22 March 2020

Counter Interview Question Answer - 2



16
How many flip flops are required for 2N count?


One flip flop count up to 2 pulses, two flip flops count up to 22 = 4 pulses. Similarly N flip flop count up to 2N pulses.

17
What to do for invalid states in the synchronous and asynchronous counter?


The synchronous and asynchronous counter goes from invalid state to valid state by providing external logic circuit which properly reset each flip flop.

18
Explain the lock out state of the counter.


Lock out
  • The counter may move from one unused state to other unused state and never come to its valid state is called as lock out state of the counter.
  • The lock out problem occurs due to noise spikes.
  • This counter becomes useless.


19
Explain the term : Forward / Backward counter                                    


Forward / Backward counter ( Bi- directional counter )( UP / DOWN counter )
  • It is a counter which can count in both upward and downward directions.


20
Why the skipping of state may occurs in the asynchronous counter?
           OR
Write the relation between propagation delay time and clock frequency. What is importance of propagation delay time?


The relation between propagation delay and clock frequency is
      fc < ( 1 / ntpd )
     fc =  Clock frequency
     n =  Number of stages and
  tpd =  Propagation delay time
Importance of propagation delay time
  • The propagation delay time imposes a limit on frequency at which the counter can be clocked in the ripple counter.


21
Why the ripple counters are connected in cascaded? How cascaded connection is done?


Cascaded connection of ripple counter
  • The ripple counters are connected in cascaded in order to increase the modulus of the counter.
  • The cascading of the counter is done by connecting the most significant stage of the first counter to the toggling stage of the second counter.


22
Explain the term : Decoding of the counter, Glitches of the counter, Strobing


Decoding of the counter
  • The decoding of the counter is done by decoder or gates.
  • The output of the counter is in the binary form and it is converted in the decoding form by a decoder.

Glitches of the counter
  • The transitional states in the asynchronous counter produces undesirable voltage spikes of short duration at the output of the decoder is called as the glitches.
  • The transition state is creating by propagation delay.

Strobing
  • It is a method to eliminate glitches.
  • The glitch can eliminate by enabling the decoder output at a time after glitches have had time to disappear.
  • This can be achieved by using LOW level of the clock to enable the decoder.


23
Which gate is used for active HIGH decoding and active LOW decoding?


AND gate

24
What is width of glitches in the counter?


The width of glitches is equal to width of the propagation delay.

25
How the glitch affect if the decoder output drive edge triggered circuit? How to reduce it?


Effect of Glitches
  • If the decoder output drives edge triggered circuit, the glitch produces unsatisfactory performance.
  • This effect is reduced by ANDing the decoder with the clock.
  • However this will reduce the width of deglitched output to the width of clock pulse which causes gaps between decoder outputs.


26
Why the asynchronous counters are slow?


The asynchronous counters are slow because each flip flop can change state only if all the preceding flip flops have changed their state.

27
What is synchronous counter?


Synchronous counter
  • It is a counter in which all the flip flops are triggered simultaneously.


28
Give reason : The clock frequency of the asynchronous counter should be low.


Clock frequency of the asynchronous counter
  • If the clock frequency of the asynchronous counter should be high, the counter may skip some of the states and malfunction occurs.


29
Why the synchronous counter can be operated at much higher frequency than the asynchronous counter?


Synchronous counter
  • The propagation delay of the synchronous counter is equal to propagation delay of the single flip flop and gates.
  • Therefore the synchronous counter can be operated at high frequency.

Asynchronous counter
  • The propagation delay of the asynchronous counter is equal to propagation delay of all flip flops.


30
Describe the advantages and disadvantages of the synchronous counter.


Advantages
  • High speed
  • Less severe decoding problem

Disadvantages
  • It requires more circuit as compared to asynchronous counter.



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