Flip-flops and other multivibrators

East Doncaster Secondary College – Unit 3 Physics

FLIP-FLOPS AND OTHER MULTIVIBRATORS
The apparent intelligence of computers and calculators comes from their ability to “remember”. The circuit elements that “remember” are known as multivibrators. Multivibrators are used in all types of digital counting devices, such as watches, and they are the basic memory elements in random access memory (RAM) devices in computers.
Multivibrators are digital devices that vibrate or oscillate between two possible output states:

1 (high – 5 volts) and 0 (low – 0 volts).

Most multivibrators actually have two outputs, known as Q and (called Q bar or not Q). is always in the opposite state of Q (so if Q is 1, is 0 and visa versa).
Note: Do not get confused with the terminology. There are two outputs Q and to the flip-flop and each of these can be in one of two possible output states: 1 (high) or 0 (low). The outputs are always opposite.

SIMPLE FLIP-FLOP

A simple flip-flop [also known as a T (or toggle) flip-flop OR a bistable] is one type of multivibrator. It is the only type of flip-flop that we have to study in Year 12 Physics.

The simple flip-flop is “triggered” to flip between its two possible output states by a pulse at its input.

It is also called a bistable because it has two stable output states (Q = 1 or Q = 0). It will stay in whatever state it is in until it is triggered again at the input by another pulse.
In Year 12 Physics the trigger pulse is always a square wave (it is actually often a series of square waves each following each other at a regular interval). A square wave is either at 1 (high) or 0 (low) and it has a rising edge (0 – 1 transition) or a falling edge (1 – 0 transition).

Simple flip-flops can be triggered by either rising edge pulses or falling edge pulses (not both) but the Year 12 Physics course restricts this to rising edge triggered flip-flops.

This means that the output of Q (and by extension ) can only change when a rising edge transition is feed into the input. The flip-flop does NOT change state when a falling edge transition feeds into the input.

Example: Let us assume that we have 4 square pulses being fed into the input of a rising edge triggered flip-flop that initially has Q = 0 (and hence = 1). You WILL need to be told the initial state of the flip-flop outputs.

The diagrams below show the voltage states of the input trigger pulses and the output states of Q and .

first rising edge

Trigger
1		Pulse 1		Pulse 2		Pulse 3		Pulse 4
		Q changes output state from low to high on the input rising edge transition.							time, t
Q
1
				Q “flops” back again when the second rising edge transition is feed into the input					time, t
1
					one period, T				time, t

When a series of digital square waves pulses are drawn they are displayed on a voltage vs. time graph. The later pulses are therefore to the right. This means that if we imagine time moving on, the pulses would seem to be moving to the left, not the right as you probably first think. If this is not kept in mind it is easy to confuse a rising and falling edge.
It is important to note that the simple flip-flop has the effect of dividing the number of pulses it receives by 2. This has the effect that the frequency of the output at Q is half the frequency of the input. As a result the period of the output at Q is double the period of the input period.
Please Note:

Your text-book talks about monostable and astable multivibrators as well as R-S and D-type flip-flops. These are not on the course.

COUNTING WITH FLIP-FLOPS

Flip-flops connected together, with the output of either Q or feeding into the input of the next (in series), can be used to count pulses and hence count events (the number of cans passing a production line for example).
Consider the arrangement shown on the next page where 3 rising edge triggered flip-flops are connected so that the Q output of the first flip-flop is connected to the input of the second flip-flop, etc. Lights A, B & C are connected to each of the outputs (they could just as easily be attached to the Q outputs but this counts “backwards”). Initially all of the lights are off (i.e. all outputs are set to 0 and Q are all 1).
We will feed 8 square wave pulses into the input of the first flip-flop and examine the way the three lights turn on and off.

		1		2		3		4		5		6		7		8
Input
																	time
A
B
C

Rising edge pulse 1 into flip-flop A causes Q(A) to flop from 1 to 0 hence (A) goes to 1 and light A comes on. The 1 to 0 transition (falling edge) from Q(A) feeds into flip-flop B so there is no change. No signal feeds into flip-flop C. Both light B and C stay off.

Rising edge pulse 2 causes the Q(A) to flip from 0 to 1 hence (A) goes to 0 and light A goes off. The rising edge from Q(A) feeds into flip-flop B so it flops from 1 to 0 and hence (B) goes to 1 and light B comes on. The 1 to 0 transition (falling edge) from Q(B) feeds into flip-flop C so there is no change. Light C stays off. (etc., etc.)
Reading the lights in reverse order as shown on the truth table below (A is 2⁰, B is 2¹, C is 2²), the number of pulses (or events such as cars going over a traffic counter) can be counted using a three-bit binary counter. Notice that the eighth pulse resets the counter to 0,0,0 (so it can only effectively count up to 7). A four-bit counter can count up to 15 (2⁴ – 1); 5-bit to 31; etc.

Pulse Number	C	B	A
0	0	0	0
1	0	0	1
2	0	1	0
3	0	1	1
4	1	0	0
5	1	0	1
6	1	1	0
7	1	1	1
8	0	0	0

BO -