How many flip flops are required for 2N count?
One flip flop counts
up to 2 pulses, two flip flops count up to 22 = 4 pulses. Similarly
N flip flop count up to 2N pulses.
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.
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.
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.
Why the skipping of state may occur 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.
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.
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.
Which gate is used for active HIGH decoding and active
LOW decoding?
AND gate
What is width of glitches in the counter?
The width of glitches
is equal to width of the propagation delay.
How the glitch affects 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.
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.
What is synchronous counter?
Synchronous counter
It
is a counter in which all the flip flops are triggered simultaneously.
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.
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.
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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