How to correctly send the data I received via I2C over UART?

Question

I am using an atmega328p and writing firmware to display the data I receive via i2c in putty.

I read the data I receive as follows:

/* Receive data from slave. */                                                                                      
TWCR = (1<<TWINT) | (1<<TWEN) | (1<<TWEA);                                                                          
while(!(TWCR & (1<<TWINT)));                                                                                        

libuart_send((char*)TWDR,1);

where:

 void libuart_send(unsigned char *data, int data_size)                                                               
 {                                                                                                                       
    for(int i = 0; i < data_size; i++)                                                                                
    {                                                                                                                   
        /* Wait for empty transmit buffer */                                                                             
        while ( !( UCSR0A & (1<<UDRE0)) );                                                                              

        /* Put data into buffer, sends the data */                                                                      
        UDR0 = data[i];                                                                                               
    }                                                                                                                   
 } 

I can see with my logic analyzer that I receive 0x00:

Yet when trying to see the corresponding data with putty, I just get a weird symbol as if I am casting it incorrectly:

I have been trying to find out how the register TWDR is defined in order to try to understand whether or not I am casting something incorrectly, but I didn't find the information I was looking for. Note that my function libuart_send works perfectly fine when I do something like this libuart_send('A', 1);, but not when I replace the first argument by TWDR. In other words the baudrate is perfectly fine.

Does anybody know why I am seing this weird character in stead of the number 0 in putty?

Thanks

Answer 1

I think you are casting it wrong. Most likely what happens is that you read the value 0x00 in TWDR and cast it as a pointer from where to read the printable character.

Try this :

unsigned char tempval;
tempval = TWDR;
libuart_send(&tempval, 1);

But if the received character value is 0, this will print the NULL character, it won't print the number or character "0" which is what you may want.

char tempstring[6];
tempstring[0]='0'+(tempval/100);
tempval=tempval%100;
tempstring[1]='0'+(tempval/10);
tempval=tempval%10;
tempstring[2]='0'+tempval;
tempstring[3]=0x0d;
tempstring[4]=0x0a;
tempstring[5]=0x00;
libuart_send(&tempstring, 6);

Answer 2

You are sending a zero, the problem is that you want to see a printable representation of 0 which is character code 0x30, not raw value 0x00.

You'll either need a terminal program which can display raw vales as a hexdump, or to write code on the Atmega side which expands each byte into a two character hex representation or an up to three character decimal one.