IMPLEMENTATION STRATEGIES FOR DIGITAL ICS

GROUP MEMBERS● NAYYAR ● SIDRA● MAJIDA

TABLE OF CONTENTS

● INTRODUCTION

● VLSI DESIGN CYCLE

● MOORE’S LAW

● PRODUCTIVITY GROWTH RATE

● DESIGN IMPLEMENTATION STRATEGIES

● COMPARISON

● DISCUSSION

VLSI

VLSI – Very Large Scale IntegrationRefers to the many fields of electrical and computer engineering that deal with the analysis and design of very dense electronic integrated circuits

VLSI DESIGN CYCLE

SYSTEM SPECIFICATION

FUNCTION DESIGN

LOGIC DESIGN

CIRCUIT DESIGN

PHYSICAL DESIGN

DESIGN VERIFICATION

FABRICATION

PACKAGING,TESTING AND DEBUGGING

A SIMPLE PROCESSOR

MEMORY

DATAPATH

CONTROL

INPU

T-OU

TPUT

BUILDING BLOCKS FOR DIGITAL ARCHITECTURE

Arithmetic unitBit-sliced data path ( Adder, comparator, shifter, multiplier etc.) Memory- RAM, ROM, Buffers, Shift registers

Control- Finite state machine (PLA, random logic.)- Counters

Interconnect- Switches- Arbiters- Bus

STEPS TO CONSTRUCTION IN VLSI

• Start with a design team• Then move to a design hierarchy• Digital design is usually based on some type of Hardware Description Language (HDL)• Choice is based on the following: Design complexity Timing requirements Area requirements Power requirements Project Schedule and Resources

Gordon Moore (Intel)The number of transistors on a chip would double about every 18 months

MOORE’S LAW

Design productivity is usually very low. Typically 10 to 20 transistors per day, per designer.

58%/Yr. compoundComplexity growth rate

21%/Yr. compoundProductivity growth rate

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PRODUCTIVITY GROWTH RATE

DESIGN IMPLEMENTATION

IC designers have two options to implement a circuit block: I. Synthesis / Auto place and route (ASIC)

II. Custom circuit design / Custom Layout (Full Custom)

DIGITAL DESIGN

CUSTOM SEMI CUSTOM

FLOW DIAGRAM OF DESIGN IMPLEMENTATION

FULL CUSTOM DESIGN

INTRODUCTION FULL CUSTOM DESIGN

BASIC DESIGN METHOD

COST

TIME

RIGOROUS CIRCUITS

EXAMPLE

SEMI-CUSTOM

INTRODUCTION

In digital CMOS VLSI, full-custom design is rarely used due to the high labor cost.

Exceptions to this include the design of high-volume products such as memory chips, high-performance microprocessors and FPGA masters.

The standard-cells based design is often called semi custom design. The cells are pre-designed for general use and the same cells are

utilized in many different chip designs.

Problem: Full Custom design has long iteration time and upfront planning:

• Circuit designer has to plan every single detail of the block• Mask designer has to draw every polygon of the layout before we

can have fully routed design

Semi-custom flow uses an iterative approach:

• At any point during this process we can stop and have a fully routed design

• Result: better trade off between ‘how much to optimize’ vs. ‘how quick to finish’

• Extreme usage case:

DESIGN IMPLEMENTATION(CONT.)

STANDARD CELL BASED DESIGN

INTRODUCTION One of the most prevalent custom design styles. Also called semi-custom design style. Requires developing full custom mask set. Basic idea: All of the commonly used logic cells are developed, characterized, and stored in a standard cell library. A typical library may contain a few hundred cells. Inverters, NAND gates, NOR gates, complex AOI, OAI gates, D-latches, and flip-flops.

CHARACTERISTIC OF THE CELLS

Each cell is designed with a fixed height. To enable automated placement of the cells, and routing of inter-cell

connections. A number of cells can be abutted side-by-side to form rows. The power and ground rails typically run parallel to upper and lower

boundaries of cell. Neighboring cells share a common power and ground bus. nMOS transistors are located closer to the ground rail while the pMOS

transistors are placed closer to the power rail. The input and output pins are located on the upper and lower boundaries of

the cell.

STANDARD CELL

FLOORPLAN FOR STANDARD CELL DESIGN

Inside the I/O frame which is reserved for I/O cells, the chip area contains rows or columns of standard cells.

Between cell rows are channels for dedicated inter-cell routing. Over-the-cell routing is also possible. The physical design and layout of logic cells ensure that :1. When placed into rows, their heights match. 2. Neighboring cells can be abutted side-by-side, which provides natural

connections for power and ground lines in each row.

GATE ARRAY

INTRODUCTION

In view of the fast prototyping capability, the gate array (GA) comes after the FPGA.

Design implementation of FPGA chip is done with user programming, Gate array is done with metal mask design and processing. Gate array implementation requires a two-step manufacturing process:

a) The first phase, which is based on generic (standard) masks, results in an array of uncommitted transistors on each GA chip.b) These uncommitted chips can be customized later, which is completed by defining the metal interconnects between the transistors of the array.

CONTD.

The GA chip utilization factor is higher than that of FPGA. Chip speed is also higher. More customized design can be achieved with metal mask designs. Current gate array chips can implement as many as hundreds of

thousands of logic gates.

FIELD PROGRAMMABLE GATE ARRAY (FPGA)

INTRODUCTION

User / Field Programmability. Array of logic cells connected via routing channels. Different types of cells: Special I/O cells. Logic cells. Mainly lookup tables (LUT) with associated registers. Interconnection between cells: Using fuse switches. Using antifuse elements.

CLB FUNCTIONALITIES Two 4-input function generators Implemented using Lookup Tables using 16x1 RAM. Can also implement 16x1 memory. Two Registers Each can be configured as flip-flop or latch. Independent clock polarity. Synchronous and asynchronous Set / Reset

DESIGN FLOW

FPGA DESIGN FLOW

Design Entry In schematic, VHDL, or Verilog. Implementation Placement & Routing Bit stream generation Analyze timing, view layout, simulation, etc. Download Directly to Xilinx hardware devices with unlimited reconfigurations.

IMPLEMENTATION OF A DESIGN IN FPGA

COMPARISON AMONG VARIOUS DESIGN STYLES

DISCUSSION

● Designers tend to think of a hard boundary between the two flows: ● Block containing some non-static circuits (SRAM or dynamic) end up as

Full Custom● In reality only a portion of that block is non-static● Most blocks also contain some standard CMOS circuits (data path and

control logic)● These portions can be built as ASIC or tiled standard cells.

Goal: allow designers to mix and match aspect from both ASIC and Full Custom approaches to improve productivity

Q&A

● Thank You!