I2C-ATMEGA328p as Master and 24LC256 as slave

Question

So i am trying to establish a connection between ATMEGA328p as Master and 24LC256 EEPROM Chip using I2C protocol.

• Clock is set to 100KHz
• SDA and SCK lines are pulled up to Vcc using 4.7K Ohm resistors,
• Start condition works fine, i get a ACK in return too,
• ACK received when the right address is sent,
• Fails when MSB of the address in the chip is sent!

Code:

#define FOSC         16000000UL
#define START        0x08
#define MT_SLA_ACK   0x18
#define MT_SLA_NACK  0x18
#define DEV_ADDR     0x50


typedef enum result_t{FAIL, SUCCESS}result;

void Debug_LED_ON()
{
   PORTD|=1<<PD4; //Turn on LED on PIN4 of PORTD
}

void set_clock(int freq_in_khz)
{
   TWCR = 1<<TWEN;     //Enable TWI module
   TWSR |=(1<<TWPS0); //Prescaler set to 4
   TWBR  = FOSC/freq_in_khz;
   TWBR -= 16;
   TWBR /= 8;  //2*Prescaler_value

}

void send_start()
{
   TWCR= ( (1<<TWINT) | (1<<TWEN) | (1<<TWSTA) ); //send START
   while( !(TWCR & (1<<TWINT)) ) ;                //wait for TWINT flag SET
}

void send_stop()
{
   TWCR= ( (1<<TWINT) | (1<<TWEN) | (1<<TWSTO) ); //send STOP
   //while( !(TWCR & (1<<TWINT)) ) ;                //wait for TWINT flag SET
}

result check_start_status()
{
   if ((TWSR & 0xF8) == START )
     return SUCCESS;
   else
     return FAIL;
}

void send_address(char addr_w)
{

   TWDR=addr_w; //7bit  address + W bit (write bit)
   TWCR = (1<<TWINT) | (1<<TWEN); //Clear TWINT bit to start transmission of address
   while (!(TWCR &(1<<TWINT)));  //wait for TWINT flag SET

}
result check_MT_slave_ack()
{
   if ((TWSR & 0xF8) == MT_SLA_ACK )
     return SUCCESS;
   else
     return FAIL;
}
result check_MT_slave_nack()
{
   if ((TWSR & 0xF8) == MT_SLA_NACK )
     return SUCCESS;
   else
     return FAIL;
}

void  transmit_data(char data)
{
   TWDR=data;
   TWCR = (1<<TWINT) | (1<<TWEN); //Clear TWINT bit to start transmission of Data on the bus
   while (!(TWCR &(1<<TWINT)));  //wait for TWINT flag SET

}

int main (void)
{
   DDRC=0xff;
   PORTC=0x30;   //Enable internal pullups on PORTC PINS  SDA(PC4) , SCL(PC5)
   DDRD=0xFF;   //Port D as output


   set_clock(100); //setting clock 100KHz
   send_start();

   if(check_start_status()==FAIL)
     Debug_LED_ON();


   send_address(DEV_ADDR << 1); 

   if(check_MT_slave_ack()==FAIL)
    Debug_LED_ON();

   transmit_data(0x00);  

   if(check_MT_slave_ack()==FAIL) //fails here!
    Debug_LED_ON();     

   return 0;
}

Guess somethings wrong with the code, or the way i send the address! A2,A1,A0 bits of 24LC256 are set to ground, therefore the device address is 0x50 << 1.

Update:

The device address is correctly sent now, now when i send the MSB of Address on the chip where i want to write data, it fails!

Answer 1

The value you are setting in send_address() function is one bit off. Chip address 0x50 is 7bit number, which has to be shifted 1 bit up when sending on I2C bus (lowest bit is read/write flag). So you should use send_address(0xa0) (that is 0x50<<1) in your code, or do the shift inside send_address() function.

By "when i send the Address+/W, i neither get ACK nor NACK" you mean that your code hangs in while (!(TWCR &(1<<TWINT))); loop inside send_address()? If so, than it is different issue unrelated to the address mentioned above.