Digital Systems

Digital Systems

Chapter 1

Digital Fundamental, 10

ed.

Thomas Floyd

Pearson Group, 2013

Textbook

Analog : varies continuously

most natural quantities that we see

Analog systems can generally handle higher power than digital systems.

Digital systems can process, store, and transmit data more efficiently but can only assign discrete values to each point.

1 100

A .M .

95 90 85 80 75

2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12

P.M .

Temperature (°F)

70

Time of day

Analog Quantity

Many systems use a mix of analog and digital electronics to take advantage of each technology.

A typical CD player accepts digital data from the CD drive and converts it to an analog signal for amplification.

Digital data CD drive

10110011101

Analog reproduction of music audio

signal Speaker

Sound waves Digital-to-analog

converter Linear amplifier

Analog and Digital Systems

Digital electronics uses circuits that have two states,

- two different voltage levels called HIGH and LOW

- The voltages represent numbers in the binary system.

In binary,

- a single number is called a bit (for binary digit).

- A bit can have the value of either a 0 or a 1,

depending on if the voltage is HIGH or LOW.

HIGH

LOW

V_H(max)

V_H(min)

V_L(max)

V_L(min)

Invalid

Binary Digitals and Logic Levels

Digital waveforms change between the LOW and HIGH levels.

Digital waveforms are made up of a series of pulses.

Falling or leading edge

(b) Negative–going pulse HIGH

Rising or trailing edge LOW

(a) Positive–going pulse HIGH

Rising or leading edge

Falling or trailing edge

LOW t₀ t₁ t₀ t₁

Digital Waveforms

Actual pulses are not ideal but are described by the rise time, fall time, amplitude, and other characteristics.

90%

50%

10%

Base line

Pulse width

Rise time Fall time

Amplitude t_W

t_r t_f

Undershoot Ringing Overshoot

Ringing

Droop

Pulse Definitions

Periodic pulse waveforms are composed of pulses that repeats in a fixed interval called the period. The frequency is the rate it repeats and is measured in hertz.

f  T1

T  1f

The clock is a basic timing signal that is an example of a periodic wave.

Periodic Pulse Waveforms

In addition to frequency and period, repetitive pulse waveforms are described by the amplitude (A), pulse width (t_W) and duty cycle. Duty cycle is the ratio of t_W to T.

Volts

Time

Amplitude (A)

Pulse width (t_W)

Period, T

Pulse Definitions (cont.)

A timing diagram is used to show the relationship between two or more digital waveforms,

Clock

A

B

C

A diagram like this can be observed directly on a logic analyzer.

Timing Diagrams