analog to digital conversion - adc mark neil - microprocessor course 1 device drivers measuring...

Analog to Dig i ta l Convers ion - ADC

Mark Neil - Microprocessor Course

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Device Drivers – Measuring Voltages


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Example Product: Signal Generator

A Signal Generator should be programmable. A user can use the the LCD display and the

keyboard to change the: Frequency scale Amplitude scale Offset on/off etc. Waveform shape (Square, Sinusoidal, Triangle..)

Analog control given by potentiometerAnalog signal output by using a Digital to

Analog Converter (DAC)

ADC on the ATMega128


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The ATMega128 has a built in Analog to Digital Converter 10 bits of data for each sample

Signals can be digitized from up to 8 inputs 1 input can be digitized at a time There is a multiplexer to select which input should be

converted

Analog to Digital Converters


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An ANALOG to DIGITAL CONVERTER (ADC) is a device that measures analog signals and convert them todigital (binary) numbers:

10-bitADC

output10-bit Numbersequence

Higher Voltage Ref.

Lower Voltage Ref.

InputSignal

Input

D9

D0

Mark Neil - Microprocessor Course 5

ADCCSR – ADC Control and Status Register

ADCMUX –RegisterSelect Channel

Multiplexer to select

which channel to

digitize

8 ADC Input

Channels

ADCL – Bits 0-7ADCH – Bits 8-9

ADC: 10 bit Analog to

Digital Conversion

The ADC Registers I


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The 10-bit ADC on Atmega128 receives its inputs from PORTF.

The Reference voltage is controlled by a potentiometer.

The device can be controlled by 4 on board registers: ADCSR : ADC Status Register ADMUX : Multiplexer Control ADCH : Data Register for D9,D8 ADCL : Data Register for D0-D7

The ADCCSR


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ADEN : ADC Enable - turns on the ADC clock ADSC : Start conversionADIF : Interrupt flag (gets set during a conversion and gets cleared if you write ‘1’ to it AFTER you have read BOTH data registers)ADIE : Interrupt enable (in the software we do not used interrupts yet so keep it ‘0’) ADPS0-2 : Three bits that determine the ADC clock prescale (e.q. 3 = 1/8)

D7 D6 D5 D4 D3 D2 D1 D0ADEN ADSC 0 ADIF ADIE ADPS2 ADPS1 ADPS0

The ADMUX Register


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The ADC Multiplexer Register ADMUX is used to select a channel to digitise (3 bits for Channel 0-7) :

D7 D6 D5 D4 D3 D2 D1 D0MUX2 MUX1 MUX0

The ADCH/ADCL Registers


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The ADC Data Register ADCL is used for the 8 lower data bits (D7 - D0) :

The ADC Data Register ADCH is used for the two highest data bits (D9,D8) :

D7 D6 D5 D4 D3 D2 D1 D0D7 D6 D5 D4 D3 D2 D1 D0

D7 D6 D5 D4 D3 D2 D1 D0D9 D8

ADCL

ADCH

Task Plan


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Design and construct a Digital Voltmeter:The Atmega128 on-chip ADCs should be used

to digitise the input analog voltages.The LCD should be used to display the

voltmeter readings.Switches or a keyboard can be used to tell

the Voltmeter to capture data

Connect your Voltmeter to the potentiometer and measurethe voltage. Calibrate it against a Voltmeter in the Lab.Demonstrate that your device works !!!!

Conceptual Design


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We want to construct :

ATmega128Board

Voltage to bemeasured

Connector

LCD DISPLAYHITACHI LM032XMBL

LCD INTERFACE

PORTD Buttons

AnalogUse ADC0

+5 Volts

10K

10K

Some Important points :


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The Voltage you are trying to measure must be between 0 – 2.5 Volts

The reference voltage of the ADC should not be lower than the voltage you are trying to measure!

First you should set the reference voltage at its highest value (4.5 V) using the potentiometer and by measuring it at the Ref. pin of PORTF

Software Design of the Voltmeter


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First ?

Display a message on the LCD sayingthat the Voltmeter is ready to make a Measurement and ask the user if he

wants to measure a voltage (capture data)

Read in Input fromPORTD or the Keyboard

Yes

Yes

No

No

Start

Display the voltageon to the LCD

Initialize LCDInitialize the ADC

Decode the Inputand if it is ‘YES’then digitise a

Voltage

Getting Started


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Use the user interface you created with the LCD display and the keyboard or the PORTD switches.

Read about using the on-board ADC in the ATmega128 manual.

Use the routines in the ADC1.inc program on the web page as a starting point, adding the relevant parts to your own code

ADC Driver routines


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.def TempReg = r16

.def ADCChannel = r18

.def ADCDL = r19

.def ADCDH = r20

ADCInit: ; ******* ADC Setup Code **** ldi TempReg, $83 ; ADC Interrupt Disabled, ADC Enableout ADCSR, TempReg ; ADC Free Run Mode, Prescaler:CK/8ret

ADCsel:out ADMUX, ADCChannel ; Channel Selectionret

DCATrig:SBI ADCSR, 6 ; Cause a Conversion

ADCCLR: SBIS ADCSR, 4 ; Wait till the Interrupt flagRJMP ADCCLR ; is cleared SBI ADCSR, 4 ; Reset ADIFIN ADCDL, ADCL ; Read in Low ByteIN ADCDH, ADCH ; Read in High ByteRET

Some gotchas!


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You really want to be able to display something meaningful on the LCD The voltage in units of Volts

Scaling your binary value to the correct level will require some arithmetic Multiply operands are available on the ATmega128 Divide is not

Convert binary values to decimal for display See “extra-examples” file for some ideas on how to do

this Or set your voltage levels to make the calculation

easier…

Binary to Dec conversion using multiply


17Set Vref

so 1V reads as $100=25

6Read ADC

output as VH:VL

• VH: units of 1V

Multiply VL by 10, result to VH:VL

• VH: units of 0.1V

Multiply VL by 10, result to VH:VL

• VH: units of 0.01V

mul r16, r17; Unsigned multiply of r16 by r17; 16-bit result stored in R1:R0

analog to digital conversion - adc mark neil - microprocessor course 1 device drivers measuring...

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