Saturday, 10 November 2012

 Digital LOGIC: the truth often hurts (your Brain)  

I created this single-page reference to help me with digital logic GATES and wanted to share it. It was created from a screen-shot in New-Wave-Concepts Circuit Wizard which simulates circuits in real time.  I use circuit Wizard a lot, so it will feature a lot in my blog - it's really useful to get you quickly prototyping your circuits. You can get Circuit Wizard from Maplins in the UK or contact the publisher (New-Wave-Concepts).

The symbols represent the types of logic gate and the tables are called truth tables. A and B are inputs and Q is the output. Logic values are often called boolean values and represent two conditions TRUE (1) and FALSE (0). Very simply put, logic 1 can be represented by five volts and logic 0 by zero volts. This isn't the full story in real circuits, because each logic gate is composed of electronic switches called transistors (usually MOSFETS).   

Here is a logic gate chip, containing 6 NOT gates or inverters often called a Hexinverter:

The NOT gate or inverter

This gate "inverts" the input so a 1 becomes a 0, a 0 becomes a 1. TRUE => FALSE, FALSE => TRUE. 

The output Q is the logical inverse of the input A, ie NOT A. In  boolean algebra it is written as the symbol:

The NOT gate is the simplest form of logic gate. Note there six inverters on the chip above. You can see how the inverters are arranged with their inputs and output pins. Vcc is the power pin for the chip and is commonly +5 volts. GND is ground, or commonly zero volts. The notch in the pin tells you the correct way up to read the pins and connect them. The letter Y was used to distinguish outputs from inputs.

I can also recommend this inexpensive book I recently bought: Digital Logic Gates and Flip Flops by Ian Robertson Sinclair, which delves into this subject with a lot of depth. But, for a quick overview, here is a reference to LOGIC gates on Wikipedia. 

Logic gates form the foundations of digital circuits, all the way up to complex microprocessors. 

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