SPI Bus - Raspberry Pi Hardware Reference - page 189

These bit values are simply or-ed together to specify the options that are required.
The use of SPI_CPOL implies CPOL = 1. Its absence implies CPOL = 0. Similarly, the use
of SPI_CPHA implies CPHA = 1 (otherwise, CPHA = 0). The options SPI_MODE_x use the
SPI_CPOL and SPI_CPHA macros to define them. You don't need to use them both in your
code. The mode definitions are shown here:

#define SPI_MODE_0 (0|0)
#define SPI_MODE_1 (0|SPI_CPHA)
#define SPI_MODE_2 (SPI_CPOL|0)
#define SPI_MODE_3 (SPI_CPOL|SPI_CPHA)

The unsupported options are not shown, though one or more of these could be
supported in the future.

■ the mode values SPI_LOOP , SPI_LSB_FIRST , SPI_3WIRE , and SPI_READY are not
currently supported in the wheezy release of raspbian Linux.

Note

The following is an example that defines SPI_MODE_0 :

uint8_t mode = SPI_MODE_0;
int rc;

rc = ioctl(fd,SPI_IOC_WR_MODE,&mode);
if ( rc < 0 ) {
perror("Can't set SPI write mode.");

If you'd like to find out how the SPI driver is currently configured, you can read the
SPI mode with ioctl(2) as follows:

uint8_t mode;
int rc;

rc = ioctl(fd,SPI_IOC_RD_MODE,&mode);
if ( rc < 0 ) {
perror("Can't get SPI read mode.");