VHDL
Tutorial: Learn by Example
--
by Weijun Zhang, July 2001
NEW BOOKS, See the new books on: FPGA, Digital Design: Online Interactive zyBook, HDL, VHDL, Verilog, System-Virlog
If we hear, we forget;
if we see, we remember; if we do, we understand.
-- Proverb
ESD
book | VHDL Projects
| VHDLReference | Auburn.edu_SynopsysTutorial | ActiveHDLTutorial | VHDL_Online_Tutorial
Table of Contents
 Foreword
|
Basic
Logic Gates
Combinational
Logic Design
Typical
Combinatinal Logic Components
|
Latch
and Flip-Flops
Sequential
Logic Design
Typical
Sequential Logic Components
|
Custom
Single-Purpose Processor Design
General-Purpose
Processor Design
Appendix:
Modeling an industry core
|
Foreword (by Frank Vahid)
<>
HDL (Hardware Description Language) based design has established itself
as the modern approach to design of digital systems, with VHDL (VHSIC Hardware
Description Language) and Verilog HDL being the two dominant HDLs.
Numerous universities thus introduce their students to VHDL (or Verilog).
The problem is that VHDL is complex due to its generality. Introducing
students to the language first, and then showing them how to design
digital systems with the language, tends to confuse students. The
language issues tend to distract them from the understanding of
digital components. And the synthesis subset issues of the language
add to the confusion.
We developed the following tutorial based on the philosophy that
the beginning student need not understand the details of VHDL -- instead,
they should be able to modify examples to build the desired basic
circuits. Thus, they learn the importance of HDL-based digital design,
without having to learn the complexities of HDLs. Those complexities
can be reserved for a second, more advanced course.
The examples are mostly from the textbook Embedded
System Design by Frank Vahid and Tony Givargis. They
start from basic gates and work their way up to a simple microprocessor.
Most of the examples have been simulated by
Aldec
ActiveHDL Simulator and Synopsys
Design Analyzer, as well as synthesized with Synopsys Design Compiler .
Several sequential design examples have
been successfully tested on
Xilinx
Foundation Software and FPGA/CPLD board.
Basic Logic Gates
(ESD Chapter 2: Figure 2.3)
Every VHDL design
description consists of at least one entity / architecture pair,
or one entity with multiple architectures. The entity section of the HDL
design is used to declare the I/O ports of the circuit, while the
description code resides within architecture portion. Standardized design
libraries are typically used and are included prior to the entity declaration.
This is accomplished by including the code "library ieee;" and "use ieee.std_logic_1164.all;".
Combinational Logic Design
(ESD Chapter 2: Figure 2.4)
We use port
map statement to achieve the structural model (components instantiations).
The following example shows how to write the program to incorporate multiple
components in the design of a more complex circuit. In order to simulate
the design, a simple test bench code must be written to apply a
sequence of inputs (Stimulators) to the circuit being tested (UUT).
The output of the test bench and UUT interaction can be observed in the
simulation waveform window.
Discussion I: Signal vs. Variable:
Siganls are used
to connect the design components and must carry the information between
current statements of the design. On the other hand, variables are
used within process to compute certain values. The following example shows
their difference:
Typical Combinational Components
(ESD Chapter 2: Figure 2.5)
The following
behavior style codes demonstrate the concurrent and sequential capabilities
of VHDL. The
concurrent statements are written within the body of
an architecture. They include concurrent signal assignment, concurrent
process and
component instantiations (port map statement). Sequential
statements are written within a process statement, function
or
procedure. Sequential statement include case statement,
if-then-else
statement and loop statement.
Latch & Flip-Flops
(ESD Chapter 2.3)
Besides from the
circuit input and output signals, there are normally two other important
signals,
reset and clock, in the sequential circuit. The
reset signal is either active-high or active-low status and
the circuit status transition can occur at either clock rising-edge
or falling-edge. Flip-Flop is a basic component of the sequential
circuits.
Typical Sequential Components
(ESD Chapter 2: Figure 2.6)
Typical sequential
components consist of registers, shifters and counters. The concept of
generics
is often used to parameterize these components. Parameterized components
make it possible to construct standardized libraries of shared models.
In the behavioral description, the output transitions are generally set
at the clock rising-edge. This is accomplished with the combination of
the VHDL
conditional statements (clock'event and clock='1'). During
the testbench running, the expected output of the circuit is compared with
the results of simulation to verify the circuit design.
Sequential Logic Design
(ESD Chapter 2: Figure 2.7)
The most important
description model presented here may be the Finite State Machine (FSM).
A general model of a FSM consists of both the combinational Logic and sequential
components such as state registers, which record the states of circuit
and are updated synchronously on the rising edge of the clock signal. The
output function computes the various outputs according to different states.
Another type of sequential model is the memory module, which usually takes
a long time to be synthesized due to the number of design cells.
Discussion II: Behavior vs. RTL Synthesis
(Y
Chart)
RTL stands for Register-Transfer
Level. It is an essential part of top-down digital design process.
Logic
synthesis offers an automated route from an RTL design to a Gate-Level
design. In RTL design a circuit is described as a set of registers and
a set of transfer functions describing the flow of data between the registers,
(ie. FSM + Datapath). As an important part of a complex design,
this division is the main objective of the hardware designer using synthesis.
The Synopsys Synthesis Example illustrates that the RTL synthesis is more
efficient than the behavior synthesis, although the simulation of previous
one requires a few clock cycles.
Following section illustrates
the RTL (FSM+Datapath) method further using several design examples.
Custom Single-Purpose Processor Design
(ESD Chapter 2, Chapter 4)
The first three
examples illustrate the difference between RTL FSMD model (Finite
State Machine with Datapath buildin) and RTL FSM + DataPath model.
From view of RT level design, each digital design consists of a Control
Unit (FSM) and a
Datapath. The datapath consists of storage
units such as registers and memories, and combinational units such as ALUs,
adders, multipliers, shifters, and comparators. The datapath takes the
operands from storage units, performs the computation in the combinatorial
units, and returns the results to the storage units during each state.
This process typically takes one or two clock cycles.
Data-flow (looks
more like an Algorithm) modeling is presented in the fourth example.
The FIR digital filter algorithm is simulated and synthesized using VHDL.
A comparison of the coding styles between the RTL modeling and Algorithm
level modeling highlights the different techniques.
-
GCD Calculator (ESD Chapter2: Figure
2.9-2.11)
-
Simple Bridge (ESD Chapter 2: Figure
2.13-2.14)
-
ISA Bus Interface (ESD Chapter 4,
Chapter 6)
-
FIR Digital Filter (DSP Example)
Discussion III: Synopsys Power Analysis
Synopsys tools can be used
to perform Power Analysis for all the VHDL designs. Generally, the better
design has smaller power consumption. On the other hand, improve the power
always means sacrificing other design metrics such as performance, area
size or NRE cost. Therefore, a designer need to balance these metrics to
find the best implementation for the given application and constraints.
Please check out the power
analysis results of Adder, Counter, ISA controller, Bridge controller
and FIR Filter. As we expected, FIR digital filter has the biggest power
consumption because it has a more complex circuit doing DSP computation.
Synopsys power analysis tutorial can be found here.
Discussion IV: Synthesis withTiming Constraints
When we design and simulate
the high-level (either behavior or RTL) code, we only care about design
functionality. However, in VHDL synthesis, the timing and
the functionality of a design must always be considered together.
Therefore, once the design has been synthesized, the second goal of simulation
is to quickly verify that the gate-level implementation meets timing requirements.
We use this idea (coding -> simulation -> synthesis -> simulation) to test
all of the examples in this tutorial.
Another common way is to
apply the timing constrains on the design during synthesis. then the timing
report is checked to see if the slack, which is the required delay minus
the actual delay, is MET or VIOLATED. If VIOLATED, we should go back to
the VHDL code and re-write it to improve timing. The whole design will
be compiled and tested again.
Discussion V: Relationship between Area and Timing
During Synopsys synthesis,
ordinary combinational logic will go through several of what are known
as mapping optimizations. In a normal optimization, the synthesis tool
will optimize in relation to the set constrains. It is usual to talk about
moving along a "banana curve" on the area and time axes. This means that
the tougher the timing constrains, the larger the design will be, and vice
versa. The results from two different synthesis constrains applied on the
same design are shown below.
General-Purpose Processor Design
(ESD Book Chapter 3, Figure 3.15)
As indicated in
the previous part, an Application Specific Integrated Circuit (ASIC) is
specified with behavior descriptions which are presented in the form of
particular algorithm or flowchart. A general purpose processor, on the
other hand, is specified completely by its instruction set (IS).
A sequence of instructions is required for the computation of a mathematical
expression or any other similar computational task. To illustrate the whole
procedure, a simple
Pseudo-Microprocessor
model is used which contains seven instructions (ESD book figure 3.7).
The RT level design method from previous examples is used again to construct
this microprocessor. The CPU will fetch, decode, and execute
each instruction in order to get the final result.
For
test purposes, a short program (sequential instructions) is loaded into
the memory. After execution, this program will obtain 10 Fabonacci
Numbers, and store the results into specific memory address. The design
was implemented using Active-HDL and Synopsys Design Compiler. (Please
note that PC.vhd need a little modify to get correct synthesis result.
Just a practice for the reader.)
Discussion V: VHDL vs. Verilog
There are now two industry
standard hardware description languages, VHDL and Verilog. It is important
that a designer knows both of them although we are using only VHDL in class.
Verilog is easier to understand and use. For several years it has been
the language of choice for industrial applications that required both simulation
and synthesis. It lacks, however, constructs needed for system level specifications.
VHDL is more complex, thus difficult to learn and use. However it offers
a lot more flexibility of the coding styles and is suitable for handling
very complex designs. Here is a great
article to explain their difference and tradeoffs.
Appendix: Modeling a real industry chip
- HD 6402
(ESD Chapter 4)
I. Specification
of HD 6402
II. Behavior Modeling of
UART Transmitter
(1) Behavior Code
(2) Gate-level design
(3) Test Benches - 1,
2,
3
(4) Synopsys Simulation
Case#1: one 8-bit word,
1 start, 2 stops, and even parity, or Data=11000101, Control Word=11011.
( Gate-level Simulation
)
Case#2: three 5-bit words,
1 start, 1 stop, and no parity, or Data=11010 & 00101 & 10001,
Control Word=00100. ( Gate-level Simulation
)
Case#3: two 6-bit words,
1 start, 2 stops, and odd parity, or Data=110010 & 101101, Control
Word=01000. ( Gate-level Simulation
)
III. Behavior Modeling
of UART Receiver
(1) Behavior Code
(2) Gate-level design
(3) Test Benches - 1,
2,
3
(4) Synopsys Simulation
Case#1: two 6-bit words,
1 start, 2 stops, and even parity, (Data=111001 & 100101, Control Word=01101).
( Gate-level Design Simulation
)
Case#2: one 8-bit words,
1 start, 1 stop, and odd parity, (Data=10111001, Control Word=11000). (
Gate-level Design Simulation
)
Case#3: three 5-bit words,
1 start, 1 stop, and no parity, (Data=01001 & 01110 & 00100, Control
Word=00010. ( Gate-level Design Simulation
)
IV. Structural Modeling
of HD-6402
(1) Behavior Code
(2) Gate-level design
(3) Test Bench
(4) Synopsys Simulation
Created by Weijun
Zhang (weijun_92507@yahoo.com)
at UC, Riverside, 06/2001
|
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