8051 interfacing da converters

DA Converters

GROUP 4

DEEC –MPIN

FEUP

April, 2010

April 10

José Borges, Patrício Lima, Marcos Brito, Marek Mastyło, Jakub Nyk

Outlines

April 10 2

1. Overview of DA converter types

• Four-bit DA converter

• R-2R ladder

2. Principle of operation

3. DAC performance

4. Applications

Overview of DA converter types

April 10 3

DAC is very important in electronics

DA converter transforms digital binary input signal into

analog output signal (e.g. voltage, current)

A typical DAC converts the abstract numbers into a

concrete sequence of impulses that are then

processed by a reconstruction filter using some form of

interpolation to fill in data between the impulses


April 10 4

Digital-to-analog (D/A) converters (sometimes called

DACs) are used to present the results of digital

computation, storage, or transmission, typically for

graphical display or for the control of devices that

operate with continuously varying quantities

The output of the D/A converter is proportional to the

product of the digital input value and the reference


April 10 5

Sampled data can be reconstructed perfectly provided

that its bandwidth meets certain requirements

Reconstructed signal should be a baseband signal

with bandwidth less than the Nyquist frequency


April 10 6

Instead of impulses, usually the sequence of numbers

update the analogue voltage at uniform sampling

intervals

Numbers are written to the DAC

Numbers are lanched in sequence with clock signal


April 10 7

It causes that DAC output voltage changes rapidly

from the previous value to the value represented by

the currently latched number

Output signal value is held in time at the current value

Every next latched number casues changes of current

output signal value


April 10 8

Example: Transform of latched number to output signal

value:

Sign bit can be used

MSB – bit 1 Bit 2 LSB – bit 3 Value

0 0 0 0.0

0 0 1 0.125

0 1 0 0.25

0 1 1 0.375

1 0 0 0.5

1 0 1 0.625

1 1 0 0.750

1 1 1 0.875


April 10 9

Operation gives „staircase” (equivalent to zero-order

hold):

Reference: http://en.wikipedia.org/wiki/Digital-to-analog_converter


April 10 10

DAC output is a sequence of piecewise constant

values or rectangular pulses

Harmonics are above Nyquist frequency

Low pass filters are used as a reconstruction filter to

remove higher harmonics


April 10 11

Effective frequency response of the DAC depends on

the filter phase distortion

Not all DACs have a zero order response


April 10 12

DA conversion is used in audio and video systems

Audio signals are stored in digital form (MP3, CD) and

they must be converted into an analog signal to be

heard

Video signals from a digital source, such as a

computer, must be converted to analog form if they are

to be displayed on an analog monitor

Applications

Principle operations

April 10 13

Counting, Pusle-Width Modulator:

Reference: http://www.analog.com/library/analogDialogue/archives/39-06/data_conversion_handbook.html

Counting Pulse-width modulator

April 10 14

It is constructed using counters to generate an output

voltage proportional to a digital input word

A sampling clock starts the counter, which is loaded

with the digital word and simultaneously sets an R/S

flip-flop



April 10 15

The counter counts upward at a fast rate, and when it

reaches all "ones", the R/S flip-flop is reset

The output of the R/S flip flop is therefore a pulse-

width-modulated (PWM) pulse whose width is

proportional to the complement of the binary word




April 10 16

An R-2R Ladder is a simple and inexpensive way to perform

digital-to-analog conversion, using repetitive arrangements of

precision resistor networks in a ladder-like configuration

Five-bit R-2R resistor ladder:

Reference: http://en.wikipedia.org/wiki/Resistor_ladder

R-2R Ladder

April 10 17

Bit4 MSB (most significant bit) to Bit0 LSB (least

significant bit) are driven from digital logic gates

The R-2R network causes the digital bits to be

weighted in their contribution to the output voltage Vout

In our example 5-bits are used, so it gives 32

possibilities of output values


R-2R Ladder

April 10 18

Depending on which bits are set to 1 and which to 0

the output voltage will be a stepped value between 0

volts and Vref minus the value of the minimum step

For a digital value VAL, of a R-2R DAC of N bits of

0 V/Vref, the output voltage Vout is:

Vout = Vref × VAL / 2N


R-2R Ladder


April 10 19

Let N = 5 and hence 2N = 32. With Vref = 3.3 V (typical

CMOS logic 1 voltage), Vout will vary between 00000,

VAL = 0 and 11111, VAL = 31

Minimum (single step) VAL = 1, we have

Vout = 3.3 × 1 / 32 = 0.1 volts

R-2R Ladder - example

April 10 20

Maximum output (11111 VAL = 31), we have

Vout = 3.3 × 31 / 25 = 3.2 volts



April 10 21

R-2R ladder features:

Cheap – needs only two resistor values (or even

one, if we use parallel conection)

Fast

Fixed output

DAC tends to be restricted to a relatively small

number of bits



April 10 22

Small inaccuracies in the higher significant bit resistors

can entirely overwhelm the contribution of the less

significant bits

Depending on the type of logic gates used and design

of the logic circuits, there may be transitional voltage

spikes at such major crossings even with perfect

resistor values

High output impedance gates may be unsuitable in

some cases



April 10 23

Resistors used with the more significant bits must be

proportionally more accurate than those used with the

lower significant bits

On a printed circuit board, using discrete components,

high precision resistors of 1% accuracy may be

employed for a 5 bit circuit

Both the ratio tolerances of the individual resistors

within the ladder and the resistance of the switches

used at the ladder inputs can affect DAC output

accuracy



April 10 24

Circuit Diagram for an R-2R Ladder Network DAC:

Reference: http://www.ecelab.com/circuit-dac-r2r.htm




April 10 25

DAC with summing amplifier:

Reference:

http://hyperphysics.phyastr.gsu.edu/hbase/electronic/dac.html#c2

Four-bit DA Converter


April 10 26

Example of 5-bit Binary-Weighted DAC:


Binary-Weighted DAC

April 10 27

By connecting various combinations of parallel shorting

switches, 32 possible values of series resistance can

be obtained ranging from 0 to 31·R

32 possible levels of light intensity are given


Binary-Weighted DAC

April 10 28

Example Voltage-mode Binary-Weighted Resistor DAC (The output impedance of the voltage-mode binary DAC changes with the input code ):



Binary-Weighted DAC

Main Technical Specifications

April 10 29

Static Performance

• Resolution (N)

• Differential Nonlinearity (DNL)

• Relative Accuracy or Integral Nonlinearity (INL)

• Zero-Scale Error (VZSE)

• Full-Scale Voltage (VFS)


April 10 30

This is the number of possible output levels the DAC is

designed to reproduce

This is usually stated as the number of bits it uses

Resolution is related to the effective number of bits

which is a measurement of the actual resolution attained

by the DAC

Resolution

M.T.S.

April 10 31

Shows how much two adjacent code analog

values deviate from the ideal 1LSB step

Differential Nonlinearity (DNL)

M. T. S.

April 10 32

Shows how much the

DAC transfer

characteristic deviates

from an ideal one. That is,

the ideal characteristic is

usually a straight line;

INL shows how much the

actual voltage at a given

code value differs from

that line, in LSBs (1LSB

steps).

Integral nonlinearity (INL)


April 10 33

Analog Output

• Output Current (Iout)

• Load Regulation at Full Scale (LREG)

• Is the capability to maintain a constant voltage (or current)

level on the output channel of a power supply despite changes

in load

• Capacitance load (CL)


April 10 34

Logic Inputs

• Logic Low Voltage (VIL)

• Logic High Voltage (VIH)

• Input Leakage Current (IIL)

• Input Capacitance (CIL)


April 10 35

Interface timing specifications

• Clock Width High (tCH) / Low (tCL)

• Load Pulse Width (tLDW)

• Data Setup (tDS) / Hold (tDH)

• Clear Pulse Width (tCLRW)

• Load Setup (tLD1) / Hold (tLD2)

• Select (tCSS) / Deselect (tCSH)


April 10 36

Interface Timing Specifications


April 10 37

AC characteristics

• Voltage Output Settling Time (ts)

• DAC Glitch

• Digital Feedthrough


April 10 38

Supply characteristics

• Positive Supply Current (IDD)

• Positive Dissipation (PDISS)

• Positive Supply Sensitivity (PSS)

DAC performance

April 10 39

This is the number of possible output levels the DAC is

designed to reproduce

This is usually stated as the number of bits it uses

Resolution is related to the effective number of bits

which is a measurement of the actual resolution attained

by the DAC

Resolution

DAC performance

April 10 40

Measurement of the maximum speed at which the

DACs circuitry can operate and still produce the

correct output

Signal must be sampled at over twice the frequency of

the desired signal – Nyquist sampling theorem

Maximum sampling frequency

DAC performance

April 10 41

Ability of a DAC's analog output to move only in the

direction that the digital input moves

This characteristic is very important for DACs used as

a low frequency signal

Monotonicity

DAC performance

April 10 42

Measurement of the distortion and noise added to the

signal by the DAC

It is expressed as a percentage of the total power of

unwanted harmonic distortion and noise that

accompany the desired signal

THD+N(Total harmonic distortion)

DAC performance

April 10 43

Measurement of the distortion and noise added to the

signal by the DAC

It is expressed as a percentage of the total power of

unwanted harmonic distortion and noise that

accompany the desired signal

Measurement of the difference between the largest

and smallest signals the DAC can reproduce

expressed in decibels

Dynamic range

DAC performance

April 10 44

Phase distortion and sampling period instability are

also sometimes important in DA conversion process

Phase distortion

Application Example

April 10 45

Requirements:

Generate four waveforms:

• Square,

• Triangle ,

• Sawtooth

• Sine

Amplitude choise

DC Level choise

Frequency choise

Waves Generator

Application Example

April 10 46

Schematic circuit

Waves Generator

Waves Generator

April 10 47

Choosing DA Converter:

Two families widely used:

DAC 0808 ( parallel)

DAC 8512 (serie)

Choose for this circuit: DAC0808

National Semiconductors

How to generate

wave?

Application Example

April 10 48

Interface :

SET FUNC: to select functions:

• Waveform

• Amplitude

• Frequency

PLUS

MINUS

RST

Waves Generator

Application Example

April 10 49

Interface:

Led’s indicates selected function

Waves Generator

Waves Generator

April 10 50

Square wave:

Characteristics:

Vmax

Vmin

Pulse-width

How to generate waves?

Waves Generator

April 10 51

Square wave:

Is represented by tow voltage leves :Vmin and Vmax

The value to set in microcontroller output is :

How to generate waves?

Waves Generator

April 10 52

Algorithm to generate Square Wave:

1. Calculate Vmax and Vmin from VPP and DC level

2. Calculate delay from requested frequency

3. Loop :

• Set Vmin

• Delay

• Set Vmax

• Jump to loop

How to generate

waves?

Waves Generator

April 10 53

Triangle wave:

How to generate

wave?

Waves Generator

April 10 54

Triangle wave:

Equation definition

How to generate

wave?

Waves Generator

April 10 55

Tabling values of magnitude:

How to generate

wave?

Waves Generator

April 10 56

Algorithm to generate Triangle Wave :

1. Calculate Vmax and Vmin

2. Table the values of Wave

3. Calculate delay time

• Delay time is function of frequency

4. Infinit loop:

• Sending upward wave

• Sending downward wave

How to generate

wave?

Waves Generator

April 10 57

HOW TO GENERATE TAWTOOTH WAVE?

I just need to set the upward of triangle wave!!

How to generate

wave?

Waves Generator

April 10 58

Sine Wave

How to generate

wave?

Waves Generator

April 10 59

Algorithm to generate Sine Wave:

Generating a table with sine values (Sk) between 0º and 90º

Steps to set sine wave:

1. Send upward wave stored in table

2. Send downward wave

3. Do VDC- upward wave and send to output

4. Do VDC-downward wave and sendo to output

How to generate

wave?

Waves Generator

April 10 60

Calculating values of Sk:

How to generate

wave?

0 pi/16 2pi/16 3pi/16 4pi/16 5pi/16 6pi/16 7pi/16 8pi/16

0 10 20 28 36 42 47 50 51

𝑵𝒌 = 𝑽𝒅𝒄 +𝑽𝒑𝒑

𝟐∗ 𝑺𝒌

April 10 61

Circuit schematic• Interface with DA converter serial port and motor controller.

8051 Car

Application Example

Application Example

April 10 62

Circuit schematic

8051 Car

Connecting 8051 to:

• DA Converter

• Motor Controller

Application Example

April 10 63

Circuit schematic• VREF+ – Reference voltage (Maximum)

• VREF- – Reference voltage (Minimum)

• IOUT – Output current of the converted data

• VEE – Power supply

• COMP – Compensation

• A1 to A8 – Digital input pins

8051 Car

Application Example

April 10 64

Circuit schematic

• Output Signal

8051 Car

𝐼𝑜𝑢𝑡 =𝑉𝑅𝑒𝑓

𝑅𝑅𝑒𝑓∗ 𝐷𝑖 ∗ 2

𝑖−8

8

𝑖=1

𝐼𝑜𝑢𝑡 = 𝐼𝑅𝑒𝑓 ∗ (𝐷12+𝐷24+𝐷38+𝐷416

+𝐷532

+𝐷664

+𝐷7128

+𝐷8256

)

Ex:

𝐷 = 10001001

𝐼𝑅𝑒𝑓 = 1𝑚𝐴

𝐼𝑜𝑢𝑡 = 1𝑚𝐴 ∗ 1

2+1

32+

1

256 = 1𝑚𝐴 ∗

137

256= 0.535𝑚𝐴

Application Example

April 10 65

Circuit schematic

• Motor interface

• The amplifier operates as a

current to voltage converter.

8051 Car

Application Example

April 10 66

Circuit schematic

• Motor interface

• The voltage is used to generate

the necessary current to feed

The motors..

8051 Car

Application Example

April 10 67

Coding

• Main program

• Check the command saved on 8051 memory and execute it.

• Commands supported:

• Speed – arg

• Front – arg

• Front right – arg

• Front Left – arg

• Back – arg

• Back right – arg

• Accelerate

8051 Car

Application Example

April 10 68

Coding

• Serial port

• Commands comes from a terminal connected to the serial port

• The supported commands, will be saved on memory

• In the end the code is compiled and a warning message tells

us if the syntax is correct

8051 Car

Application Example

April 10 69

Coding

• Main program Flowchart

8051 Car

Memory Value Mnemonic

30h f front

31h 5 5s

32h b back

33h 6 6s

34h s speed

35h 9

36h 0

37h j jump

38h 0 0

39h e end

90

Application Example

April 10 70

Coding

• Serial port interrupt routine

• Flowchart

8051 Car

References

April 10 71

Sites: http://www.ecelab.com/circuit-dac-r2r.htm

http://en.wikipedia.org/wiki/Digital-to-analog_converter

http://en.wikipedia.org/wiki/Resistor_ladder

http://hyperphysics.phyastr.gsu.edu/hbase/electronic/dac.html#c2

http://www.the-crankshaft.info/2009/08/dac-interfacing.html

http://www.8051projects.net/

http://www.labcenter.co.uk/download/prodemo_autodl_general.cfm

Softwares: ISIS Proteus

Uvision

http://www.ecelab.com/circuit-dac-r2r.htm











http://en.wikipedia.org/wiki/Resistor_ladder

http://hyperphysics.phyastr.gsu.edu/hbase/electronic/dac.html






http://www.8051projects.net/



http://www.keil.com/uvision/

April 10 72

Thank you !

8051 interfacing da converters

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