Tab Electronics Guide to Understanding Electricity ... - Sciences Club

Digital Electronics
349
Multivibrators
Multivibrators are used extensively in digital electronics to provide clock
signals (oscillators), count and store data, and control timing sequences.
They can be divided into three major groups, or types: astable multivibrators
(called clocks or oscillators), bistable multivibrators (flip-flops), and
monostable multivibrators (one-shots).
You should already be familiar with astable multivibrators from our previous
discussions regarding their use in sound circuits. The term astable
means “not stable”; they cannot come to rest in either a high or low state.
In other words, they oscillate. Because their outputs are in the form of a
square wave, they are naturally suited to digital systems. IC forms of
astable multivibrators are designed to operate at very high speeds.
Three common examples of bistable multivibrators are illustrated in Fig.
13-1. As the name suggests, bistable multivibrators have two stable states:
“set” and “reset.” They are usually called “flip-flops” (F-Fs).
Referring to Fig. 13-1, notice that each F-F has a “Q” and “NOT Q” output.
The NOT Q output is always logically opposite of the Q output.
When an F-F is “reset,” the Q output will be a logic 0, thus meaning that
the NOT Q is a logical 1. If an F-F is “set,” the logical states of the Q and
NOT Q will be reversed.
The first F-F illustrated in Fig. 13-1 is a “set-reset (RS) F-F,” or RS flipflop.
If a logical 1 is applied to the R (reset) input, the Q output goes to
logical 0. Similarly, if a logical 1 is applied to the S (set) input, the Q output
will go to a logical 1. RS flip-flops have limited use in most digital
systems. Their importance lies in their ability to latch, or remember, a
logical status (if the logic levels to the RS inputs are not altered).
The second type of F-F illustrated is the JK flip-flop. The JK inputs are
“clocked inputs.” This means that the logical levels applied to the JK
inputs have no effect without a coincidental pulse applied to the clock
input. For example, if the K input is 1 and the J input is 0, the F-F will
reset as soon as a clock pulse is applied to the clock input. If the K input
changes to 0, and the J input goes to 1, the F-F will set as soon as (but
not before) another pulse is received at the clock input.
If both the J and K inputs are held at logical 1, a JK flip-flop
becomes a toggle F-F. The output, or Q status, of a toggle F-F will change
state every time the correct transitional change occurs at the clock input.
Toggle F-F’s are designed to change state, or toggle, on either the “leading
edge” or “trailing edge” of input clock pulses. For example, if a toggle F-F
is specified to toggle on the trailing edge of the input clock pulses, a