26-digital design

7/30/2019 26-Digital Design

1/32

CS 150 Spring 2007 - Lec #26 Digital Design 1

Digital Design and System Implementation

Overview of Physical Implementations

CMOS devices

CMOS transistor circuit functional behavior

Basic logic gatesTransmission gates

Tri-state buffers

Flip-flops vs. latches revisited

7/30/2019 26-Digital Design

2/32

CS 150 Spring 2007 - Lec #26 Digital Design 2

The stuff out of which we make systems

Overview of Physical Implementations

Integrated Circuits (ICs) Combinational logic circuits, memory elements, analog interfaces

Printed Circuits (PC) boards substrate for ICs and interconnection, distribution of CLK, Vdd, and

GND signals, heat dissipation

Power Supplies Converts line AC voltage to regulated DC low voltage levels

Chassis (rack, card case, ...)

1-25 conductive layers holds boards, power supply, fans, provides physical interface to user

or other systems

Connectors and Cables

7/30/2019 26-Digital Design

3/32

CS 150 Spring 2007 - Lec #26 Digital Design 3

Integrated Circuits

Primarily Crystalline Silicon 1mm - 25mm on a side

200 - 400M effective transistors

(50 - 75M logic gates")

3 - 10 conductive layers

2007 feature size ~ 65nm = 0.065 x 10-6 m45nm coming on line

CMOS most common -complementary metal oxide semiconductor

Package provides: Spreading of chip-level signal paths to

board-levelHeat dissipation.

Ceramic or plastic with gold wires

Chip in Package

7/30/2019 26-Digital Design

4/32CS 150 Spring 2007 - Lec #26 Digital Design 4

Multichip Modules (MCMs)

Multiple chips directly connected to a substrate(silicon, ceramic, plastic, fiberglass) without chip packages

Printed Circuit Boards

Fiberglass or ceramic

1-20in on a side

IC packages aresoldered down

7/30/2019 26-Digital Design

5/32CS 150 Spring 2007 - Lec #26 Digital Design 5

Integrated Circuits Moores Law has fueled innovation for the last 3 decades

Number of transistors on a die doubles every 18 months.

What are the consequences of Moores law?

7/30/2019 26-Digital Design

6/32CS 150 Spring 2007 - Lec #26 Digital Design 6

Integrated Circuits

Uses for Digital IC technology today:Standard microprocessors

Used in desktop PCs, and embedded applications (ex: automotive)Simple system design (mostly software development)

Memory chips (DRAM, SRAM)

Application specific ICs (ASICs)custom designed to match particular applicationcan be optimized for low-power, low-cost, high-performancehigh-design cost / relatively low manufacturing cost

Field programmable logic devices (FPGAs, CPLDs)customized to particular application after fabrication

short time to marketrelatively high part cost

Standardized low-density componentsstill manufactured for compatibility with older system designs

7/30/2019 26-Digital Design

7/32CS 150 Spring 2007 - Lec #26 Digital Design 7

Cross Section

The gate acts like a capacitor. A high voltage onthe gate attracts charge into the channel. If avoltage exists between the source and drain acurrent will flow. In its simplestapproximation, the device acts like a switch.

Top View

MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

nFET

pFET

CMOS Devices

7/30/2019 26-Digital Design

8/32CS 150 Spring 2007 - Lec #26 Digital Design 11

Logic and Layout: NAND Gate

7/30/2019 26-Digital Design

9/32CS 150 Spring 2007 - Lec #26 Digital Design 12

Transmission gates are the way to build switches in CMOS

In general, both transistor types are needed:

nFET to pass zeros

pFET to pass ones

The transmission gate is bi-directional (unlike logic gates)

Does not directly connect to Vdd and GND, but can be combined withlogic gates or buffers to simplify many logic structures

Transmission Gate

7/30/2019 26-Digital Design

10/32CS 150 Spring 2007 - Lec #26 Digital Design 13

Pass-Transistor Multiplexer

2-to-1 multiplexer:

c = sa + sb

Switches simplify the

implementation:s

sb

a

c

7/30/2019 26-Digital Design

11/32

CS 150 Spring 2007 - Lec #26 Digital Design 14

4-to-1 Pass-transistor Mux

The series connection ofpass-transistors in eachbranch effectively forms theAND of s1 and s0 (or theircomplement)

20 transistors

7/30/2019 26-Digital Design

12/32

CS 150 Spring 2007 - Lec #26 Digital Design 15

Alternative 4-to-1 Multiplexer

This version has lessdelay from in to out

Care must be taken to

avoid turning on multiplepaths simultaneously(shorting together theinputs)

36 Transistors

7/30/2019 26-Digital Design

13/32

CS 150 Spring 2007 - Lec #26 Digital Design 16

Example: Tally Circuit

N inputs: How many of these are asserted?

Tally

In

I1

N

TwoOneZero

E.g., 1 input, 2 outputs: One, ZeroE.g., 2 inputs, 3 outputs: Two, One, ZeroN inputs, N+1 outputs: N, , One, Zero

7/30/2019 26-Digital Design

14/32

CS 150 Spring 2007 - Lec #26 Digital Design 17

Example: Tally Circuit

01

7/30/2019 26-Digital Design

15/32

CS 150 Spring 2007 - Lec #26 Digital Design 18

Example: Tally Circuit

7/30/2019 26-Digital Design

16/32

CS 150 Spring 2007 - Lec #26 Digital Design 19

Example: Tally Circuit

7/30/2019 26-Digital Design

17/32

CS 150 Spring 2007 - Lec #26 Digital Design 20

Example: Tally Circuit

2 inputs, 3 outputs:Two, One, Zero

I1

0

1

1

0

7/30/2019 26-Digital Design

18/32

CS 150 Spring 2007 - Lec #26 Digital Design 21

Example: Tally Circuit

2 inputs, 3 outputs:Two, One, Zero

I1

0

1

0

7/30/2019 26-Digital Design

19/32

CS 150 Spring 2007 - Lec #26 Digital Design 22

Example: Tally Circuit

2 inputs, 3 outputs:Two, One, Zero

7/30/2019 26-Digital Design

20/32

CS 150 Spring 2007 - Lec #26 Digital Design 23

Example: Tally Circuit

2 inputs, 3 outputs:Two, One, Zero

7/30/2019 26-Digital Design

21/32

CS 150 Spring 2007 - Lec #26 Digital Design 24

Example: Tally Circuit

2 inputs, 3 outputs:Two, One, Zero

7/30/2019 26-Digital Design

22/32

CS 150 Spring 2007 - Lec #26 Digital Design 25

Example: Tally Circuit

2 inputs, 3 outputs:Two, One, Zero

7/30/2019 26-Digital Design

23/32

CS 150 Spring 2007 - Lec #26 Digital Design 26

Example: Crossbar Switch

N inputs, N outputs, N x N control signals

CrossBar

Busi

Outi

Note: circuit like thisused inside Xilinxswitching matrix

7/30/2019 26-Digital Design

24/32

CS 150 Spring 2007 - Lec #26 Digital Design 27

Barrel Shifter

BarrelShifter

BusOut

Shift

Bus Shift

7/30/2019 26-Digital Design

25/32

CS 150 Spring 2007 - Lec #26 Digital Design 28

Example: Barrel Shifter

N inputs, N outputs, N control signals

Shift 0

7/30/2019 26-Digital Design

26/32

CS 150 Spring 2007 - Lec #26 Digital Design 29

Example: Barrel Shifter

N inputs, N outputs, N control signals

Shift 1

7/30/2019 26-Digital Design

27/32

CS 150 Spring 2007 - Lec #26 Digital Design 30

Example: Barrel Shifter

N inputs, N outputs, N control signals

RotatingShift 1

7/30/2019 26-Digital Design

28/32

CS 150 Spring 2007 - Lec #26 Digital Design 33

Tri-state Based Multiplexer

Multiplexer

If s=1 then c=a else c=b

Transistor Circuit forinverting multiplexer:

7/30/2019 26-Digital Design

29/32

CS 150 Spring 2007 - Lec #26 Digital Design 34

D-type Edge-triggered Flip-flop

The edge of the clock is used tosamplethe "D" input & send it to"Q (positive edge triggering) At all other times the output Q is

independent of the input D (juststores previously sampled value)

The input must be stable for ashort time before the clock edge.

7/30/2019 26-Digital Design

30/32

CS 150 Spring 2007 - Lec #26 Digital Design 35

Transistor-level Logic Circuits

Positive Level-sensitive latch:

Latch Transistor Level:

clk

clk

clk

clk

Positive Edge-triggered flip-flop built from two level-sensitive latches:

7/30/2019 26-Digital Design

31/32

CS 150 Spring 2007 - Lec #26 Digital Design 36

State Machines in CMOS

Two Phase Non-Overlapping Clocking

CombinationalLogic

R

EG

R

EG

In Out

State

P1 P2

CLK

P1

P2

1/2 Register 1/2 Register

Di it l D si d I l t ti

7/30/2019 26-Digital Design

32/32

CS 150 Spring 2007 - Lec #26 Digital Design 37

Digital Design and ImplementationSummary

CMOS preferred implementation technology

Much more than simple logic gatesTransmission gate as a building block

Used to construct steering logic

Very efficient compact implementations of interconnectionand shifting functions

Simple storage building blocksD-type flip flop behavior with cross-coupled inverters and

two phase clocking

Heart of Xilinx implementation structures