4. Electrons and electronics

1
4. Electrons and electronics

• 4.5 Digital electronics

2
Digital electronics

• 1. Analog signals The electrical signals which
  are continuous and which can have any value over
  a given range are called analog signals. In an
  analog signal, the values of current or voltage
  vary continuously with time.

3
Digital electronics

• 2. Digital signal A digital signal is a
  discontinuous function of time. It has two
  discrete values or voltage levels 0 and 1.The
  zero refers to some low value while 1 refers to
  some high value.

4
Digital electronics

• Analog watch

• Digital watch

5
Digital electronics

• 3. What is logic?
• Logic is a form of reasoning which tells us
  whether a statement is true or false when certain
  conditions are true or false.
• Eg
• AND operation
• OR operation
• NOT operation

6
Digital electronics

• Electrical analog of AND operation

7
Digital electronics

• Electrical analog of OR operation

8
Digital electronics

• Electrical analog of NOT operation

9
Digital electronics

• 4. Logic circuit A logic circuit is an
  electronic circuit, which represents a logic
  function.
• Digital instruments use logic circuits to measure
  a quantity and then display it in a numerical
  reading form.

10
Digital electronics

• Logic gate A logic gate is an electronic
  circuit with one or more inputs and only one
  output. There are three basic logic gates
• 1. OR gate
• 2. AND gate and
• 3. NOT gate

11
Digital electronics

• Truth table A table which gives all the possible
  combinations of input variables and the
  corresponding output for a logic gate is called a
  Truth table.

12
Digital electronics

• The OR gate
• An OR gate has two input signals and only one
  output signal. The logic symbol of a two input OR
  gate is shown in the figure
• Output Y A B

13
Digital electronics
14
Digital electronics

• The AND gate
• The AND gate has two or more inputs and only
  one output. It is a circuit whose output is one
  if and only if all the inputs are one.

15
Digital electronics
Output Y A B
16
Digital electronics

• The NOT gate
• A NOT gate has only one input and one output.
  It is a device whose output is always the
  complement of the input i.e. the output of a NOT
  gate is one when its input is zero and vice-versa.

17
Digital electronics
18
Digital electronics

• UNIVERSAL GATES
• By combining the three basic gates a large
  number of gates can be obtained. Hence these
  three gates are called Universal gates.

19
Digital electronics

• NAND gate
• If the output of an AND gate is connected to
  the input of a NOT gate, the resultant gate so
  obtained is known as NAND gate. NAND is the
  abbreviation for NOT-AND

20
Digital electronics

• NAND gate

21
Digital electronics

• NOR gate
• When the output of an OR gate is connected to
  the input of a NOT gate the resultant combination
  is known as a NOR gate. NOR is the abbreviation
  for NOT-OR.

22
Digital electronics

• NOR gate

23
Digital electronics

• NOT gate using NAND gate

24
Digital electronics

• OR gate using NAND gates

This is same as
25
Digital electronics

• The OR gate is the NAND gate with the inputs
  inverted

26
Digital electronics
AND gate using NAND gate
This same as
27
Digital electronics

• NAND-NOT AND

28
Digital electronics

• NOR gate using NAND gates

29
Digital electronics

• This completes the description of the basic logic
  gates. These gates can be combined to make the
  larger circuits needed to make a computer
  processor. These larger circuits are multiplexors
  (data selectors), registers, the adder, and the
  logic that controls the computer.