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The size and complexity of the circuits FPGAs can contain would become very
cumbersome should you actually draw out a schematic. Instead, you can describe
the behavior of the circuit you want and FPGA tools will use this to create a circuit
that matches that behavior.

FPGAs can only create digital circuits. Some of the newer FPGAs include on-board
analogue to digital converters, convert the analogue input into a digital signal as
soon as possible.

PROGRAMMABLE LOGIC GATES within an FPGA

There are a handful of different logic gates but the most common ones are NOT
AND, OR, and XOR, and NOT. Each of these takes digital inputs, performs its logical
function, and outputs a digital value.
A NOT gate is the simplest gate. It has An AND gate takes two inputs and
one input and simply outputs the outputs a 1 only when the first
opposite of whatever it is. So a 1 input and the second input are 1. If
becomes a 0 and a 0 becomes a 1. either input is 0, the output is 0. The
symbol of an AND gate looks like this:

An OR gate takes two inputs and An XOR gate is similar to an OR gate
outputs a 1 when either the first but only outputs a 1 when either the first
input or the second input is 1. Only input or the second input are 1, but not
when both are 0 is the output 0. Here’s when both are 1. It can also be thought
the OR gate symbol: of as outputting a 1 when the inputs are

different.

Figure 35-22
There are variations of the basic gates known as NAND, NOR, and XNOR. These
are simply the standard versions with their outputs inverted.

FPGAs require configuration so the device’s logic circuits and interconnects know
what role they should play in the implementation of a specific application. Using
specialised software (generally provided by the FPGA supplier), developers design
the logic to be implemented in the FPGA using either graphical design capture or a
hardware description language (HDL).
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