very large scale integration (vlsi) - german …eee.guc.edu.eg/courses/electronics/elct904 very...

Dr. Ahmed H. Madian-VLSI 1

Very Large Scale Integration (VLSI)

Dr. Ahmed H. [email protected]

Lecture 5


ContentsBody effects & delayWiring techniques

Width and spacingLayer selectionShieldingRepeaters

Transmission GatesTG problems

Array subsystemsGate arrays technologySea-of-gatesStandard cellMacrocell TechnologyFPGA Technology


Body effect & delay

• If A goes from 0 to 1 while B, C and D are 1, then all the intermediate nodes in the pulldown chain have already been discharged and the top MOSFET sees only a small body effect.

• If D goes from 0 to 1 while A, B and C are 1, then the intermediate nodes are all one Vt below Vdd and the upper MOSFETs see a larger body effect.


Wire Engineering

Goal: achieve delay, area, power goals with acceptable noise.Degrees of freedom:

Width SpacingLayerShielding


Wiring techniques

Width and spacingIncreasing width of wire minimize its resistance but increase it’s capacitance platesIncreasing spacing minimize the capacitance without affecting the resistance.The best trade off depends mainly in your design


Wiring techniques (cont.)Layer selection

Modern processes have six or more metal layers.

The lower layers are thin and optimized for a tight routing pitchMiddle layers often slightly thicker for lower resistance and better current-handling capability.Upper layers may be even thicker to provide a low-resistance power grid and fast global interconnect.

I/O pads, clock, power, ground

Metal 6

Interconnect between cells, critical signals

Metal 4/5

Interconnect between cells

Metal 2/3

Interconnect within cells

Metal 1purposeLayer


Power and Ground bounceMetal power-carrying conductors have to be sized for three reasons:

• metal migration• power supply noise• RC delay

General rule:• limit current density• contact replication


Wiring techniquesShielding

Coupling from adjacent lines impacts the delaysCoupling could be avoided if the critical signals that has a switching characteristics shielded with power or ground wires.

VDD a1 a2 GND a3 a4 VDD a1 a2GND VDD GNDa3


Wiring techniques

RepeatersBoth resistance and capacitance increase with wire length lThis increase the delay


Wiring techniquesRepeaters

The delay could be reduced by splitting the wire into N segments and inserting an inverter or buffer called repeater to actively drive the wire.

wwCRRC

Nl 2=Best length between repeaters could be obtained as,

To obtain this delay the inverter should use a nMOS transistor of width

CRRC

Ww

w=


Transmission Gates

TG Circuits: 4 to 1 MUX


TG Circuits: Problemsdifficult to get compact layoutoutputs behave like bi-directional signalsmany TG in series cause large delays


Gate Arrays and Sea-of-Gates

This means to construct a common base array of transistors and personalize the chip by altering the metallization (metal and via masks) that is placed on top of the transistors.


Gate Arrays Technologyprefabricated wafers

I/O stages predefinedregular array of fets and interconnection channelsinterconnection defines functionality

featuressize: 100 - 1M gatesshort turn around timecheap at medium quantitiesUnsuitable for regular structures like RAM, PLA, ALU


Sea-of-Gate Technologyprefabricated wafers

I/O stages predefinedregular array of fets, no reserved interconnection channelsinterconnection defines functionality

featuressize: 100 - 1M gatesshort turn around timecheap at medium quantitiessuitable for regular structures like RAM, PLA, ALU


Standard Cell Technologycomplete fabrication process

predefined library of base functionsmodular similar to TTL families

featureschip size limits complexitycheap at high quantitiesstandardized cell heightunsuitable for regular structuresmore flexible and compact than gate array


Full Custom Technology

complete fabrication processtotal flexibility, only limited by layout rules manual design

featureschip size limits complexitylong design and fabrication timeefficient use of silicon areacheap only at highest quantities (ex. uP, memories, ...)


Macrocell Technologycomplete fabrication process

combines semi- and full custom technologiespredefined library of base functionsgenerators for regular structures

featureschip size limits complexityshort design, long fabrication timecheap at high quantitieshigh flexibility, compact layouts


FPGA Technologyfield programmable device

no fabrication needed for customizingpredefined logic blocks

featuressize: up to 2‘000’000 logic gates (see Virtexfrom Xilinx)large silicon area necessary (72 million fets)short design and customize timecheap for small quantitiescompared to ASICs, FPGAs have a reduced clock speedcircuit configuration downloadable (RAM or PROM)


Field Programmable Gate Array (FPGA)

Programming technologyProgrammable logic cellsProgrammable interconnect

Anti fuse (One time programmable, ex. Actel)Static RAM (volatile, ex. Xilinx)EPROM or EEPROM (non-volatile, ex. Altera)

Programmable I/O’s


Programmable Logic Cells The logic cells could be programmed to realize a logic function the main blocks are:

Multiplexer based (ACTEL)Look-up tables (XILINX)AND-OR array (ALTERA)

What is the suitable number of input for the logic cell?


Programmable Logic CellsMultiplexer Based logic cells

The logic function could be written using shanon’sexpansion theorem:

01 ..,..),,( == += AA FAFACBAF


Programmable Logic Cells

Realize the following logic function using Multiplexer Based logic cells

DBCBAF ++= ..



).().(

1

).(..

DCBDABF

BBwhere

BBDBCBAF

+++=

=+

+++=

FB=1 = A+D = A(1) + A(D)FB=0 = C+D = C(1) + C(D)



Combinational module (C-module)Sequential module (S-module)

(C-module) (S-module)

very large scale integration (vlsi) - german …eee.guc.edu.eg/courses/electronics/elct904 very...

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