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Digital Design > Lecture 2 > Flashcards

Lecture 2 Flashcards

1
Q

Logic Gates

A

Simple digital circuits that take one or more binary inputs and produce a binary output

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2
Q

Relationships between inputs and outputs described with:

A

Truth table

Boolean equation

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3
Q

Truth table

A

Lists inputs on the left and corresponding output on the right

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4
Q

Boolean equation

A

A mathematical expression using binary variables

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5
Q

NOT Gate

A

Output inverse of input

If A is FALSE, Y is TRUE

If A is TRUE, Y is False

Y equals not A

Line over A or A’

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6
Q

Buffer Gate

A

Copies input to output

If A is FALSE, Y is FALSE

If A is TRUE, Y is TRUE

Same as wire (logical PoV)

Able to deliver large current amounts to motor, able to quickly send its output to many gates, digital abstraction hides the real purpose of buffer (analog POV)

Triangle symbol (circle on output called a bubble, indicates inversion)

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7
Q

AND Gate

A

Two inputs, A and B
One output, Y

Produces a TRUE output only if both A and B are TRUE; otherwise output is FALSE

Intersection

Y equals A and B (Y=AB)

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8
Q

OR Gate

A

Two inputs, A and B
One output, Y

Produces a TRUE output if either A or B or both are TRUE; otherwise, output is FALSE

Y equals A or B (Y = A + B)

Union

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9
Q

XOR Gate

A

Exclusive OR

Two inputs, A and B
One output, Y

Produces a TRUE output if either A or B, but not both, are TRUE; otherwise, output is FALSE

Module-2 addition

Y = A^B

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10
Q

NAND Gate

A

NOT AND

Produces a TRUE output unless both inputs are TRUE; otherwise, output is FALSE

Y = ~(AB)

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11
Q

NOR Gate

A

NOT OR

Produces a TRUE output if neither input A nor B is TRUE; otherwise, output is FALSE

Y = ~(A+B)

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12
Q

XNOR Gate

A

NOT XOR

Produces a TRUE output if both inputs are FLASE or both are TRUE; otherwise FALSE

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13
Q

Multiple-input gates: AND

A

Produces a TRUE output when all N inputs are TRUE

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14
Q

Multiple-input gates: OR

A

Produces a TRUE output when at least one input is TRUE

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15
Q

Multiple-input gates: XOR

A

Called a parity gate

Produces TRUE if odd number of inputs are TRUE

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16
Q

DC Transfer Characteristics

A

Describe the output voltage as a function of the input voltage when the input is changed slowly enough that the output can keep up

Called “transfer characteristics” because they describe relationship between input and output voltage

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17
Q

Transfer Characteristics of NOT gate

A

Would have an abrupt switching threshold at Vdd/2

For V(A) < Vdd/2, V(Y) = 0
Vih = Vil = Vdd/2
Voh = Vdd
Vol = 0
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18
Q

Real inverter changes more gradually between extremes

A

When V(A) = 0, output V(Y) = Vdd

When V(A) = Vdd, output V(Y) = 0

Transition between these endpoints is smooth and not exactly centered at Vdd/2

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19
Q

How to define the logic levels?

A

Where the slope of the transfer characteristic dV(Y)/dV(A) = -1

These two points are called the unity gain points

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20
Q

Choosing logic levels at unity gain points

A

Usually maximizes noise margins

If Vil were reduces, Voh would only increase by a small amount

If Vil were increased, Voh would drop precipitously

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21
Q

Static Discpline

A

Requires that, given logically valid inputs, every circuit element will produce logically valid outputs (to avoid inputs falling into the forbidden zone)

Digital designers sacrifice freedom of using arbitrary analog circuit elements in return for the simplicity and robustness of digital circuits

They raise the level of abstraction, and increase design productivity by hiding needless detail

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22
Q

All gates belong to a logic family obey the static discipline when used with other gates in the family

A

“Snap together like Legos” in that they use consistent power supply voltages and logic levels

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23
Q

Major logic families that predominated from 70’s to 90’s

A

TTL: transistor-transistor logic

CMOS: complementary metal-oxide-semiconductor logic

LVTTL: low-voltage TTL

LVCMOS: low-voltage CMOS

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24
Q

TTL

A 
Vdd: 5(4.75-5.25)
Vil: 0.8
Vih: 2.0
Vol: 0.4
Voh: 2.4
25
Q

CMOS

A 
Vdd: 5(4.5-6)
Vil: 1.35
Vih: 3.15
Vol: 0.33
Voh: 3.84
26
Q

LVTTL

A 
Vdd: 3.3(3-3.6)
Vil: 0.8
Vih: 2.0
Vol: 0.4
Voh: 2.4
27
Q

LVCMOS

A 
Vdd: 3(3-3.6)
Vil: 0.9
Vih: 1.8
Vol: 0.36
Voh: 2.7
28
Q

TTL: communication with other families

A

TTL: OK
CMOS: NO Voh < Vih
LVTTL: MAYBE
LVCMOS: MAYBE

29
Q

CMOS: communication with other families

A

TTL: OK
CMOS: OK
LVTTL: MAYBE
LVCMOS: MAYBE

30
Q

LVTTL: communication with other families

A

TTL: OK
CMOS: NO Voh < Vih
LVTTL: OK
LVCMOS: OK

31
Q

TTL: communication with other families

A

TTL: OK
CMOS: NO Voh < Vih
LVTTL: OK
LVCMOS: OK

32
Q

Transistors

A

Electrically controlled switches that turn ON or OFF when a voltage or current is applied at a control terminal

Modern computers use transistors because they are cheap, small, reliable

33
Q

Types of transistors

A

Bipolar transistors

MOSFETs: metal-oxide semiconductor field effect transistors

34
Q

MOSFETs

A

The building blocks of almost all digital systems

FET

Today, engineers can pack roughly 1 billion MOSFETS onto a 1cm2 chip of silicon

Costs<10 microcents

Capacity and cost improve by an order of magnitude every ~8 years

35
Q

FET

A

Field-effect transistor

Creates an electric field that turns ON or OFF a connection between source and drain

36
Q

Silicon

A

Used to build MOSFETs

Predominant atom in rock and sand

Group IV atom (4 valence e, forms bonds with 4 adjacent atoms, results in a cubic crystalline lattice)

By itself, a poor conductor b/c all electrons are tied up in covalent bonds (becomes a better conductor when small impurities called dopant atoms are carefully added)

37
Q

Adding group V dopant to Si

A

As

Gives dopant atoms an extra electron uninvolved in bonds

Electron can easily move about the lattice

Carries negative charge, so n-type dopant

38
Q

Adding a group III dopant to Si

A

B

Leaves a hole at neighboring silicon atom

Hole can migrate around the lattice

Lacks a negative charge, so it acts like a positively-charged particle - p-type dopant

39
Q

Diode

A

The junction between p-type (anode) and n-type (cathode) silicon

40
Q

Forward-biased diode

A

When voltage on anode rises above voltage on cathode

Current flows through the diode from the anode to the cathode

41
Q

Reverse-biased diode

A

When the anode voltage is lower than the voltage on the cathode

No current flows

42
Q

Capacitor

A

Two conductors separated by an insulator

When voltage V is applied to one conductor

  • the conductor accumulates electric charge Q
  • the other conductor accumulates -Q
43
Q

Capacitance of a capacitor

A

The ratio of charge to voltage: C = Q/V

Proportional to size of the conductors

Inversely proportional to the distance between them

Important b/c charging or discharging a conductor takes time and energy (more capacitance means a circuit is slower and requires more energy to operate)

44
Q

MOSFETs consists of:

A

A conducting layer (gate)

On top of an insulating layer of SiO2

On top of the silicon wafer (substrate)

Sandwich of several layers of conducting and insulating materials (built on thin flat wafers of silicon about 15-30cm in diameter)

45
Q

nMOS

A

N-type transistors with regions of n-type dopants adjacent to gate called source and drain built on a p-type substrate

46
Q

pMOS

A

Consisting of p-type source and drain regions in an n-type semiconductor substrate

47
Q

nMOS Transistor Operation

A

Substrate of nMOS typically tied to GND, lowest voltage

48
Q

nMOS Transistor Operation: gate at 0V

A

Diodes between source or drain and substrate are reverse-biased

No path for current to flow b/w source and drain, so transistor is OFF

49
Q

nMOS Transistor Operation: gate at Vdd

A

Positive voltage applied to top plate of capacitor

Establishes electric field, attracts positive charge on top plate and negative charge on bottom plate

If voltage is sufficiently large, region inverts from p-type to become n-type

Inverted region - channel

Electrons flow from source to drain. Transistor is ON

50
Q

pMOS operation

A

Substrate tied to Vdd

Gate at Vdd, transistor is OFF

When gate at GND, channel inverts to p-type and transistor ON

51
Q

MOSFETs are not perfect switches

A

nMOS transistors pass 0’s well but pass 1’s poorly

When the gate of an nMOS transistor is at Vdd, the drain will only swing b/w 0 and Vdd-Vt

pMOS transistors pass 1’s well and pass 0’s poorly

52
Q

To build both nMOS and pMOS transistors on the same chip

A

Start with p-type wafer, then implant n-type regions called wells

53
Q

CMOS

A

Complementary MOS processes provide both types of transistors, and are used to build the majority of all transistors fabricated today

Gives 2 types of electrically controlled switches

54
Q

CMOS operation

A

Voltage at gate (g) regulates flow of current between source (s) and drain (d)

nMOS transistors OFF when gate is 0, ON when gate is 1

pMOS transistors ON when gate is 0, OFF when gate is 1

55
Q

CMOS NOT Gate

A

Triangle indicates GND

Flat bar indicates Vdd

N1 connected b/w GND and Y output
P1 connected Vdd and Y output

Both transistor gates controlled by the input A

If A = 0, N1 OFF and P1 ON. Y connected to Vdd but not to GND and is pulled up to a logic 1. P1 passes a good 1

If A=1, N1 ON and P1 OFF. Y pulled down to a logic 0. N1 passes a good 0

56
Q

CMOS NAND Gate

A

Wires are always joined at 3-way junctions (only joined at a 4-way junction if a dot is shown)

N1 and N2 connected in series, both must be ON to pull output down to GND

P1 and P2 are in parallel, only one must be ON to pull output up to Vdd

A=1 and B=0:

  • N1 ON but N2 OFF, blocking path from Y to GND
  • P1 OFF but P2 ON, creating path from Vdd to Y
  • Y pulled up to 1
57
Q

Power consumption

A

The amount of energy used per unit time (battery life of portable systems limited by it)

Digital systems draw both dynamic and static power

58
Q

Dynamic power

A

Used to charge capacitance as signals change b/w 0 and 1

Energy drawn from power supply to charge capacitance C to Vdd to CVdd^2

If voltage on capacitor switches at frequency f, it charges it f/2 times power second and discharges it at same rate. Discharging does not draw energy from the power supply

59
Q

Static power

A

Used even when signals do not change and the system is idle

Total static current Idd (leakage current or quiescent supply current) b/w Vdd and GND

Digital Design (13 decks)

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