hardware description languages ece 3450 m. a. jupina, vu, 2014

Hardware Description Languages

ECE 3450 M. A. Jupina, VU, 2014

Lecture Objective

A brief (very brief) discussion of VHDL and the future of VHDL-AMS and mixed signal ICs.

References:1. Fundamentals of Digital Logic, Sections 2.10 and 4.12 and Appendix A.2. Document files at the course web site


VHDL - VHSIC (Very High Speed Integrated Circuit) Hardware

Description Language


• VHDL is code that is used to describe hardware rather than a program to be executed on a computer.

• For example, other HDLs: Verilog and AHDL

• VHDL is a language used for simulation and synthesis of digital logic.

• A VHDL description of a digital system can be transformed into a gate level implementation. This process is known as synthesis.

VHDL for Combinational Logic


Logic Synthesis: convert a description of a digital system in a Hardware Description Language (HDL) to an implementation technology.

library ieee;use ieee.std_logic_1164.all;

entity majority is port ( A, B, C : in std_logic; Y: out std_logic );end majority;

ARCHITECTURE a of majority is

begin

Y <= (A and B) or (A and C) or (B and C);end a;

HDL description Gates

Synthesis

Logic Synthesis


• VHDL has a reputation as a complex language (it is!)• We will use a small subset of the language for our

purposes• Some VHDL constructs:

– Signal Assignment: A <= B;– Comparisons = (equal), > (greater than), < (less

than), etc.– Boolean operations AND, OR, NOT, XOR– Sequential statements (CASE, IF, FOR)– Concurrent statements (when-else)

VHDL Statements


• Every VHDL model is composed of an entity and at least one architecture .

• Entity describes the interface to the model (inputs, outputs)

• Architecture describes the behavior of the model

• Can have multiple architectures for one entity (we will only use one in this class).

VHDL Combinational Template


entity model_name is port ( list of inputs and outputs ); end model_name; architecture arch_name of model_name is begin concurrent statement 1 concurrent statement 2 ... concurrent statement N;

end arch_name;

All of the text not in italics are VHDL keywords. VHDL is NOT case sensitive. (ENTITY is same as entity is same as EnTiTy).

A VHDL Template for Combinational Logic


library ieee;use ieee.std_logic_1164.all;

entity example is port ( A, B, C : in std_logic; Y: out std_logic );end example;

-- this is the architecture declaration, uses only -- one concurrent statement.

ARCHITECTURE a of example is

begin

Y <= (A and B) or (A and C) or (B and C);end a;

Description Implementation

A VHDL Example of an XOR Gate


• Brown and Vranesic textbook (Sections 2.10 and 4.12 and Appendix A)

• Course Website VHDL overview from Hamblen’s book Dr. Kresch’s VHDL overview VHDL Files from the textbook

VHDL References/Resources


Analog and Mixed Signal VHDL (VHDL-AMS)

• The IEEE 1076.1 language (VHDL-AMS) is a superset of the IEEE Std 1076-1993 (VHDL) that provides capabilities for describing and simulating analog and mixed-signal systems.

• VHDL-AMS was developed to provide the industry with a high-level design language to master future challenges in both mixed digital and analog system design as well as multi-physics applications.

• Currently, VHDL-AMS provides simulation only. Future software could provide synthesis. For example, field-programmable analog arrays (FPAA) are being developed. A FPAA containing many (~20) analog function blocks could be configured as DC level shifters, rectifiers, amplifiers, filters, oscillators, comparators, equalizers, etc.


FPAA Reconfiguration Example

While FPAA technology is very young, and still expensive, it could someday be useful for creating communications products that would work on any standard.

For instance, a cell phone with a re-programmable analog-to-digital converter would search the entire communications band for a signal, then grab the necessary program from memory to make itself into a code-division multiple access (CDMA), time-division multiple access (TDMA), or global system for mobile communications (GSM) phone. Such a phone could be used in Japan, Europe, or the US without modification.


VHDL-AMS Code Example-- VHDL-AMS model of an Analog Schmitt Trigger -- Description -- This Schmitt trigger uses a signal to model a simple hysteresis -- loop. When the input voltage exceeds the high threshold, the internal -- state switches to 5 Volts and when the input drops below the low threshold, -- the state switches to 0 Volts. -- The output is an ideal voltage source which contributes to the output -- terminal OutTerminal. -- LIBRARY DISCIPLINES;LIBRARY IEEE;

USE DISCIPLINES.ELECTROMAGNETIC_SYSTEM.ALL;USE IEEE.MATH_REAL.ALL;

entity AnalogSchmitt is end entity AnalogSchmitt;

architecture Hysteresis of AnalogSchmitt is TERMINAL n1,n2 : ELECTRICAL;


VHDL-AMS Example Continued-- declare a signal to memorize the hysteresis state: SIGNAL State : REAL := 0.0; -- initial state is low QUANTITY vin ACROSS iin THROUGH n1;

-- declare a through branch for the output voltage source quantity vout across iout through n2;

begin -- the architecture consists of two architecture statements: -- a conditional concurrent signal assignment to implement the hysteresis -- a simultaneous statement to implement the equation for the output source vin == 5.0 * sin(2.0 * math_pi*0.5E3*NOW);

-- hysteresis: break State => 0.0 when Vin'above(1.0); -- trigger event when Vin > 1.0 break State => 5.0 when not Vin'above(2.4); -- trigger event when Vin < 2.4

-- output voltage source equation: vout == State'Ramp(1.0e-9,1.0e-9); -- the use of ramp assures that when a discontinuity -- in State arises, it is announced to the simulator end architecture Hysteresis;


hardware description languages ece 3450 m. a. jupina, vu, 2014

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