Store 2 bytes from I2C Slave

Question

struggling with what is probably a straightforward piece of code to copy 2 bytes received from an I2C slave into an array so I can then convert to INT and perform maths on the data.

This code successfully (according to my I2C analyser) reads 2 consecutive bytes from one register address on a 16bit ADC.

void ADC_Initialise( void )
{
    _DINT() ;                              // disable all maskable interrupts

    I2C_SCL_HIGH() ;
    I2C_SCL_OUTPUT() ;
    I2C_SDA_HIGH() ;
    I2C_SDA_OUTPUT() ;

    I2C_Write_Register_3B( ADC, 0x01, 0xC0, 0x83) ;  // Write 2 bytes to ADC Config Register

    for(i=0; i < 10000; i++);

    I2C_Read_Register( ADC, 0x00 );      // Read the 2 byte ADC value

    for(i=0; i < 10000; i++);
    _EINT() ;                             // re-enable the interrupts
}

static unsigned char I2C_Read_Register( char Device_Address, char Register_Address )

{
    unsigned char Value ;

    I2CM_Start( ) ;

    if( I2CM_Out( Device_Address << 1 ) )    //send write control byte + chip address
        return 0 ;

    else if( I2CM_Out( Register_Address ) )  //send register number
        return 0 ;

    I2CM_Start( ) ;                          // Restart

    if( I2CM_Out(( Device_Address << 1 ) | 0x01 ))      //send read control byte + chip address
    return 0 ;

    I2CM_In( &Value, 2 ) ;                    //RJ  6.8.19 input 2 byte ADC value to 'buf'

//      return Value ;                          //rj placed after IC2M_In
    I2CM_Stop( ) ;

    return Value ;                              //rj placed after IC2M_Stop
}


static void I2CM_In( unsigned char* buf, int count )
{
    unsigned char data ;

    for( ; count--; )  // How do I store these 2 bytes in a char[] or INT
    {
            data = 0 ;
            I2C_SDA_INPUT() ;

        volatile unsigned int i = 0 ;

        for( ; i < 8 ; ++i )
        {
            //Set Clock High
            I2C_SCL_HIGH() ;              

            //shift the bit over
            data <<= 1 ;

            if( I2C_SDA_IS_HIGH() )
            {
                data |= 0x01 ;
            }
            //Set Clock Low
            brief_pause( 0x04 ) ;
            I2C_SCL_LOW() ;
        }
        //put the input data byte into the buffer, inc buffer pointer
        *buf++ = data ;

        //take sda to output ack
        I2C_SDA_OUTPUT() ;

        //Set Clock High
        I2C_SCL_HIGH() ;

        //Set Clock Low
        brief_pause( 0x04 ) ;
        I2C_SCL_LOW() ;
    }
}

So, presumably I then need an array (unsigned char adcarray [2] to collect / store the resulting reads, so I tried -

*buf++ = data ;

adcarray[] = data ;

adcarray[] = buf ;

'expected an expression' presumably means I need some form of for loop to populate the array on each pass, even though the *buf line is already inside a count 2 loop.

Assistance appreciated (C in IAR on MSP430 and ADS1115 ADC). Regards, Ralph

Answer 1

If you try to store the data in another array with the I2CM_In function, you are using it wrong.

static void I2CM_In( unsigned char* buf, int count )
{
    unsigned char data ;

    for( ; count--; )
    {
        ....
        //put the input data byte into the buffer, inc buffer pointer
        *buf++ = data ;
    }
}

The data received is already stored in a buffer: buf . It is your job to provide a buffer that is suitable to store the result.

static unsigned char I2C_Read_Register( char Device_Address, char Register_Address )
{
    unsigned char Value ;
...
    I2CM_In( &Value, 2 ) ;  // << Value is 1 byte!
...
  return Value ;
}

If you want to read a register with 16 bit, you cannot return the value as an unsigned char . You must use a data type capable of holding (at least) 2 bytes.

By telling the function that the buffer is 2 bytes long, you also cause undefined behaviour.

static uint16_t I2C_Read_Register( char Device_Address, char Register_Address )
{
    uint16_t Value ;
...
    I2CM_In( &Value, 2 ) ;
...
    return Value ;
}

Depending on the endianess you might also need to switch byte order:

static uint16_t I2C_Read_Register( char Device_Address, char Register_Address )
{
    uint8_t tmp_Value[2] ;
...
    I2CM_In( tmp_Value, 2 ) ;
    uint16_t Value;

#if MSB_FIRST
    Value = ((uint16_t)tmp_Value[0]) << 8 || tmp_Value[1];
#else // LSB_FIRST
    Value = ((uint16_t)tmp_Value[1]) << 8 || tmp_Value[0];
#endif
...
    return Value ;
}