Analog to digital conversion in AVR Microcontrollers (Chapter 13 of - - PowerPoint PPT Presentation

Analog to digital conversion in AVR Microcontrollers

Microprocessors, Lecture 10:

(Chapter 13 of the text book)

University of Tehran 2

Contents

• ADC units of ATmega32
• ADC programming in C

University of Tehran 3

ADC in AVR

University of Tehran 4

ADC

• Typical usage of ADC (analog to digital

convertors)

University of Tehran 5

ADC

• A typical ADC
• Vref= the maximum allowable volatge

University of Tehran 6

ADC resolution

• Step size: the difference between two

consecutive output nembers

• For a 8-bit output and Vref=5v,

– step size= 5/246

University of Tehran 7

ADC internal structure

• Successive approximation method

– See page 468 of the book for algorithm details

• n steps for an n-bit output ADC
• Requires n cycles to calculate digital output

University of Tehran 8

ADC in ATmega32

• 7 analog input

channels – Just one ADC multiplexed by input channels

• 10-bit output

– Kept in ADCL and ADCH registers – 6 unused bits, can be set the upper or lower 6 bits

University of Tehran 9

AVR ADC Registers

• ADCL and ADCH to keep digital data
• ADCSRA to control ADCs
• ADMux to select one input channel for

conversion

• SPIO, special function register

University of Tehran 10

ADMUX register

University of Tehran 11

ADMUX register

• Select the reference voltage (the maximum

acceptable input channel voltage)

University of Tehran 12

ADMUX register

• Which bits of ADCH and ADCL are unused

University of Tehran 13

ADMUX register

• Which channel is selected to the ADC

– Can also work in differential mode

University of Tehran 14

ADCSRA register

• ADC control and status register

– Control and monitor the ADC

University of Tehran 15

ADCSRA register

• ADATE bit
• Sets the operation mode of ADC
• 0= single conversion

– Converts the input just one time every time ADSC becomes 1

• 1= free running

– Continuously converts the input to digital values with some frequency – For ADATE=1 we have more ADC options, Take a look at AVR documents for more details if you are interested!

University of Tehran 16

ADCSRA register

• Selecting the ADC clock

University of Tehran 17

ADC clock

• Determines the speed of sampling the input

data

– Each conversion takes around 13 ADC clocks

• Larger frequency has smaller accuracy
• In AVR, frequency have to be less than 200

kHz

• Set it to smaller frequency (ADPS[0..2]=111) if

speed is not critical

University of Tehran 18

ADC programming in C

University of Tehran 19

ADC programming in C

A program that coverts the analog voltage of ADC0 and copies it to portD and portB

• -polling method
• The ADSC bit

should be 1 for the ADC to start conversion

• Returns to 0

automatically

• nce the

conversion is completed

University of Tehran 20

ADC programming in C

A program that coverts the analog voltage of ADC0 and copies it to portD and portB

• -interrupt method

University of Tehran 21

Connecting sensors to AVR

• Transducer (sensor): convert

environment parameters (temperature, pressure, velocity,…) to electrical quantities (voltage, current, capacitance, resistance)

• Signal conditioning:

converting electrical quantities to voltage

University of Tehran 22

Connecting sensors to AVR- example

University of Tehran 23

Connecting LM35 to AVR

• In AVR:

– Vref=2.56v – 10 bit output, 1024 values step size=2.56/1024= 2.5 mv

• In LM34:

– for 1 degree increase in temperature, we have 10mv increase in output voltage If the temperature increases by one degree: – 10mv increase in sensor output – 10/2.5=4 increase in ADC output

• Divide the ADC result by 4 to get actual

temperature (shift right 2 times)

University of Tehran 24

Connecting LM35 to AVR

• Example: temp= 20 degrees, sensor output= 200mv (10mv for every

degree), this voltage is converted to 0x1010000=80 (200mv/2.5mv=80) by the ADC

• Divide by 4 to get actual temperature by ADC

University of Tehran 25

ADC programming in C in CodeVision

• Set different values in the project setup wizard