VHDL Tutorial: Learn by
Example
-- by Weijun Zhang
If we hear, we forget; if we see, we remember; if we do, we understand.
-- Proverb
ESD book | Dalton Project | VHDL Reference | Synopsys Tutorial | ActiveHDL Tutorial | Xilinx Tutorial
Table of Contents
Foreword
Basic Logic Gates Latch and Flip-Flops
Combinational Logic Sequential Logic Design
Design
Typical Combinatinal Typical Sequential
Logic Components 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;".
Behavior Behavior Behavior Behavior
Driver Inverter
Code Simulation Code Simulation
Behavior Behavior Behavior Behavior
OR gate NOR gate
Code Simulation Code Simulation
AND Behavior Behavior NAND Behavior Behavior
gate Code Simulation gate Code Simulation
XOR Behavior Behavior XNOR Behavior Behavior
gate Code Simulation gate Code Simulation
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.
Combinational Behavior Test Behavior Synthesis Gate-level
Logic Code Bench Simulation Schematic Simulation
Behavior Test Behavior Synthesis Gate-level
Tri-State Driver
Code Bench Simulation Schematic Simulation
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:
Signal/Variable Behavior Behavior
Example Code Simulation
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.
Behavior Test Behavior Synthesis Gate-level
Multiplexor Code Bench Simulation Schematic Simulation
Behavior Test Behavior Synthesis Gate-level
Decoder
Code Bench Simulation Schematic Simulation
Behavior Test Behavior Synthesis Gate-level
Adder
Code Bench Simulation Schematic Simulation
Behavior Test Behavior Synthesis Gate-level
Comparator
Code Bench Simulation Schematic Simulation
Behavior Test Behavior Synthesis Gate-level
ALU
Code Bench Simulation Schematic Simulation
Behavior Test Behavior Synthesis Gate-level
Multiplier
Code Bench Simulation Schematic Simulation
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.
Simple Behavior Test Behvaior Gate-level Gate-level
Latch Code Bench Simulation Implementation Simulation
D Flip- Behavior Test Behavior Gate-level Gate-level
Flop Code Bench Simulation Implementation Simulation
JK Flip- Behavior Test Behavior Gate-level Gate-level
Flop Code Bench Simulation Implementation Simulation
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.
Behavior Test Behavior Gate-level Synthesis Structural
Register Code Bench Simulation Implementation Schematic Simulation
Shift Behavior Test Behavior Gate-level Synthesis Structural
Register Code Bench Simulation Implementation Schematic Simulation
Behavior Test Behavior Gate-level Synthesis Structural
Counter
Code Bench Simulation Implementation Schematic Simulation
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.
FSM Behavior Test Behavior Gate-level Synthesis Gate-level
Model Code Bench Simulation Implementation Schematic Simulation
o Memories (ESD Chapter 5)
RAM Behavior Test Behavior Gate-level Synthesis Gate-level
Module Code Bench Simulation Implementation Schematic Simulation
ROM Behavior Test Behavior Gate-level Synthesis Gate-level
Module Code Bench Simulation Implementation Schematic Simulation
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.
GCD Behavior RTL Code
Comparison
Caculator Code (FSM+D)
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.
o GCD Calculator (ESD Chapter2: Figure 2.9-2.11)
FSMD RTL Test RTL Code Gate-level Synthesis Gate-level
Modeling Code Bench Simulation Implementation Schematic Simulation
FSM +
RTL Test RTL Code Gate-level Synthesis Gate-level
Datapath
Code Bench Simulation Implementation Schematic Simulation
Modeling
o Simple Bridge (ESD Chapter 2: Figure 2.13-2.14)
FSMD RTL Test RTL Code Gate-level Synthesis Gate-level
Modeling Code Bench Simulation Implementation Schematic Simulation
FSM +
RTL Test RTL Code Gate-level Synthesis Gate-level
Datapath
Code Bench Simulation Implementation Schematic Simulation
Modeling
o ISA Bus Interface (ESD Chapter 4, Chapter 6)
FSM +
RTL Test RTL Code Gate-level Synthesis Gate-level
Datapath
Code Bench Simulation Implementation Schematic Simulation
Modeling
o FIR Digital Filter (DSP Example)
Data-
Behavior Test Behavior Gate-level Synthesis Gate-level
Flow
Code Bench Simulation(1,2) Implementation Schematic Simulation
Modeling
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 with Timing 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.
Behavior Sythesis Script Timing
Counter
Code File Report
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.
FIR Sample Comparison Banana
filter Synthesis Script Table Curve
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.)
Top Level Design Test Bench of CPU
Structural Code Hierarchy Design
microprocessor.vhd cpuhierarchy.jpg TB_mp.vhd
Control Unit Block Diagram Data Path
ctrl_unit.vhd cpublock.jpg datapath.vhd
controller.vhd
Memory smallmux.vhd reg_file.vhd
PC.vhd
memory.vhd alu.vhd obuf.vhd
IR.vhd bigmux.vhd
Synopsys Sythesis Script Files
cpusyn.scr
syn_ctrl_unit.inc, syn_PC.inc, syn_IR.inc, syn_datapath.inc,
syn_reg_file.inc
syn_alu.inc, syn_memory.inc
syn_controller.inc, syn_obuf.inc, syn_bigmux.inc,
syn_smallmux.inc
Behavor Simulation Gate-Level
Simulation Waveform Simulation
Script File cpusim1.jpg Script File
cpusim.scr cpusim2.jpg cpugatesim.scr
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
