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. The counter's output is indexed
by one LSB every time the counter is clocked.
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A simple
implementation of a 4bit counter is shown in Figure 1, which consists of
4 stages of cascaded JK flipflops. This is a binary counter, since the
output is in binary system format, i.e., only two digits are used to
represent the count, i.e., '1' and '0'. With only 4 bits, it can
only count up to '1111', or decimal number 15.
As one can see
from Figure 1, the
J and K inputs of all the flipflops are tied to '1', so that they will
toggle between states every time they are clocked. Also, the
output of each flipflop in the counter is used to clock the next
flipflop. As a result, the succeeding flipflop toggles between '1'
and '0' at only half the frequency as the flipflop before it.
Figure 1.
A Simple Ripple Counter Consisting of JK Flipflops
Thus, in Figure
1's 4bit example, the last flipflop will only toggle after the first
flipflop has already toggled 8 times. This type of binary counter is
known as a 'serial', 'ripple', or 'asynchronous' counter. The name
'asynchronous' comes from the fact that this counter's flipflops are not
being clocked at the same time.
A 4bit
counter, which has 16 unique states that it can count through, is also
called a modulo16 counter, or mod16 counter. By definition, a
modulok or basek counter is one that returns to its initial state after
k cycles of the input waveform. A counter that has N flipflops is
a modulo 2^{N}
counter.
An asynchronous
counter has a serious drawback  its speed is limited by the cumulative
propagation times of the cascaded flipflops. A counter that has N
flipflops, 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.
A better
counter, therefore, is one whose flipflops are clocked at the same time.
Such a counter is known as a synchronous counter. A simple 4bit
synchronous counter is shown in Figure 2.
Not all
counters with N flipflops are designed to go through all its 2^{N}
possible states of count. In fact, digital counters can be used to
output decimal numbers by using
logic gates to force them to reset when
the output becomes equal to decimal 10. Counters used in this manner are
said to be in binarycoded decimal (BCD).
Figure 2.
A Simple Synchronous Counter Consisting of JK Flipflops and AND gates
See Also:
What is a
Semiconductor?;
Flipflops; Shift
Registers
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