Embedded Systems - philadelphia.edu.jo Syste… · Analog to digital converter ... – PIC16F877...

Embedded Systems

Interfacing PIC with external devicesAnalog to digital Converter

Eng. Anis NazerFirst Semester 2017-2018

● What is the time?

● What is the time ?

Definition

● Analog: can take any value

● Digital: takes a set of fixed values

● Examples of analog “things”:

– temperature

– velocity

– distance

– … etc● Examples of digital “things”:

– light: ON, OFF

– number of heartbeats in a minute

– number of students in lecture

Example: Analog Signal

Time

Vol

tage

Example: Digital Signal

[ 0 - 2 )

[ 2 - 4 )

[ 4 - 6 )

[ 6 - 8 )

[ 8 - 10 )

[ 10 - 12 )

[ 12 - 14 )

[ 14 - 16 )

[ 16 - 18 )

[ 18 - 20 ]

0 2 4 6 8 10 12 14

0

0

2

5

9

12

4

7

5

3

Grade Distribution

Number of students

Gra

de R

ange

Advantages

● What are the advantages of digital systems?

● What are the advantages of analog systems?

Problem

● Almost all real life signals are analog

● How to deal with them using digital systems ?

1) convert analog signal to digital signal

2) process converted signal using a digital processor (CPU, microcontroller, …)

3) convert signal back to analog

ADC

Dig

ital O

utpu

tin

bin

ary

ADC

Analog Input

Analog to digital converter

● ADC: a device that takes an analog value and converts it to a digital value

● Analog input:

– A value within a known range (Vmin to Vmax)● Digital output:

– a number between 0 and Nmax

ADC input/output

Vmin

Vmax

0

Nmax

Analog Input

1

2

3

Digital Output

Analog to digital converter

● Nmax is determined by the number of digital output pins of the converter:

– ex: an 8 bit converter has an output range:● min: b'0000 0000' → 0● max: b'1111 1111' → 255

– what is the range if the converter has 6 bits, 10 bits?

ADC operation

● the analog input is converted to a number

● this means that a range of analog input is converted to the same digital output

● Definitions:

– Vmin, Vmax: range of possible analog input

– Nmax: maximum digital outuput

– Step size = (Vmax – Vmin)/Nmax

ADC operation

● How to calculate the output given a certain input Vin ?

● output = (Vin – Vmin) / step size

– note: the result should be rounded up or down● Ex: Assume that Vmin = 1V, Vmax = 3 V and we have an 8 bit ADC

– what is the step size ?

– what is the digital output if the input is

1 V, 3 V, 1.5V, 2.3 V, 1.2V ?

– what is the input if the digital output is

b'0110 1101', b'1100 1010' ?

Solution

Ex: Assume that Vmin = 1V, Vmax = 3 V and we have an 8 bit ADC

– what is the step size ?

step size = ( 3 – 1 ) / 255 = 2 / 255 = 0.007843V

Solution


– what is the digital output if the input is 1 V, 3 V, 1.5V ?

– Vin = 1 → Out = ( 1-1) / 0.007843 = 0

→ Out = b'0000 0000'

Vin = 3 V → Out = (3-1) / 0.007843 = 255

→ Out = b'1111 1111'

Vin = 1.5 V → Out = (1.5-1) / 0.007843 = 63.75

→ Out = 63 or 64 = b'0011 1111' or b'0100 0000'

Solution


– what is the digital output if the input is 2.3 V, 1.2V ?

Vin = 2.3 → Out = ( 2.3-1) / 0.007843 = 165.75

→ Out = 165 or 166 = b'1010 0101' or b'1010 0110'

Vin = 1.2 V → Out = (1.2-1) / 0.007843 = 25.5

→ Out =25 or 26 = b'0001 1001' or b'0001 1010'

Solution


– what is the input if the digital output is b'0110 1101' ?

Out = (Vin – Vmin) / step size

→ Vin = Out * step size + Vmin

Out = b'0110 1101' = 109

Vin = 109 * 0.007843 + 1 = 1.8549V

The input is a range

→ input range:

1.8549 < Vin < 1.8627 (if the ADC rounds down)or 1.8471 < Vin < 1.8549 (if the ADC rounds up)

Solution


– what is the input if the digital output is b'1100 1010' ?

Out = b'1100 1010' = 202

Vin = 202 * 0.007843 + 1 = 2.5843 V

The input is a range

→ input range:

2.5843 < Vin < 2.5922 (if the ADC rounds down)2.5765 < Vin < 2.5843 (if the ADC rounds up)

Practical considerations

● An ADC takes some time to convert an input value

– Usually the converter has clock signal input (ADC_CLK)

– Each bit takes one pulse to convert

– So, the conversion time is approximately ADC_CLK period times number of bits

● The analog input should not change during this time


● during the conversion, the analog input should be held constant... why ?

● A special circuit is used to hold the analog value

● Sample and hold circuit:


● So, to perform a conversion, you need to

1) start conversion

2) hold the analog value

3) wait until conversion is finished

– this time depends on the number of bits4) proccess the result of conversion

PIC internal ADC

● Some PICs have an internal ADC module

● PIC16F877 has a 10 bit converter

● It has an 8 channel ADC:

– you can convert up to 8 analog signals

– the channels are multiplexed to a single ADC

– you can select the channel in the program

– only one conversion is possible at a given time

ADC module

● Pins that can be used as Analog inpupts:

– AN <0:7>

– To use the pin as analog input, you have to select the pin as an input through the TRIS register

● Registers to control ADC:

– ADCON0 (in bank 0)

– ADCON1 (in bank 1)● Registers that hold the result:

– ADRESH: Most significant (in bank 0)

– ADRESL: Least significant (in bank 1)

ADC module

● ADCON0:

ADON: turn the ADC on

0 : ADC module is shut off and consumes no power

1 : ADC module is operating

ADC module

● ADCON0:

GO/DONE :

1 → conversion is in progress, setting this to 1 will start the conversion process

0 → conversion is not in progress, bit is automatically cleared when conversion in finished

ADC module● ADCON0:

CHS<2:0>: select ADC channel

000 → Channel 0 (AN0)








ADC module

● ADCON0:

ADCS0, ADCS1: ADC clock select

00 → F_OSC / 2

01 → F_OSC / 4

10 → F_OSC / 8

11 → Internal ADC module RC oscillator

ADC module

● ADCON1:

ADFM: result format,

0 : left justified (ADRESH<7:0> with ADRESL<7:6> )

1: right justified (ADRESH<1:0> with ADRESL<7:0> )

PCFG3-PCFG0: ADC configuration bits

ADC module

● Steps to use ADC module in the PIC 16F877A:

– Configure ADC ● configure pins, Vref, Analog pins (ADCON1)● select channel (ADCON0)● select ADC clock (ADCON0)● turn ADC on (ADCON0)

– Wait for the ADC to acquire the analog value (Sample time)

– Start conversion (set GO/DONE bit ADCON0)

– Wait until conversion is finished ( wait until GO/Done is 0 )

– Read result ( ADRESH:ADRESL )

ADC application

● Assume you have an analog input between -2V and 2 V, we need to convert this analog input and display the equivalent digital value on the output pins

– Questions: ● Which channel is suitable● Which ports are used (inputs/outputs)● How to connect the circuit

ADC application


– Which channel is suitable?

we need Vref- =-2V , and Vref+ = 2V

and we need a single channel, from the table

→ PCFG<3:0> could be one of the following:

1000, 1011, 1100

Assume we want to use ADC channel 4

→ PCFG<3:0> = 1100,

→ CHS<2:0> = 100

ADC application


– Which ports are used (inputs/outputs)● input:

– AN4 (RA5), so we need to set TRISA<5> to 1– RA2, RA3 inputs for Vref- and Vref+

● outputs: 10 bits we can use PORTB with two bits from PORTC

ADC application


– How to connect the circuit● PORTA<5> connected to the analog input● PORTC<1:0> and PORTB<7:0> outputs connected

to LEDS● Vmin connected to AN2● Vmax connected to AN3

ADC application● Circuit diagram:

ADC application


– Which channel is suitable?

we need Vref- =-2V , and Vref+ = 2V

and we need a single channel, from the table

→ PCFG<3:0> could be one of the following:

1000, 1011, 1100

Assume we want to use ADC channel 4

→ PCFG<3:0> = 1100,

→ CHS<2:0> = 100

ADC application● ADCON0 setup

ADON = 1

GO/DONE = 0

CHS<2:0> = channel 4 → 100

ADCS<1:0> = 00

=> ADCON0 = B'0010 0001'

ADC application● ADCON1 setup

ADFM = right justified = 1

PCFG<3:0> = 1100

=> ADCON1 = B'1000 1100'

ADC application

● Program:

– initialize ports: ● RA5, RA2, RA3 inputs● PORTB output, RC1, RC0 output

– initialize ADC (ADCON0 and ADCON1)

– Loop● delay for sampling time● Start conversion (ADCON0.GO_DONE = 1)● Wait until end of conversion ( while ADCON0.GO_DONE not 0 )● display result on LEDS

– ADRESL → PORTB– ADRESH → PORTC

ADC application - Program

● Initialize ports

● Initialize ADC

TRISA = 0xFF;TRISB = 0;TRISC = 0;

ADCON0 = 0b00100001;ADCON1 = 0b10001100;


● Start Conversion

● Wait until end of conversion

● Display the conversion result

while(ADCON0.GO_DONE != 0) ;

PORTB = ADRESL;PORTC = ADRESH;

ADCON0.GO_DONE = 1;

ADC application

● Complete program

void main() {

TRISA = 0xFF; TRISB = 0; TRISC = 0; ADCON1 = 0b10001100; ADCON0 = 0b11100001;

while (1) { Delay_ms(10); // delay for acquisition time ADCON0.GO_DONE = 1; // start conversion

while(ADCON0.GO_DONE != 0) ; // wait till conversion is done

PORTB = ADRESL; // put result on output ports PORTC = ADRESH; }}

ADC application – Extra :)

● Complete program in assembly:

#include "p16f877a.inc"C1 equ 0x20C2 equ 0x21

ORG 0x00 GOTO start

ORG 0x04start BSF STATUS, RP0 MOVLW 0xFF MOVWF TRISA CLRF TRISB CLRF TRISC MOVLW B'10001100' MOVWF ADCON1 BCF STATUS, RP0 MOVLW B'00100001' MOVWF ADCON0

loop CALL delay BSF ADCON0, GO_DONEwait BTFSC ADCON0, GO_DONE GOTO wait BSF STATUS, RP0 MOVF ADRESL, 0 BCF STATUS, RP0 MOVWF PORTB MOVF ADRESH, 0 MOVWF PORTC GOTO loop delay ... ... ; delay subroutine RETURN

END

Example

● You want to display a message on an LCD screen based on the temperature. You have the following parts

– PIC16F877 microcontroller that has a 10 bit ADC

– A temperature sensor that measures temperatures between -50 °C and 150 °C. The sensor gives an output voltage between 0 and 4 volts

– A 16x2 LCD● The system should display the following messages:

– “Cold weather” if the temperature is less than 10 °C

– “Warm weather” if the temperature is between 10 °C and 25 °C

– “HOT, turn A/C!!” if the temperature is above 25 °C

Example – continued

● What is the voltage of the sensor if the temperature is 10 °C

● What is the converted digital value if the temperature is 25 °C

● What should be the value of ADCON0 and ADCON1 if the sensor is connected to AN5

● Draw the detailed connection between the microcontroller and the LCD

● Write a program in C language to operate the system and display the messages as required

ADC application 2

● Assume that you want to measure the value of a resistor ( 0-15K W ) and you want to display the value of the resistor on an LCD (like a digital multimeter)

Questions: ● how many inputs / Outputs are needed ?● Which PIC is suitable ?● How to connect the pins of the PIC ?

ADC application

● Use the ADC module of the PIC16F877 to measure a resistor value ( between 0-15K) and display result on an LCD

– Questions:● How many pins are required ?

– 1 analog input– 6 for LCD

● How to connect the pins ?– AN0 : analog input– PORTC to LCD

ADC application

● The resistance to be measured is connected to form a voltage divider with a resistor

● Review voltage divider:

V X=( RX

R+RX)V DD

Voltage divider

● Assume that VDD = 5 V, R = 5 K

– What is Vx if RX = 3 K ?

– What is Vx if RX = 10 K ?

– What is the minimum, maximum value of Vx ?

Voltage divider

● Write a formula to calculate Rx given a value of Vx :

V X=( RX

R+RX)V DD ⇒ V X (R+RX)=RXV DD

V X R+V X RX=RX V DD ⇒ V X R=RX V DD−V X RX

V X R=RX (V DD−V X ) ⇒ RX=V X R

V DD−V X

ADC application

● How to calculate the value of R

● Inside PIC we have a number N that reflects the voltage Vx

N is between 0 and 1023

Vx is between 0 and 5V

⇒ V X=5 N

1023

N=V X−0

step size=

V X

51023

=1023V X

5

ADC application: Circuit digram

ADC application

● ADCON0 setup:

– channel 0 → CHS<2:0> = 000

– ADON = 1

– GO_DONE = 0

– ADCS<1:0> = 10

=> ADCON0 = B'1000 0001' = 0x81● ADCON1 setup:

– ADFM = 1 (right justified)

– PCFG<3:0> = 1110 (AN0 analog , Vmin=0, Vmax = 5V)

– => ADCON1 = B'1000 1110' = 0x8E


void main() { int N; float Vx, Rx; char s[20];

TRISA = 0x01; // configure ADC ADCON1 = 0x8E; ADCON0 = 0x81;

TRISC = 0;

Lcd_Init(&PORTC); Lcd_Cmd(LCD_CLEAR); Lcd_Cmd(LCD_CURSOR_OFF); Lcd_Out(1,1,"R = ");

while ( 1 ) { Delay_ms(1); ADCON0.GO_DONE = 1; // start

// wait till end of conversion while( ADCON0.GO_DONE == 1);

N = ADRESH; N = N << 8; // shift left 8 bits N = N + ADRESL;

// calculate value of resistor Vx = 5.0 * N / 1023.0 ; Rx = ( 5.0*Vx ) / (5.0 Vx);

// display value on LCD FloatToStr(Rx, s); Lcd_Out(1,5,s); Lcd_Out_Cp(" K");

Delay_ms(100); }}

Embedded Systems - philadelphia.edu.jo Syste… · Analog to digital converter ... – PIC16F877...

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