FPGA's and VHDL

There are multiple methods available to describe digital hardware: some examples are:

• Truth Table
• function Table
• boolean equations

Hardware Description languages like ABEL, VHDL and Verilog also belong to the Hardware description systems.
So ABEL is a Hardware Description Language, and is short for Advanced Boolean Equation Language
This language knows many different variants and stands close to the PLD's (Programmable Logic Devices).

IT is designed by Data I/O Coporation for PLD's, That is why it is extremely usefull when describing smaller Digital hardware schematics which can be realised with PLD's.
ABEL permits you to describe the behavior of the digital system in different ways like:

• logic equations
• truth tables
• state diagrams

FPGA stands for Field Programmable Gate Array.
It is a chip which can be programmed with A HDL
A FPGA looks a lot like a PLD but a FPGA supports thousends of logical ports while a PLD only supports a few hundreds.
When a FPGA has not yet been programmed all ports are connected with each other, and when it is programmed, the connections will be erased.

The Image below tries to explain how a FPGA works

The building blocks in the FPGA can be anything, ranging from simple AND / OR / XOR Functions to more complex structures like Flip flops, Small PLA's Multiplexers etc...
The advantage of a FPGA compared to a PAL or PLA is the immence complexity and the low cost per module.
and the Advantage compared to a Gate array is that its faster to program.

Photo of an FPGA

And its insides

The FPGA also has some disadvantages:
A FPGA has a limited routing capability compared to a Gate Array and is more expensive, esspecially with the larger FPGA's.
also a FPGA is slower then a Gate Array.

The FPGA loads its software which it uses to program the connections from external memory, this varies from a rom or an EEPROM.
It is even possible to use a microcontroller to load the software to the FPGA thus creating a really flexible system which can be used for lots of different aplications.

A statemachine is used to describe an object and has 3 states:

• open
• closed
• locked

Both a flop flop with a Q and a Q' but with a Mealy machine the result depends on the Q, Q' and its inputs.
With the Moore machine the value changes when the Q or Q' of the flip flob outputs are changed.

The Mealy machine can change on every moment even when the Clock pulse is low.

The image below tries to explain this

VHDL stands for VHSIC(Very High Speed Integrated Circuit) Hardware Description Language.
This Language is often used for describing digital electronical circuits which can host all kinds of functions like a encoder/decoder, ethernet chip or even a real processor see opencores for all kinds of cool descriptions.

The American Defence initiated the development of VHDL so it would be easy for humans to read and write this language and which would be easy to convert in code which could be loaded in fysical chips.

The most Important aspect would have to be that the language could support parralel hardware, and sequential functions for complex functions.

It was standarised in 1987 by the IEEE(Institute of Electrical and Electronics Engineers) and updated in 1993

PAL stands for Paradox Application Language.

A PAL is build up out of programmable AND