Software Developers View of Hardware

Understanding Flip-Flops

What is a Flip-Flop?

A flip-flop is a bistable device. This means that output from the device can

be one of two possible states and will remain that way even after input signals are removed.

A flip-flop will only change state when commanded.

Because of this they form the basis of memory.

Components

A flip-flop has three (3) basic components:1. Latch

2. Clock

3. Trigger

Latches

The most basic of all latches is the RS latch. There are TWO types of RS latches, they

are:

NOR Latch NAND Latch

Latches

The basic understanding behind a latch is that if 1 is sent as the SET input then the output (Q) will be 1.

Likewise, if 1 is sent to the RESET input then the output (Q) will be 0.

The most important part is FEEDBACK which enables the latch to carry out the task of enabling memory.

NOR Latch

Firstly, lets look at the RESET condition. (Send 1 as RESET input)

1

0

Look at the Truth!!!!!!

A B X

0 0 1

0 1 0

1 0 0

1 1 0

Look at the Truth!!!!!!

A B X

0 0 1

0 1 0

1 0 0

1 1 0

NOR Latch

So therefore, if an at least one input is a 1 then the only possible output can be 0.

1

0

NOR Latch

So therefore, if an at least one input is a 1 then the only possible output can be 0.

1

0

0 0

11

NOR Latch

Next, lets look at the SET condition. (Send 1 as SET input)

0

1

NOR Latch

Next, lets look at the SET condition. (Send 1 as SET input)

01

0

01 1

NOR Latch

Next, lets look at the HOLD condition. (Will hold what ever was the previous output)

0

0

0

1

NOR Latch

Next, lets look at the HOLD condition. (Will hold what ever was the previous output)

0

0

0

10

1

NOR Latches

This is where the see saw effect comes into play:

NOR Latch

Finally, for a NOR latch when 1 and 1 are both entered this violates logic rules because Q and NOT Q cannot be the same.

It is referred to as being illegal.

NOR Latch Truth Table

NAND Latch

Firstly, lets look at the RESET condition. (Send 1 as RESET input)

0

1

Look at the Truth!!!!!!

A B X

0 0 1

0 1 1

1 0 1

1 1 0

Look at the Truth!!!!!!

A B X

0 0 1

0 1 1

1 0 1

1 1 0

NAND Latch

Firstly, lets look at the RESET condition. (Send 1 as RESET input)

0

1

1 1

0 0

NAND Latch

Next, lets look at the SET condition. (Send 1 as SET input)

1

01 1

0 0

NAND Latch

Next, lets look at the if 1 is sent to both inputs.

0

1

NAND Latch

A point to remember is that a NAND flip flop works oppositely to a NOR flip flop so an input of (0 0) is illegal. However, an input of (1 1) cause the gate to remember the previous input.

NAND Latch

Next, lets look at the if 1 is sent to both inputs.

1

1

What you will notice is that it depends on the see saw.

NAND Latch

Next, lets look at the if 1 is sent to both inputs.

1

1

What you will notice is that it depends on the see saw.

NAND Latch

Next, lets look at the if 1 is sent to both inputs.

1

1

What you will notice is that it depends on the see saw.

1

0

NAND Latch

Complete the following truth table depending on the inputs shown.

A B C D Q Q’

1 1 0 1

Clocked RS Latch

The logic behind this logic gate is the fact that there is another input called ENABLED.

This acts like a gate or a switch and when set to 1 the circuit will respond as usual. However, when it is set to zero the circuit will not respond.

Clocked RS Latch

Clocked RS Latch