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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
CMOS
``` Vdd: 5(4.5-6) Vil: 1.35 Vih: 3.15 Vol: 0.33 Voh: 3.84 ```
26
LVTTL
``` Vdd: 3.3(3-3.6) Vil: 0.8 Vih: 2.0 Vol: 0.4 Voh: 2.4 ```
27
LVCMOS
``` Vdd: 3(3-3.6) Vil: 0.9 Vih: 1.8 Vol: 0.36 Voh: 2.7 ```
28
TTL: communication with other families
TTL: OK CMOS: NO Voh < Vih LVTTL: MAYBE LVCMOS: MAYBE
29
CMOS: communication with other families
TTL: OK CMOS: OK LVTTL: MAYBE LVCMOS: MAYBE
30
LVTTL: communication with other families
TTL: OK CMOS: NO Voh < Vih LVTTL: OK LVCMOS: OK
31
TTL: communication with other families
TTL: OK CMOS: NO Voh < Vih LVTTL: OK LVCMOS: OK
32
Transistors
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
Types of transistors
Bipolar transistors MOSFETs: metal-oxide semiconductor field effect transistors
34
MOSFETs
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
FET
Field-effect transistor Creates an electric field that turns ON or OFF a connection between source and drain
36
Silicon
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
Adding group V dopant to Si
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
Adding a group III dopant to Si
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
Diode
The junction between p-type (anode) and n-type (cathode) silicon
40
Forward-biased diode
When voltage on anode rises above voltage on cathode Current flows through the diode from the anode to the cathode
41
Reverse-biased diode
When the anode voltage is lower than the voltage on the cathode No current flows
42
Capacitor
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
Capacitance of a capacitor
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
MOSFETs consists of:
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
nMOS
N-type transistors with regions of n-type dopants adjacent to gate called source and drain built on a p-type substrate
46
pMOS
Consisting of p-type source and drain regions in an n-type semiconductor substrate
47
nMOS Transistor Operation
Substrate of nMOS typically tied to GND, lowest voltage
48
nMOS Transistor Operation: gate at 0V
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
nMOS Transistor Operation: gate at Vdd
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
pMOS operation
Substrate tied to Vdd Gate at Vdd, transistor is OFF When gate at GND, channel inverts to p-type and transistor ON
51
MOSFETs are not perfect switches
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
To build both nMOS and pMOS transistors on the same chip
Start with p-type wafer, then implant n-type regions called wells
53
CMOS
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
CMOS operation
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
CMOS NOT Gate
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
CMOS NAND Gate
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
Power consumption
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
Dynamic power
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
Static power
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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