static int get_slave_ack(void)
{
    int r;
    sda_high(); 
    scl_high();
    r = read_sda();
    scl_low();
    return r;
}

static int send_byte(unsigned char byte)
{
    int i;
    // SCL is currently low
    // Send byte in MSB first order
    for(i = 7; i >=0; i--) {
        if (byte & (1 << i)) sda_high();
        else sda_low();
        
        scl_high();
        scl_low();
    }
    // Now get the acknowledgement.
    return get_slave_ack();
}