end up in erroneous states.
    B. Simpler to construct.
    C. Faster
    D. Consumes less power.

  2. The combinational element of a counter can be made with...
    A. Flip-flops.
    B. Logic Gates.
    C. PLA (Programmable Logic Array)
    D. ROM (Read-Only Memory)

  3. What types of flip-flops can be used to implement the memory elements of a counter?
    A. T flip-flips.
    B. JK flip-flops.
    C. D flip-flops.
    D. XY flip-flops.

  4. What are the advantages of using a microprocessor to implement a counter rather than the conventional method (flip-flop and logic gates)?
    A. Speed.
    B. Cost.
    C. Flexibility.
    D. Simpler to construct.

  5. Take a look at the diagram below.

    What kind of counter is it?

    A. Ripple counter.
    B. Johnson counter.
    C. BCD counter.
    D. Twisted-ring counter.

  6. What is the principal advantage of Gray Code over straight (conventional) binary?
    A. No intermediate states occur during transition.
    B. Bit positions in Gray Code have greater weights.
    C. Gray Codes are much easier to process.
    D. All of the above.

  7. Convert 3267 to its BCD equivalent.
    A. 0011 0010 1111 0000
    B. 0011 1111 0110 0111
    C. 0011 0010 0110 1110
    D. None of the above.

  8. What is the principal function of the prescaler in the "pulse swallowing" technique?
    A. To divide a fast incoming clock.
    B. To divide a binary number.
    C. Stops counter when too many pulses are received.
    D. None of the above.

  9. What does Pipelining do?
    A. Predicts an event.
    B. Separates the detection of an event and the setting of certain output/s resulting from the event into two different clock periods.
    C. To store a value that is required in the next clock period.
    D. None of the above.

  10. In Xilinx synchronous counters, how does the use of a prescaler increase the speed of the counter?
    A. It increases the speed at which the bits toggle.
    B. It predicts subsequent values.
    C. It allows the carry ripple chain time to settle.
    D. All of the above.

Answers :

A 1 :
A. True. Since all inputs are synchronised with a common clock, no interrupts can occur in the middle of a state transition.

B. False. Asynchronous counters are actually easier to construct as they usually require less combinational logic.

C. True. In synchronous counters, all flip-flops change simultaneously and in asynchronous counters, the propagation delay of the flip-flops add up to produce the overall delay. Although synchronous counters usually have more combinational logic, the propogation delay through these gates are small compared to the propogation delay through many stages of flip-flops.

D. False. Both types of flip-flops are made with similar components.

A 2 :

A. False. Flip-flops are the memory elements of a counter. They cannot be used to implement combinational logic.

B, C & D. True. All of these can be used to implement combinational logic. The PLA and ROM are similar except that the ROM needs to have every minterm defined, even though the system may never enter that state. The PLA only needs to have the minterms which are actually used defined.

A 3 :

A, B & C. True. Flip-flops are used mainly to "remember" the state of a sequential machine. All of these flip-flops can be used to implement this memory function.

D. False. There is NO SUCH THING as an XY flip-flop.

A 4 :

A. False. A microprocessor is usually much slower than a dedicated hardware device. A dedicated device can produce an output immediately on receiving an input signal while a microprocessor has to read in the signal, process the signal, increment program counter and write output etc. A dedicated device is usually a few orders of magnitude faster than a microprocessor.

B. True/False. Whether a counter is more cost efficient to implement using a microprocessor or dedicated hardware depends on the size of the counter(s). For small systems, if would be cheaper to use dedicated hardware and vice versa. This is because for larger systems, a dedicated hardware system would need to add more flip-flops and logic gates whereas for microprocessors, we only need to write a different program for it. The number of states that can be implemented in a microprocessor is only limitted by the size of the program and the available memory area.

C. True. A microprocessor is much more flexible. A dedicated hardware device, as its name suggests, can only perform the function it was designed to do. A single microprocessor can perform many different functions.

D. False. A microprocessor often needs several support chips, such as memory chips, to create a functioning system. It also needs to be programmed before it can be used.

A 5 :

B & D. True. The diagram shows a Johnson counter, which is also known as a twisted ring counter. It has a feedback from the negated output back to its JK/T input.

A 6 :

The answer is A. It is not only the principal advantage of Gray Code over straight binary but also the defining characteristic of Gray Code. The use of such a code limits errors to one bit position. This reduces the likelihood of unwanted transients during a state transition. This code is particularly suitable when analogue(continuous) data is being represented in binary.
A 7 :
The answer is D. The BCD equivalent is 0011 0010 0110 0111. To calculate this, each decimal digit is simply converted to its binary equivalent.
A 8 :
The answer is A. The main function of the prescaler is to divide a fast incoming clock and thereby provide a slower "clock" to the different sections(units and tens). The different sections only respond to this "clock" depending on the prescaler Mode(divide-by-11 or divide-by-10 etc).
A 9 :
A,B and C are essential functions of Pipelining. It predicts an event by detecting the state preceeeding the required state. Following this detection, a value is set and fed to the input of a Flip-flop(usually D-type). This effectively stores the value. The value will appear as the required output on the next clock pulse. The separation of the two events(detection and output setting) into two clock periods results in a much shorter clock period. Had the above actions been performed in a single clock period, the minimum clock period would have been significantly limited.
A 10 :
The answer is C. The prescaler generates an enable signal at a rate lower than(usually a factor of) the incoming clock rate. This difference in speed will give the carry ripple chain time to settle. If this were not done, the time taken for the carry ripple chain to settle has to be taken into account. This significantly limits the minimum clock period.

References :

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