VHDL tutorial

FPGA programming using a hardware description language is not a commonly taught skill in physics programs, but is a necessary skill for designing the electronics required for this project. This tutorial aims to layout the design process and teach the basics of hardware description language; in particular VHDL. The main competitor to VHDL is Verilog; tutorials and information regarding Verilog can be found through Google web searching.

Design example

To illustrate the discussions in this tutorial, a design example is discussed along the way. The design example is the [for the AD5535 DAC]. As each step of the design process is discussed, the DAC emulator will be used for illustration.

Where to start

The first part of the design process is completely independent of any code. The first step is to define the "black box" of your circuit; that is, draw a box and say what goes in and what comes out. VHDL allows three types of pins (connections to the outside world):

• in: An in pin can be read from but never written to.
• out: An out pin can be written to but never read from.
• inout: An inout pin can be both read from and written to, providing the flexibility to allow bidirectional communication on a single line. At first this seems the ideal choice and that you would always want inout pins; in actual fact you want to avoid inout pins unless you absolutely need them for bidirectional communication.

Having defined your block box, you need to fill in your black box.

VHDL Resolution Table

VHDL Resolution Table

	U	X	0	1	Z	W	L	H	-
U	U	U	U	U	U	U	U	U	U
X	U	X	X	X	X	X	X	X	X
0	U	X	0	X	0	0	0	0	X
1	U	X	X	1	1	1	1	1	X
Z	U	X	0	1	Z	W	L	H	X
W	U	X	0	1	W	W	W	W	X
L	U	X	0	1	L	W	L	W	X
H	U	X	0	1	H	W	W	H	X
-	U	X	X	X	X	X	X	X	X

VHDL Logic States

• U: uninitialized
• X: forcing unknown
• 0: forcing 0
• 1: forcing 1
• Z: high impedance
• W: weak unknown
• L: weak 0
• H: weak 1
• -: don't care