Tag: logic gates

The SR Flip-Flop – Practical Introduction

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This post is used for discussion in class.

In class demonstration

Teacher draws a SR latch using NOR gates  with top gate as g1 and bottom gate as g2.

R-S

Teacher draws a truth table for quick reference:

ABA NOR B
001
010
100
110

  • Scenario, Circuit Startup S=0, R=0 , Q=0  (Q’ is irrelevant and can be proven in a later scenario)
    • Input G2 becomes 0,
    • Q’ becomes 1
    •  Input G1 becomes 1
    • Q stays at 0 , circuit is stable in a consistent state.

  • Scenario: circuit in previous state, S becomes 1, r stays at 0, Q was 0 and Q’ is 1



  • Scenario: Same as previous state,  however S returns to 0.
    (Memory is Achieved)

  • Same as previous State, but R is set to 1


  • Same as previous state but R returns to 0

  • CHAOS!!! S and R set to 1.

Media Attrbution

No machine-readable author provided. Arturo Urquizo assumed (based on copyright claims)., CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

© 2021  Vedesh Kungebeharry. All rights reserved. 

Logic Gates – Formal Introduction

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In the previous example we used statements as propositions.  Computer logic is accomplished by using logic gates as building blocks.  A logic gate is a physical circuit which has electrical inputs and usually a single output.

Used in truth tables, we label each input and use truth values to represent weather or not an input is on or off. 

We use the value true , T , 1 to represent on and false , F , 0 to represent off.

Sometimes, On is refered to as a high voltage and off is referred to as a low or no voltage.

The most common gates are shown in the table below:

Gate

AND

OR

NOT

NAND

NOR

XOR

Symbol

AND ANSI Labelled

OR ANSI Labelled

Buffer ANSI Labelled

NAND ANSI Labelled

NOR ANSI Labelled

XOR ANSI Labelled

Inputs

 

A

B

Q=AB

Q=A+B

Q = A

Q= AB

Q=   A+B

Q= A ⊕ B

0

0

0

0

1

1

1

0

0

1

0

1

1

0

1

1

0

0

1

0

1

0

1

1

1

1

1

0

0

0

 

Description

Outputs 1 when all inputs are 1

Outputs 1 when ANY input is a 1

Inverts the input

Outputs 0 where all inputs are 1

Outputs 1 then both inputs are 0

Outputs 1 when exactly one input is 1

Outputs 0 otherwise

Outputs 0 otherwise

 

Outputs 1 otherwise

Outputs 0 otherwise

Outputs 0 otherwise

Typing the mathematical notation into most word processors can be challenging. Sometimes these worded expressions or symbols are used :

GateWorded expressionSymbol Expression
ANDA AND BA^B
ORA OR BA v B
NOTNOT A~A
NANDNOT( A AND B) ~(A^B)
NORNOT (A OR B)~(A v B)
XORA XOR BA ∨ B

© 2021  Vedesh Kungebeharry. All rights reserved. 

Logic Gates

vkungeLeave a comment

Logic gates are electronic combinational circuits which contain one or more electrical inputs and usually one output.

A combinational circuit’s output depends on the state of it’s inputs.

We use truth tables to document the  behaviour of a logic gates.

In class demonstration

Demonstrate an AND gate and describe with a truth table and in words

(Teacher constructs the truth table from description for a 2 input and a 3 input gate)

“An AND gate’s  output is true only when both inputs are true”

Homework/In class exercise

Draw the truth table for  the following gates

  1. AND
  2. OR
  3. NOT
  4. ExOR
  5. NAND

Describe their outputs or mode of operation using a well formed sentence in English. E.g “An AND gate’s  output is true only when both inputs are true”

© 2021  Vedesh Kungebeharry. All rights reserved.