How to interface LM75A temperature IC with AVR - avr

Question

I am trying to get the temperature data of LM75A which is connected to atmega8 microcontroller using i2c, and display the data to docklight using serial communication. I have written the code and the output I am getting is
FF 7F 0F
According to the datasheet, if I ignore FF then 7F 0F will lead to +125 C temperature. But i dont know if its right or wrong(and why to ignore FF). So i am confused in cracking the output I am getting. The code which I think is correct but if it is wrong please correct it.
CODE:
#ifndef F_CPU
#define F_CPU 8000000UL
#endif
#include <avr/io.h>
#include<util/delay.h>
#include <stdio.h>
//Serial tansmit
void serial_avr(char *str)
{
UCSRB=(1<<TXEN);
UCSRC=(1<<UCSZ1)|(1<<UCSZ0)|(1<<URSEL);
UBRRL=51;
for (unsigned int i=0;str[i]!=0;i++)
{
UDR=str[i];
while(!(UCSRA&(1<<UDRE)));
}
_delay_ms(500);
}
void i2c_init(void)
{
TWSR=0x00;
TWBR=0x47;
TWCR=0x04;
}
void i2c_start(void)
{
TWCR = (1 << TWINT) | (1 << TWSTA) | (1 << TWEN);
while ((TWCR & (1 << TWINT)) == 0);
}
void i2c_write(unsigned char data)
{
TWDR = data ;
TWCR = (1<< TWINT)|(1<<TWEN);
while ((TWCR & (1 <<TWINT)) == 0);
}
unsigned char i2c_read(unsigned char ackVal)
{
TWCR = (1<< TWINT) | (1<<TWEN) | (ackVal<<TWEA);
while (!(TWCR & (1 <<TWINT)));
return TWDR ;
}
void i2c_stop()
{
TWCR = (1<< TWINT)|(1<<TWEN)|(1<<TWSTO);
}
void main(void)
{
int i =23;
unsigned char temp[20];
i2c_init();
i2c_start();
i2c_write(0b10010001); //slave address for LM75A
i2c_stop();
i2c_init();
i2c_start();
i2c_write(0b00000000); //pointer register address of LM75A
i2c_stop();
i2c_init();
i2c_start();
temp[20] = i2c_read(1);
i2c_stop();
while(1)
{
serial_avr(temp);
_delay_ms(2000);
}
}
I am reading the temperature in array and I am getting the output as FF 7F 0F and when I initialise it as a normal char variable then I am getting the output as C4. I am confused, i dont know where I am missing the point. If there is any error in the code then please tell me and how to crack the output.??
Please help, thanks.!

Answer 1

The first obvious error is how you treat the array temp[20].
You only read one byte from the sensor, but then write the value off the end of the array. (The only valid spots in the array are temp[0] to temp[19]. temp[20] is past the memory allocated.) You should be reading 3 bytes from the sensor and storing them at temp[0] to temp[2].
The next error with temp is how you write it out over the serial. You should be writing all the bytes of the array, not all the bytes until a 0. You don't know the last byte of the array is 0, because it never had a string in it. A convenient thing to do would be to #define a value for the length of the array so you could refer to it in the declaration of the array and in the write function.
Until you fix these problems, it is hard to tell if the rest is working. I don't see how you can even know that the values form the sensor are FF 7F 0F.