ADC, DAC and Sensor Interfacingsaqazi.com/EEE342/FA19_MP_Lecture18_20191126_ADC_DAC... · 2019. 12....

ADC, DAC and

Sensor InterfacingLecture# 18

Microprocessor System and Interfacing

Dr. Sohaib Ayyaz QaziCOMSATS University Islamabad

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2

Submit

Assignment No 2

Next Class i.e. Thursday

ADC Characteristics


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ADC Devices

Analog to Digital Converters, widely used in data

acquisition

To convert physical world input (analog) to computer

compatible (discrete, binary)

Physical world include Temperature, Pressure, Humidity,

Velocity etc

Physical quantity is converted to electrical voltage using

Sensors.

An ADC is required to convert this voltage/current to a

digital number to be processed by AVR/Computer

ADC Characteristics


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Major Characteristics of ADC

Resolution

An ADC has n-bit resolution

n can be 8, 10, 12, 16 or even 24

Higher Resolution = Smaller step size

Step size the is smallest change possible

No. of steps = 2n

Max. Voltage = 5 V

n-bit No. of Steps Step Size (mV)

8 256 5/256 = 19.53

10 1024 5/1024 = 4.88

12 4096 5/4096 = 1.2

16 65536 5/65536 = 0.076

ADC Characteristics


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Conversion Time

A major factor to check the performance of ADC

Defined as the time taken by ADC to convert the analog

input to a digital number.

It is dependent on the clock source used for AVR

ADC Characteristics


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Reference Voltage (Vref)

Vref is input voltage pin, used for reference voltage

Voltage connected to Vref pin and resolution of ADC define

the step size

Step Size = vref / 2n

Example:

If Analog voltage is between range 0 – 4V.

Vref will be 4 Volts

Step size for 8-bit ADC will be = 4 / 256 = 15.62mV

ADC Characteristics


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Digital Data Output

To calculate the output voltage use this formula

Dout = Vin / (step size)

Dout is analog data output

Vin = analog input voltage

Step size is the minimum change in voltage

ADC Characteristics


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Parallel vs Serial ADC

Parallel ADC: 8 or more bits are used to bring out data

Serial ADC: Only one pin for data out.

Inside serial ADC a parallel-in-serial-out shift register is used

ADCs with more than 8 bits conversion size use same D0-D7

bits to send lower and higher bytes

Serial ADCs are preferred where space is a critical issue

Fewer Pins, Smaller Packages, Less Space

ADC Characteristics


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Analog Input Channels

Many applications need more than one ADC

Chips with 2, 4, 8 or 16 ADC channels are available

Multiplexing is used for the multiple ADC channels

ADC Characteristics


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Start and End of Conversion Signals

Multiple input channels and a single output channel needs

some synchronization.

Start of Conversion (SC) and End of Conversion (EOC) is

defined.

SC activated, conversion starts

After some time when conversion is complete, EOC signal tells

data is ready to be copied

ADC Characteristics


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Successive Approximation ADC

Widely used method because

Conversion time is fixed, hardware compatible

Three main components

Successive approximation register (SAR)

Comparator

Control Unit

Example 1 – Successive Approximation ADC


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Assume a step size of 10 mV and 8-bit SA-ADC and analog input is 1

Volts. The following steps will be followed to convert to digital output

1. Start with binary 1000 0000 = 128 in dec

128 x 10 mV = 1.28 V

1.28V > 1 V

Clear D7 bit (0000 0000)

2. Take next binary 0100 0000 = 64

64 x 10 mV = 640 mV

640mV < 1 V

Retain D6 bit (0100 0000)


96 x 10 mV = 960 mV

960mV < 1 V



112 x 10 mV = 1120 mV

1120 mV > 1 V

Clear D4 bit (0110 0000)


108 x 10 mV = 1010 mV

1080mV > 1 V

Clear D3 bit (0110 0000)


100 x 10 mV = 1000 mV

1000 mV = 1 V


We have reached the result, still it will iterate the

next two bits


102 x 10 mV = 1020 mV

1020 mV > 1 V

Clear D1 bit (0110 0100)


101 x 10 mV = 1010 mV

1010mV > 1 V

Clear D0 bit (0110 0100)

ADC Programming in AVR


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ADC Hardware Considerations

For digital signals small change in voltage level is not a big

deal (<0.5 is considered low)

For ADCs, small change means a lot.

A voltage change equal to step size changes the output

Need a stable ADC supply voltage and Vref variations

Which can be achieved using

Inductor and Capacitor

between AVCC and GND

Capacitor between AVREF

and GND



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Registers for ADC Operation

ADCH (High Data Byte)

ADCL (Low Data Byte)

ADCSRA (ADC Control and Status Register)

ADMUX (ADC Multiplexer Selection Register)

SPIOR (Special Function IO Register)



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Vref Source (Three Options)

AREF pin (0 0)

AVCC pin (0 1)

Internal 2.56 V (1 1)

REFS1 REFS0 ADLAR MUX4 MUX3 MUX2 MUX1 MUX0



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ADC Input Channel Source

Single Ended input

ADC0 – ADC7 Channel

Differential Input

Can select Op-amp gain

1x, 10x or 200x

+ve pin of Op-amp (ADC0 -7)

-ve pin of Op-amp (ADC0-2)




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ADLAR bit Operation

AVRs normally have 10-bit ADC

Result is stored in 16-bits (ADCH, ADCL)

6-bits are unused

ADLAR = 1 result is left-justified

ADLAR = 0 result is right-justified




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ADCSRA (Status and Control Register)

ADC Start Conversion bit

AVR has a special circuit to trigger start conversion

ADSC is one to be used for start conversion

SFIOR register control other modes

For this course we are only focused on normal operation of ADC

ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0



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ADCSRA (Status and Control Register)

A/D Conversion Time

Using ADPS 2:0 A/D conversion time can be controlled

ADEN ADSC ADATE ADIF ADIE ADPS2 ADPS1 ADPS0

ADSP2 ADSP1 ADSP0 Clock

0 0 0 Reserved

0 0 1 clk/2

0 1 0 clk/4

0 1 1 clk/8

1 0 0 clk/16

1 0 1 clk/32

1 1 0 clk/64

1 1 1 clk/128



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Steps in Programming the A/D Converter

1. Make the pin for selected ADC channel as input pin

2. Turn on ADC module (ADEN in ADCSRA) , it is disabled uponreset

3. Select Conversion Speed (ADPS2:0 in ADCSRA)

4. Select voltage reference (REFS1:0 in ADMUX) and ADC inputchannels (MUX4:0 in ADMUX)

5. Activate start conversion (ADSC in ADCSRA)

6. Wait for conversion complete (ADIF in ADCSRA)

7. When ADIF is HIGH, Read Data (ADCL, ADCH)

1. Read ADCL before ADCH

8. To read same Channel again, Go to Step 5

9. Read using diff. Vref or input channel, Go to Step 4

Example 1


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Get data from Channel 0 (ADC0) of ADC and display the result

on PortB and PortD.

# include <avr/io.h>

int main (void)

{

DDRB = 0xFF;

DDRD = 0xFF;

DDRA = 0x00;

ADCSRA = 0x87; ADC enable & clk/128

ADMUX = 0xC0; 2.56 Vref, ADC0 channel

Right Justified

while (1)

{

ADCSRA |= (1 << ADSC);

while ((ADCSRA & (1 << ADIF)) == 0);

PORTD = ADCL;

PORTB = ADCH;

}

return 0;

}

Exercise 1


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Perform same operation of last example using interrupts.

# include <avr/io.h>

# include <avr/interrupts.h>

ISR (ADC_vect)

{

PORTD = ADCL;

PORTB = ADCL;


}

int main (void)

{

DDRB = 0xFF;

DDRD = 0xFF;

DDRA = 0x00;

ADCSRA = 0x8F; ADC enable & clk/128 and

ADIE enable

ADMUX = 0xC0; 2.56 Vref, ADC0 channel

Right Justified

sei (); Enable Global Interrupt


while (1);

return 0;

}

Exercise 2


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Get data from Channel 0 (ADC0) of ADC, divide 10-bit result by 4to fit in 8-bits and send result to PortB. Then get data from Channel1 (ADC1), divide 10-bit result by 4 to fit in 8-bits and send to PortD.Repaet this process again and again. Use ADC clock pre-scalingwith a factor of 32.# include <avr/io.h>

int main (void)

{

DDRB = 0xFF;

DDRD = 0xFF;

DDRA = 0x00;

ADCSRA = 0x85; ADC enable & clk/32

while (1)

{

ADMUX = 0xE0; 2.56 Vref, ADC0 channel

Left Justified



PORTB = ADCH;

}

return 0;

}

ADMUX = 0xE1; 2.56 Vref, ADC1 channel

Left Justified



PORTD = ADCH;

}

return 0;

}

ADC, DAC and Sensor Interfacingsaqazi.com/EEE342/FA19_MP_Lecture18_20191126_ADC_DAC... · 2019. 12....

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