EC2354 -VLSI DESIGN

Syllabus

UNIT I CMOS TECHNOLOGY

A brief History-MOS transistor, Ideal I-V characteristics, C-V characteristics, Non ideal IV effects, DC transfer characteristics - CMOS technologies, Layout design Rules, CMOS process enhancements, Technology related CAD issues, Manufacturing issues

 

UNIT II CIRCUIT CHARACTERIZATION AND SIMULATION

Delay estimation, Logical effort and Transistor sizing, Power dissipation, Interconnect, Design margin, Reliability, Scaling- SPICE tutorial, Device models, Device characterization, Circuit characterization, Interconnect simulation

UNIT III COMBINATIONAL AND SEQUENTIAL CIRCUIT DESIGN

Circuit families –Low power logic design – comparison of circuit families – Sequencing static circuits, circuit design of latches and flip flops, Static sequencing element methodology- sequencing dynamic circuits – synchronizers

 

Syllabus Contd.,

UNIT IV CMOS TESTING

Need for testing- Testers, Text fixtures and test programs- Logic verification- Silicon debug principles- Manufacturing test – Design for testability – Boundary scan

UNIT V SPECIFICATION USING VERILOG HDL

Basic concepts- identifiers- gate primitives, gate delays, operators, timing controls, procedural assignments conditional statements, Data flow and RTL, structural gate level switch level modeling, Design hierarchies, Behavioral and RTL modeling, Test benches, Structural gate level description of decoder, equality detector, comparator, priority encoder, half adder, full adder, Ripple carry adder, D latch and D flip flop.

 

References

TEXTBOOKS:

• Weste and Harris: CMOS VLSI DESIGN (Third edition) Pearson Education, 2005

• Uyemura J.P: Introduction to VLSI circuits and systems, Wiley 2002.

•Samir Palnitkar; Verilog HDL - Guide to Digital design and synthesis, III edition, Pearson Education, 2003

REFERENCES:

•D.A Pucknell & K.Eshraghian Basic VLSI Design, Third edition, PHI, 2003

• Wayne Wolf, Modern VLSI design, Pearson Education, 2003

• M.J.S.Smith: Application specific integrated circuits, Pearson Education, 1997

•J.Bhasker: Verilog HDL primer, BS publication,2001

•Ciletti Advanced Digital Design with the Verilog HDL, Prentice Hall of India, 2003

WHAT IS VLSI

WHY VLSI

VLSI

VLSI stands for "Very Large Scale Integration". This is the field which involves packing more and more logic devices into smaller and smaller areas.

Evolution of IC’s

Vacuum tubes

ULSI –Ultra large scale integration – more than thousands of gates in a single package.

Many other factors grow exponentially

Ex: clock frequency, processor performance

ADVANTAGES OF VLSI DESIGN

The most important message here is that the logic complexity per chip has been (and still is) increasing exponentially. The monolithic integration of a large number of functions on a single chip usually provides:

•Less area/volume and therefore, compactness

•Less power consumption

•Less testing requirements at system level

•Higher reliability, mainly due to improved on-chip interconnects

•Higher speed, due to significantly reduced interconnection length

•Significant cost savings

VLSI Design Flow

Design Specification

•specifications describe abstractly the functionality, interface and overall architecture of the digital circuit to be designed.

•to describe the circuit in terms of its behavior.

Behavioral Description and RTL Description

•The design at the behavioral level is to be elaborated in terms of known and acknowledge functional blocks.

•Written with HDLs(Hardware Description Language)

•Behavioral converted to Register Transfer Level(RTL) description.

Functional Verification and Testing

• to check whether all the functions are carried out as expected and rectify them.

•All such activities are carried out by the simulation tool.

•The tool also has an editor to carry out any corrections to the source code.

•Simulation involves testing the design for all its functions, functional sequences, timing constraints and specifications

Logic Synthesis

•The hardware realization is carried out by a synthesis tool.

•Logic synthesis tools convert the RTL description to a gate-level netlist.

•A gate-level netlist is a description of the circuit in terms of gates and connections between them.

•Logic synthesis tool ensure that the gate level netlist meets timing, area and power specifications.

Physical Design

FPGA-Field Programmable Gate Array

System Partitioning

•The design is partitioned into convenient compartments or functional blocks.

•The main objective of system partitioning is to minimize the number of external connections between the functional blocks.

Floor Planning

•Calculate the sizes of all the functional blocks and assign them locations.

•The main objective is to keep the highly connected blocks physically close to each other.

• Blocks with I/O pins are kept close to the periphery; those which interact frequently or through a large number of interconnections are kept close together, and so on.

Placement

•Assign the interconnect areas and the location of all the logic cells within the flexible blocks• Minimize all the critical net delays.• Minimize power dissipation.• Minimize cross talk between signals.• Minimize the total estimated interconnect length.• Meet the timing requirements for critical nets.• Minimize the interconnect congestion

Routing

•routing step determines the channels to be used for each interconnect

•Minimize total interconnect length and area.

• Minimize the number of layer changes that the connections have to make.

• Minimize the delay of critical paths

Post layout Simulation & Verification

•The performance specifications like silicon

area, power consumed, path delays, etc., can be computed.

•Equivalent circuit can be extracted at the component level and performance analysis carried out.

• This constitutes the final stage called verification

Design flow -summary• Design Specification – Describes abstractly functionality, interface

& overall architecture of digital circuit.• Behavioral Description – Design in terms of functionality,

performance compliance to standards, & other high level issues. Written with HDLs(Hardware Description Language)

• RTL Description (HDL) – Behavioral converted to Register Transfer Level(RTL) description.

• Logic synthesis / Timing verification – Converts RTL to gate – level net list.

• Gate level net list – Describes circuits in terms of gates & connections between them.

• Logical verification & testing – ensures gate level net list meets timing, area & power specifications.

• Floor planning ,Automatic place & Route, Physical Layout – Automatic place & route which creates layout.

• Layout verification ,Implementation – Layout verification & fabrication in to chip.

*Bushnell: Digital Systems Design

Lecture 7*

FPGA Modern Design Methodology

always

mumble

mumbleblahblahSynthesizable Verilog

Synthesis

Tech

nology

Mapping

LE 1LE 2

Place and Route

gates, gates, gates, …

Logic Elements in FPGA ChipBushnell: Digital Systems Design Lecture 7

HDL (Hardware Description Language)

PALASM,ABEL & CUPL•PALASM (from “PAL assembler”) language was used to express boolean equations . The PALASM compiler was written in FORTRAN IV on an IBM 370/168.

•The Advanced Boolean Expression Language (ABEL) is a Hardware description language and an associated set of design tools for programming PLDs.

-It was created in 1983 by Data I/O Corporation, in Redmond, Washington.

-ABEL includes both concurrent equation and truth table logic formats as well as a sequential state machine description format.

•Logical Devices, Inc. released the Universal Compiler for Programmable Logic (CUPL), which runs under MSDOS on the IBM PC.

HDL (Hardware Description Language)

•All modern digital designs start with a designer writing a hardware description of the IC (using HDL or) in Verilog/VHDL.

•A Verilog or VHDL program essentially describes the hardware (logic gates, Flip-Flops, counters etc) and the interconnect of the circuit blocks and the functionality.

•VHDL stands for "VHSIC Hardware Definition Language", where VHSIC stands for "Very High Speed Integrated Circuit".

•VHDL & VerilogHDL is an IEEE standard(it reduces confusion and makes interfaces between tools, companies, and products easier).

What is Verilog?

• Hardware Description Language (HDL)

• Developed in 1984

• Standard: IEEE 1364, Dec 1995

• Verilog was created by Phil Moore in 1983-4 at

Gateway Design Automation.

• The syntax is deliberately close to that of C.

History Of Verilog HDL

MAJOR FEATURES

• Built in primitive gates – AND,OR,NAND etc..• Allows flexibility for creating user defined

primitives (UDP) • Built in Switch level modeling primitive gates

PMOS,NMOS,CMOS etc..• Four different styles Or abstraction level • Gate level – uses primitive

gates(AND,OR,NAND )• Switch level – uses pMOS, nMOS,

CMOS• Dataflow – uses assignment (LHS =

RHS)• Behavioral – uses 2 structural

procedural statement always & initial

•

•Description of digital systems only

Basic Limitation of Verilog

Abstraction Levels in Verilog

Behavioral

RTL

Gate

Layout (VLSI)

Our focus

Design Methodologies

• 1.Top – down Design• 2. Bottom – up Design• Top – down : Define top level block, identify sub- block,

further subdivide sub-blocks into leaf cells (cells that cannot be further subdivided.)

• define the top-level block and identify the sub-blocks necessary to built the top-level block.

• further subdivide the sub-blocks until to leaf cells, which are the cells that cannot be further divided

Bottom-Up design

In a bottom-up design methodology, first identify the building blocks that are available and built bigger cells, using these building blocks. These cells are then used for higher-level blocks until we built the top-level block in the design

Example

MODULES

• Basic building block.

• Can be collection of lower – level design blocks or an element.

• Describes the functionality on structure of a design & also ports through which it communicates.

User Identifiers

• Formed from {[A-Z], [a-z], [0-9], _, $}, but ..

• .. can’t begin with $ or [0-9]• myidentifier • m_y_identifier • 3my_identifier • $my_identifier • _myidentifier$

• Case sensitivity• myid Myid

Comments

• // The rest of the line is a comment

• /* Multiple line• comment */

• /* Nesting /* comments */ do NOT work

*/

Operators

Numbers in Verilog (i)

• <size>’<radix> <value>

• 8’h ax = 1010xxxx• 12’o 3zx7 = 011zzzxxx111

No of bits

Binary b or BOctal o or ODecimal d or DHexadecimal h or H

Consecutive chars 0-f, x, z

Numbers in Verilog (ii)

• You can insert “_” for readability• 12’b 000_111_010_100 • 12’b 000111010100• 12’o 07_24

• Bit extension• MS bit = 0, x or z extend this

• 4’b x1 = 4’b xx_x1

• MS bit = 1 zero extension• 4’b 1x = 4’b 00_1x

Represent the same number

Numbers in Verilog (iii)

• If size is ommitted it • is inferred from the value or• takes the simulation specific number of bits or• takes the machine specific number of bits(at least 32)

• If radix is ommitted too .. decimal is assumed• 15 = <size>’d 15

Data Types

Nets (i)

• Can be thought as hardware wires driven by logic

• Equal z when unconnected• Various types of nets

• wire

• wand (wired-AND)• wor (wired-OR)• tri (tri-state)

• In following examples: Y is evaluated, automatically, every time A or B changes

Nets (ii)

AB

Ywire Y; // declarationassign Y = A & B;

B

A

Y

wand Y; // declarationassign Y = A; assign Y = B;

wor Y; // declarationassign Y = A; assign Y = B;

A Y

drtri Y; // declarationassign Y = (dr) ? A : z;

Registers

• Variables that store values• Do not represent real hardware but ..• .. real hardware can be implemented with

registers• Only one type: reg

• reg A, C; // declaration• // assignments are always done inside a procedure

• A = 1;

• C = A; // C gets the logical value 1

• A = 0; // C is still 1• C = 0; // C is now 0

• Register values are updated explicitly!!

Vectors

• Represent buses• wire [3:0] busA;• reg [1:4] busB; • reg [1:0] busC;

• Left number is MS bit• Slice management

• busC[1] = busA[2];

• busC[0] = busA[1];

• Vector assignment (by position!!)• busB[1] = busA[3];

• busB[2] = busA[2];• busB[3] = busA[1];• busB[4] = busA[0];

busB = busA;

busC = busA[2:1];

Integer & Real Data Types

• Declaration• integer i, k;

• real r;

• Use as registers (inside procedures)• i = 1; // assignments occur inside procedure

• r = 2.9;• k = r; // k is rounded to 3

• Integers are not initialized!!• Reals are initialized to 0.0

Time Data Type

• Special data type for simulation time

measuring

• Declaration

• time my_time;

• Use inside procedure

• my_time = $time; // get current sim time

• Simulation runs at simulation time, not real

time

Arrays (i)

• Syntax• integer count[1:5]; // 5 integers• reg var[-15:16]; // 32 1-bit regs• reg [7:0] mem[0:1023]; // 1024 8-bit regs

• Accessing array elements• Entire element: mem[10] = 8’b 10101010;• Element subfield (needs temp storage):

• reg [7:0] temp;• ..• temp = mem[10];• var[6] = temp[2];

Arrays (ii)

• Limitation: Cannot access array subfield or

entire array at once• var[2:9] = ???; // WRONG!!

• var = ???; // WRONG!!

• No multi-dimentional arrays• reg var[1:10] [1:100]; // WRONG!!

• Arrays don’t work for the Real data type• real r[1:10]; // WRONG !!

Strings

• Implemented with regs:• reg [8*13:1] string_val; // can hold up to 13

chars

• ..

• string_val = “Hello Verilog”;

• string_val = “hello”; // MS Bytes are filled with 0

• string_val = “I am overflowed”; // “I ” is truncated

• Escaped Characters:• \n newline

• \t tab

• %% %

• \\ \

• \“ “

Parameters

System Tasks

• $display(“..”, arg2, arg3, ..); much like printf(), displays formatted string in std output when encountered

• $monitor(“..”, arg2, arg3, ..); like $display(), but .. displays string each time any of arg2, arg3, .. Changes

• $stop; suspends sim when encountered• $finish; finishes sim when encountered• $fopen(“filename”); returns file descriptor (integer); then, you can

use $fdisplay(fd, “..”, arg2, arg3, ..); or $fmonitor(fd, “..”, arg2, arg3, ..); to write to file

• $fclose(fd); closes file• $random(seed); returns random integer; give her an integer as a

seed

Always written inside procedures

$display & $monitor string format

Compiler Directives

•`include “filename” inserts contents of file into current file; write it anywhere in code ..

•`define <text1> <text2> text1 substitutes text2;•e.g. `define BUS reg [31:0] in declaration part:

`BUS data;

•`timescale <time unit>/<precision>•e.g. `timescale 10ns/1ns later: #5 a = b;

50ns

Modules

Ports

Port Connection Rules

Port Connection Rules

Test Bench