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6
ACCESS THE GPIO CONTROLLER
The TI TCA9555 provides 16 GPIO pins, with 12 connected to the Embedded PC 3000 Input and Output pins (DI0-5, DO0-5) as
described in previous sections. The remaining 4 pins are unconnected. Each can be configured as an input or an output pin. The TI
TCA9555 presents several couples of two 8-bit registers (i.e. 16 bits in each couple) with each couple dedicated to input, output and
more. To simplify the programming model, the read/write byte operations on a couple of 8-bit register will be combined into a single 16-
bit operation.
The following source code example implements the functions for reading and writing 16-bit words from and to the I2C bus via IOCTL.
int smbus_read_word(int fd, int reg, __u16 *word)
{
union i2c_smbus_data data;
struct i2c_smbus_ioctl_data ioctl_data;
int rc;
ioctl_data.read_write = I2C_SMBUS_READ;
ioctl_data.command = reg;
ioctl_data.size = I2C_SMBUS_WORD_DATA;
ioctl_data.data = &data;
rc = ioctl(fd, I2C_SMBUS, &ioctl_data);
if (rc)
fprintf(stderr, "Failed to read word from SMBUS\n");
else
*word = data.word;
return rc;
}
int smbus_write_word(int fd, int reg, __u16 word)
{
union i2c_smbus_data data;
struct i2c_smbus_ioctl_data ioctl_data;
int rc;
ioctl_data.read_write = I2C_SMBUS_WRITE;
ioctl_data.command = reg;
ioctl_data.size = I2C_SMBUS_WORD_DATA;
ioctl_data.data = &data;
data.word = word;
rc = ioctl(fd, I2C_SMBUS, &ioctl_data);
if (rc)
fprintf(stderr, "Failed to write word to SMBUS\n"”);
return rc;
}
THE REGISTERS ON GPIO CONTROLLER
The TI TCA9555 consists of four couples of 8-bit registers. From the programmer’s perspective, there are four 16-bit registers
comprising of configuration, input, output, and polarity registers.
Each bit in the configuration register represents the direction, the input or output of a GPIO pin.
 Each bit in the output register specifies the level of an output pin to be high or low.
 Each bit in the input register indicates the read value of the current level of a GPIO pin.