Thermometer using LM35

Hello there in this post I’ll be talking about how to interface a temperature sensor to msp430 launchpad. So we’ll be requiring a temperature sensor for this project. I am using LM35 IC. Well as I always say that one needs the datasheet of the IC he is using. Here is the link to the datasheet. The LM35 series are precision integrated-circuit Calibrated Directly in ° Celsius (Centigrade) temperature sensors, with an output voltage linearly proportional to the Centigrade temperature. Thus the LM35 has an advantage over linear temperature sensors calibrated in ° Kelvin, as the user is not required to subtract a large constant voltage from the output to obtain convenient Centigrade scaling.

• Linear + 10 mV/°C Scale Factor

• 0.5°C Ensured Accuracy (at +25°C)

• Rated for Full −55°C to +150°C Range

• Suitable for Remote Applications

These are few features of this sensor. The pin out is given below.

Please take care while connecting the +Vs and GND. The given view is bottom view. So just hold the IC properly to identify the terminals. I connected them thinking that this was top view(idiotic on my part), but still so that you don’t make the same mistake I am sharing this.

The connection diagram is given below.

We have a voltage reading available from lm35. So we use the inbuilt 10 bit ADC of msp430g2553. Using this ADC we can get a digital value for the analog voltage. Now we need to find the relation between this digital value and the centigrade scale. I’ve chosen 3.6V as the Vref, so 1024 corresponds to 3.6V. Now the scale is  10 mV/°C, so 3.6V corresponds to 360°C.

So we need to multiply the digital reading by 0.35, but since there is no inbuilt multiplication hardware we multiply by 35 then divide by 100 to avoid floating multiplication. Now all that is left is to display this reading on an LCD.

I’ve started using Energia recently so I’ll posting its code here as well. Regarding the ADC initialization I’ll be writing a separate tutorial for that. (Note the LCD header file used has a new function send_integer to send integer values.)

The circuit:
=================================
LCD pin              Connect to
———————————
01 – GND             GND, pot
02 – VCC             +5V, pot
03 – Contrast        Pot wiper
04 – RS               P1.4
05 – R/W              P1.5
06 – EN               P1.6
07 – DB0             GND
08 – DB1             GND
09 – DB2             GND
10 – DB3             GND
11 – DB4             P2.0
12 – DB5             P2.1
13 – DB6             P2.2
14 – DB7             P2.3
15 – BL+             +5V
16 – BL-             GND
=================================

Temperature sensor output goes to P1.0 (A0) pin.

CCS Code

/*
*  temperature_sensor.c
*  Created on     : 01-Jan-2014 1:26:21 PM
*  Author         : Manpreet Singh Minhas
*  Website        : https://learningmsp430.wordpress.com
*/
#include "lcd.h"
void main()
{

WDTCTL = WDTPW|WDTHOLD; //Stop watchdog timer
lcd_init();// Initialize the LCD
ADC10CTL0 |= ADC10ON;//ADC setup
ADC10CTL1 |= INCH_0|ADC10SSEL_1|CONSEQ_1;
ADC10AE0  |= BIT0|BIT1;
ADC10CTL0 |= ENC|ADC10SC;
TA0CCR0 = 0x1000;
TA0CTL = TASSEL_1|ID_3|MC_1|TAIE;// Timer setup
send_command(0x80);
send_string("Temperature:");
_BIS_SR(LPM3_bits + GIE);// Enter Low power mode

}
#pragma vector=TIMER0_A1_VECTOR
__interrupt void Timer_A(void)
{
ADC10CTL0 |= ENC|ADC10SC;
send_command(0xC0);
int temp = (ADC10MEM*35)/100;
send_integer(temp);
send_data(0xDF);
send_string("C");
TA0CTL &= ~(TAIFG);
}

lcd.h code

// Author : Manpreet Singh Minhas
// This file is for 4 bit mode LCD interfacing with msp430g2553 chip
// 16x2 LCD is used   
#include <msp430g2553.h>
#define DR     P1OUT = P1OUT | BIT4  // define RS high
#define CWR        P1OUT = P1OUT &(~BIT4)// define RS low
#define READ       P1OUT = P1OUT | BIT5  // define Read signal R/W = 1 for reading
#define WRITE      P1OUT = P1OUT &(~BIT5)// define Write signal R/W = 0 for writing
#define ENABLE_HIGH     P1OUT = P1OUT | BIT6  // define Enable high signal
#define ENABLE_LOW      P1OUT = P1OUT &(~BIT6)// define Enable Low signal
unsigned int i;
unsigned int j;
void delay(unsigned int k)
{
    for(j=0;j<=k;j++)
    {
        for(i=0;i<100;i++); } } void data_write(void) { ENABLE_HIGH; delay(2); ENABLE_LOW; } void data_read(void) { ENABLE_LOW; delay(2); ENABLE_HIGH; } void check_busy(void) { P2DIR &= ~(BIT3); // make P2.3 as input while((P2IN&BIT3)==1) { data_read(); } P2DIR |= BIT3; // make P2.3 as output } void send_command(unsigned char cmd) { check_busy(); WRITE; CWR; P2OUT = (P2OUT & 0xF0)|((cmd>>4) & 0x0F);   // send higher nibble
        data_write();                              // give enable trigger
        P2OUT = (P2OUT & 0xF0)|(cmd & 0x0F);      // send lower nibble
        data_write();                            // give enable trigger

}

void send_data(unsigned char data)
{
        check_busy();
        WRITE;
        DR;
        P2OUT = (P2OUT & 0xF0)|((data>>4) & 0x0F);    // send higher nibble
        data_write();                                // give enable trigger
        P2OUT = (P2OUT & 0xF0)|(data & 0x0F);       // send lower nibble
        data_write();                              // give enable trigger
}

void send_string(char *s)
{
    while(*s)
    {
        send_data(*s);
        s++;
    }
}
void send_integer(int a)
{
    {
        int temp;
        int rev=0;
        int dummy =a;
         while (dummy)
           {
              rev = rev * 10;
              rev = rev + dummy%10;
              dummy = dummy/10;
           }
        while(rev)
        {
            temp=rev%10;
            send_data(0x30+temp);
            rev /=10;
        }
    }
}
void lcd_init(void)
{
        P2DIR |= 0xFF;
        P1DIR |= BIT4|BIT5|BIT6;
        P2OUT &= 0x00;
        send_command(0x33);
        send_command(0x32);
        send_command(0x28); // 4 bit mode
        send_command(0x0E); // clear the screen
        send_command(0x01); // display on cursor on
        send_command(0x06); // increment cursor
        send_command(0x80); // row 1 column 1
}

Energia Code

#include LiquidCrystal.h;
/*
*  temperature_sensor.c
*  Created on    : 01-Jan-2014
*  Author          : Manpreet Singh Minhas
*  Website        : https://learningmsp430.wordpress.com
*/

// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(P2_0, P2_1, P2_2, P2_3, P2_4, P2_5);
byte degree[8] = {
0b00111,
0b00101,
0b00111,
0b00000,
0b00000,
0b00000,
0b00000,
0b00000
};

void setup() {
// set up the LCD's number of columns and rows:
lcd.begin(16, 2);
// Print a message to the LCD.
lcd.print("Temperature:");
Serial.begin(9600);
pinMode(A0,INPUT);
lcd.createChar(1, degree);
}

void loop() {
// set the cursor to column 0, line 1
// (note: line 1 is the second row, since counting begins with 0):
lcd.setCursor(0, 1);
// print the number of seconds since reset:

int temperature = analogRead(A0);
Serial.println(temperature);
int temp = (temperature*35)/100;
lcd.print(temp);
lcd.write(1);
lcd.print('C');
delay(3000);
}

Output