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DIGITAL COUNTERS
A digital counter, or simply counter, is a semiconductor device that is used for
counting the number of times that a digital event has occurred
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A simple implementation of a 4-bit counter is shown in Figure 1, which consists
of 4 stages of cascaded J-K flip-flops
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e
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e
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With only 4 bits, it can only count up to '1111', or decimal number 15
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Also, the output of
each flip-flop in the counter is used to clock the next flip-flop
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Figure 1
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This type of binary counter is known as a 'serial',
'ripple', or 'asynchronous' counter
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A 4-bit counter, which has 16 unique states that it can count through, is also
called a modulo-16 counter, or mod-16 counter
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A
counter that has N flip-flops is a modulo 2N counter
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A counter that has N flip-flops,
each of which has a propagation time t, must therefore wait for a duration equal to N x t
before it can undergo another transition clocking
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Such a counter is known as a synchronous counter
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Not all counters with N flip-flops are designed to go through all its 2N possible
states of count
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Counters
used in this manner are said to be in binary-coded decimal (BCD)
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A Simple Synchronous Counter Consisting of J-K Flip-flops and AND gates
RESISTERS
A register is a semiconductor device that is used for storing several bits of digital data
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Thus, an n-bit register has n flip-flops
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'
A special type of register, known as the shift register, is used to pass or transfer bits of
data from one flip-flop to another
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Shift registers are useful for transferring data in a
serial manner while allowing parallel access to the data
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Clocking all the flip-flops at the same time
will cause the bits of data to shift or move to the right in one direction (i
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, toward the
last flip-flop)
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Figure 1
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In
some applications there is a need to bring this back to the first flip-flop, in which case
the data will just be circulated within the shift register
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A more complicated version of a shift register is one that allows shifting in both
directions, left or right
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To accomplish this, a 'Mode' control line is added to the circuit
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