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 emulator for the AD5535 DAC. As each step of the design process is discussed, the DAC emulator will be used for illustration.

Where to start

As any good engineer will tell you, design twice and code once. As any bad engineer will tell you, design once and code eighteen times. Learn to love white boards, erasers, and engineering paper. I learned this the hard way, so do yourselves a favor and just humor me.

The black box

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.

Some notes on nomenclature and notation:

The term signal is sometimes used to generically refer to a line, pin, or bus.
Signals come in active-high and active-low varieties. Active-high means that a logical 1 is "on" and a logical 0 is "off". It is also known as "positive logic". Active-low is the exact opposite; logical 0 is "on" and logical 1 is "off", and it is alternately known as "negative logic".
Some signals serve double-duty. As active-high logic they perform one operation, but as active-low logic they perform another operation. This links these operations as complimentary pairs. For example a shift register may shift its output or it may load a new value. If it's not shifting then it's loading. So the name would be something like "Shift/Load" to signify that shifting is active-high and loading is active-low. Carrying over this notation, any signal that is written "/Name" is an active-low signal. This notation is not always used, but it is quite common and I shall attempt to maintain this practice throughout the tutorial. When writing on paper, an active-low signal is frequently denoted by an overbar instead of a leading slash. Unless otherwise specified, a signal that is not marked as active-low is assumed to be active-high by default. Some designs assume active-low as the default, but that will either be marked or implied by context (i.e. all active-high lines marked as such).
A pin is an input, output, or inout. A line is a single pin or a single bit of data flowing along a wire. A bus is properly a multidrop line, but through common usage (due to the way circuits are commonly designed), in digital logic at this level the term has come to mean multiple lines bound together into a bundle. On a diagram, a bus appears as a thick line with a slash through it. Near the slash will be a number denoting how many lines are bundled into that bus.

Example: the block box

For the DAC emulator, the inputs are clearly defined for us. The AD5535 data sheet discusses the serial interface protocol in detail. We need four input lines:

/Reset: an asynchronous, active-low reset line
D_in: serial data line
/Sync: an active-low flag to being transmission
SClk: a serial clock

Our outputs are not so clearly defined. This emulates the circuit itself, so the output of the black box is purely for our benefit to aid in testing. So I decided to define as outputs 32 channels, each 14-bits wide, which display the value being fed to each DAC at any given time.

The block diagram

Having defined your block box, you need to fill in your black box. But before doing that, we need to note the different between two types of logic:

Combinational logic merely recombines lines into new lines. For example, signal Q may be the logical AND of signals A, B, and C. There is no reference to a clock in combinational logic.
Sequential logic is any logic that makes use of a clock for latches, flip-flops, registers, or other devices. Sequential logic changes only when the clock changes. Often circuits are wired so that all sequential logic changes together, either on a falling edge of a clock or a rising edge of a clock. Advanced designs can change some components on a rising edge and other components on a falling edge, but this is significantly more difficult due to the tighter timing restrictions imposed.

A "good" design does its best to separate combinational and sequential logic. All combinational logic takes time; each gate has a delay associated with it. Highly complex combinational logic can take long enough to skew the timing of your circuit; some lines will clock in too late and will take effect on the following clock cycle, completely ruining your synchronization and causing sporadic or faulty behavior in your circuit. For this reason it is best to separate the two types of logic as best you can, although it is not always possible to fully separate them. Generally the sequential block will feed the combinational block, and the combinational block will loop back to the sequential block for any required feedback or recursion.

Bearing this in mind, you need to partition your design into functional blocks. Each functional block will be a new black box within the larger design, with two well-defined attributes: I/O pins and functionality. Sometimes these functional block diagrams will become layered, with a functional block in the top-most diagram having a functional block diagram describing its own internals. For complex designs, there can be many layers and many engineers, so that each engineer is only working on a small subset of the components so that the I/O ports and functionality must be precisely defined and followed so that integration of the components requires a minimum of component redesign. The functional block diagram shows the I/O ports of the overall design (if you have no inouts, it is conventional to put inputs on the left and outputs on the right at all times to clarify data flow), all blocks (with I/O ports labeled), and signals connecting each block as appropriate.

Notes:

Since clock lines are required for most designs (every sequential block needs one), most engineers no longer write the world "clock" or even the abbreviation "CLK" on a block diagram. Instead it is understood that a clock line is represented by a small carat or divot on the side of the block (often placed in the top left corner of the block, but that is not a requirement).
Many devices are general purpose, so giving a descriptive label to a pin would be pointless as the description will change depending on the application. A common shorthand for such blocks is to use "D" as the input signal and "Q" as the output signal. This is often seen on registers, multiplexers, flip-flops, etc.
Pens are the devil. Click erasers are a divine blessing. Engineering paper (or just plain old graph paper) is practically a holy artifact. You will draw this diagram several times (especially as you try to route lines across the paper and realize you left too much space on one side and not enough on the other), so draw it out in pencil then trace over in pen once you're satisfied (for particularly large designs that can be the point where you break your last pencil in half and decide its not worth driving to the store for more).

Example: the block diagram

Functional block diagram of the DAC emulator.

The block diagram is shown to the right, and each block is described on the FPGA programming page. Note that the repeated blocks (the 32 terminal registers) are not all drawn. Two or three are generally sufficient to illustrate connections (for example, does a line feed all 32, or are there 32 separate lines?). Ellipses are perfectly acceptable.

Enter the code monkey

See also: code monkey

Some basics

Now we lay down some actual code. First things first: comments! If you've ever done any programming, you know how wonderful comments are. If you've not done much coding, then remember that comments are not as pointless as they seem. Add descriptive comments or be murdered in your sleep 15 years from now by an irate engineer who has to decipher your legacy code. VHDL has no block comments, only line comments (the comment goes from the comment marker to the end of the line). The comment marker is a double dash with no spacing between them. Many development environments (for example Xilinx ISE) will auto-generate a large block of comments at the top of each file to be used to describe the file (who, what, where, when, why, how, and so on).

Secondly are the libraries. These are like the include statements in C/C++. Honestly I forget which libraries have exactly what tools in them, but a standard block will cover mostly any design you work on:

-- Lines beginning with "--" are comments
-- This standard block will cover you for most designs.
-- Some development environments will auto-generate library "use" statements that may or may not include this set
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

Coding your black box

Every VHDL file defines an entity. Every entity has two parts to it: a port list and an architecture. The port list defines the black box of your component. First you declare that you are defining an entity, then inside the entity you declare your ports.

As discussed above, there are three types of ports: in, out, inout. There are also lines and buses. A line is referred to as a STD_LOGIC and a bus is referred to as a STD_LOGIC_VECTOR. There are also two flavors of STD_LOGIC_VECTORS: downto and to. A "downto" bus has the most significant bit (MSB) associated with the largest subscript, and a "to" bus has the least significant bit (LSB) associated with the largest subscript. A bus with N lines need not have subscripts running from 0 to N-1. They can run from 1 to N or N+84 downto 85; there is complete freedom as to the subscript offset. Most people who learned coding in C or Java or something similar will generally stick to 0 to N-1 or N-1 downto 0 out of habit unless the situation calls for something else (for clarity, usually). Often an engineer will choose either "to" or "downto" (I happen to prefer "downto") and stick with it. However mixing "to" and "downto" has it's uses; for example connecting a "downto" to a "to" will reverse the order of the bits in the bus. For a beginner, I recommend picking one and sticking to it.

entity component_name is
    Port ( line_in : in STD_LOGIC;                      -- a single input line
           bus_in1 : in STD_LOGIC_VECTOR (7 downto 2);  -- a 6-bit "downto" input bus
           bus_in2 : in STD_LOGIC_VECTOR (0 to 8);      -- a 9-bit "to" input bus
           line_out : out STD_LOGIC;                    -- a single output line
           bus_inout : inout STD_LOGIC_VECTOR (1 to 10) -- a 10-bit bidirectional bus
         );
end component_name;

Note that each port in the list is separated by a semicolon, but the last port does not have a semicolon after it. The semicolons in this case are not end-of-line markers, but are list delimiters.

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