Jai Henwood

Introduction to VHDL

ECE 5571

4/24/03

Dr. Veton Këpuska

Goals and Objectives

- The goal of my project was to further my knowledge of VHDL

- Provide some history of the development of VHDL

- Introduce VHDL syntax and a few key concepts about VHDL.

Overview

History of VHDL

Go through a few example of VHDL:

- 8 bit shifter

- 4 to 1 Mux

- State machine

VHDL

• VHDL is an International IEEE Standard Specification Language (IEEE 078-2001) for Describing Digital Hardware Used by Industry Worldwide

• VHDL stands for VHSIC (Very High Speed Integrated Circuit) Hardware Description Language

History of VHDL

• In the 1970’s the initial idea for a Hardware Description Language was discussed.

• But, the VHSIC program wasn’t launched until 1980.

• The goal was to create a common language that would shorten the time from concept to implementation for hardware design.

History of VHDL

• In July 1983 the contract was awarded to create VHDL by the Department of Defense

- Intermetrics

- IBM

- Texas Instruments

August 1985 Version 7.2 was released

History of VHDL

• It was first in December of 1987 that IEEE standardized VHDL 1076-1987

• VHDL also became an ANSI standard in 1988

• In September of 1993 VHDL was re-standardized to clarify and enhance the language

History of VHDL

• In 1998 a committee convened to update the VHDL-1993 standard.

• In 2001 IEEE revised the 1993 standard and the new standard today is 1076-2001

Other Languages besides VHDL

• On the way to standardizing VHDL other languages were devolved and some are still used

• AHPL A Hardware Programming Language

• CDL computer design Language

• CONLAN CONsensus LANguage

Other Languages besides VHDL

• IDL Interactive Design Language

• ISPS Instruction Set Processor Specification

• TEGAS Test Generation And Simulation

• TI-HDL TI Hardware Description Language

• Verilog Gateway Design Automation: 1985

• ZEUS A HDL by GE cooperation

Basic VHDL Terms • Entity - All designs are expressed in terms of entities. An entity is the

most basic building block in a design. The uppermost level of the design is the top-level entity. If the design is hierarchical, then the top-level description will have lower-level descriptions contained in it. These lower-level descriptions will be lower-level entities contained in the top-level entity description.

• Architecture - All entities that can be simulated have an architecture description. The architecture describes the behavior of the entity. A single entity can have multiple architectures. One architecture might be behavioral, while another might be a structural description of the design.

• Configuration - A configuration statement is used to bind a component instance to an entity-architecture pair. A configuration can be considered as a parts list for a design. It describes which behavior to use for each entity, much like a parts list describes which part to use for each part in the design.

4 to 1 MUX

Mux

Sel

Inputs OutputsABCD

Q

4 to 1 MUX

library ieee; use ieee.std_logic_1164.all; -- 4 to 1 mux entity mymux is port (A, B, C, D : in

std_logic_vector(0 to 3); Sel : in

std_logic_vector ( 0 to 1 ); Q : out

std_logic_vector(0 to 3) ); end mymux;

Library you need to include

Commententity “name_of_entity” isport(all your input and output

signals);

end “name_of_entity”;

4 to 1 MUXarchitecture mux4 of mymux is constant delay : time := 100 ns; begin mux_proc : process ( A, B, C, D, Sel ) variable temp : std_logic_vector(0 to 3); begin case Sel is when "00" => temp := A; when "01" => temp := B; when "10" => temp := C; when "11" => temp := D; when others => temp := "XXXX"; end case; Q <= temp after delay; end process mux_proc; end mux4;

mux4 is the name of my architecture

Declaration of time as a constant

begin the architecture

Process mux_proc is initialized

Variable temp is declared

begin process mux_proc

case on the Sel singal

If anything else then temp is don’t care

If there was a delay on the mux

end process

end architecture

TestBench for 4 to 1 MUX

library ieee;

use ieee.std_logic_1164.all;

entity mux_testbench is

end mux_testbench;

architecture test of mux_testbench is

signal A : std_logic_vector(0 to 3);

signal B: std_logic_vector(0 to 3);

signal C: std_logic_vector(0 to 3);

signal D: std_logic_vector(0 to 3);

signal Sel: std_logic_vector(0 to 1);

signal Q: std_logic_vector(0 to 3);

still have to begin with entity

end the entity

test is the name of the architecture

the signals have to be the same as the port signals in your entity declaration of the program you want to test.

TestBench for 4 to 1 MUX

component mymux is port (A, B, C, D : in

std_logic_vector(0to3); Sel : in std_logic_vector ( 0 to 1 );

Q : out std_logic_vector(0 to 3));end component;begintest_mux: mymux port map( A => A ,

B => B, C => C , D => D, Sel => Sel, Q => Q );

This component declaration tells us that we have 5 inputs A,B,C,D and Sel; we also have one output Q

The port map is used to link the signal A from your VHDL program to the signal A in your testbench

Used incase you want to use different names

TestBench for 4 to 1 MUX

test_process: processbegin A<="1010";

B<="1011"; C<="1100";D<="1101";Sel<="00";

wait for 500 ns;Sel<="01";

wait for 500 ns;Sel<="10";

wait for 500 ns;Sel<="11";

wait for 500 ns; end process;end test;

initialize your signal

when Sel is 00 it should let A come through on Q with a delay of 100ns

when Sel is 01 Q should be B

end the process test_process

end the test

8- Bit Shifter

Shifter

clk

rstload

data 8 bits

8 bits

Q

Input Output

8 – Bit Shifter

-- 8-Bit Shift Register

library ieee;

use ieee.std_logic_1164.all;

entity shift is

port (CLK, RST, LOAD : in bit;

Data : in bit_vector(0 to 7);

Q : out bit_vector(0 to 7));

end rotate;

Comment

Library you need to include

entity “name_of_entity” is

port(all your input and output signals);

end “name_of_entity”;

8 – bit shifterarchitecture shifter1 of shift is

begin

reg : process(RST, CLK)

variable reg : bit_vector(0 to 7);

begin

if(RST = '1') then

reg := "00000000";

elsif(CLK = '1' and CLK'event) then

if(LOAD = '1') then reg := Data;

. else

reg := reg(1 to 7) & reg(0);

end if;

end if;

Q <= reg;

end process;

end shifter1;

shifter1 is the name of my architecture

shift is the name of my entity

reg is the name of my process

reg is also a local variable

Begin the process reg

if RST clear reg

else wait for a clock event then

reg = Data if LOAD signal is high

a bit shift

R gets the value of reg

end process

end architecture

TestBench for 8 bit shifter

library ieee;

use ieee.std_logic_1164.all;

entity shift_testbench is

end shift_testbench;

architecture test of shift_testbench is

Signal CLK : bit;

signal RST : bit;

signal LOAD : bit;

signal Data :bit_vector(0 to 7);

signal Q :bit_vector(0 to 7);

still have to begin with entity

end the entity

test is the name of the architecture

the signals have to be the same as the port signals in your entity declaration of the program you want to test.

TestBench for 8 bit Shifter

component shift is port (CLK, RST, LOAD : in bit; Data : in bit_vector(0 to 7); Q : out bit_vector(0 to 7));end component;begintest_shift: shift port map(

CLK => CLK , RST => RST,

LOAD => LOAD, Data => Data,

Q => Q);

port has all the input and output signals used in the VHDL program

The port map is used to link the signal Data from your VHDL program to the signal Data in your testbench

TestBench for 8 bit Shifterclock_gen : process

begin

clk <= '0', '1' after 50 ns;

wait for 100 ns;

end process;

RST<='1', '0' after 200 ns;

test_process: process

begin

Data<="00000001";

LOAD<='1', '0' after 400 ns wait;

end process;

end test;

creates a clock signal

this is a 10 MHz clock

end the process clock_gen

this is only done once to set it off

data is give a value

every 400 ns Data is reset to “00000001”

end the process test_process

end the test

System Overview

Programmable Logic Device

DIP Switches

Strobe (pushbutton)

An open switch will

have a value of ‘1’ and

closed switch value

of ‘0’

LED

•PLD monitors data stream looking for commands•PLD responds to On command by turning LED On •PLD responds to Off command by turning LED Off

Functional Overview

On Off

“AB” “50”

Details

•PLD receives 4 bits (one Hex digit) of data at a time

•Data is ready on rising edge of Strobe

•Output ‘1’ to LED for ON / ‘0’ for OFF

Programs Used

• To enter VHDL text nedit

• To compile ncvhdl made by Cadence

• To elaborate ncelab made by Cadence

• To simulate ncsim made by Cadence

• To create html and state diagrams

visual_elite made by Innoveda

Why use VHDLCompared to schematic diagrams

- More standardized & portable

- Easier to create

- More concise

- Easier to comment

- Easier to read

- Faster to simulate

schematics/layouts can be automatically generated from VHDL by logic synthesis

Reasons for Modeling

• requirements specification

• documentation

• testing using simulation

• formal verification

• synthesis