SPI Bus - Raspberry Pi Hardware Reference - page 193

The cs_change member is treated as a Boolean value. When 0, no chip select is
performed by the driver. The application is expected to do what is necessary to notify
the peripheral that it is selected (usually a GPIO pin is brought low). Once the I/O has
completed, the application then must normally unselect the slave peripheral.
When the cs_change member is true (non-zero), the slave selected will depend on
the device pathname that was opened . The bus and the slave address are embedded in the
device name:

/dev/spidev<bus>.<device>

When cs_change is true , the driver asserts GPIO 8 for spidev0.0 and asserts GPIO 7
for spidev0.1 prior to I/O and then deactivates the same upon completion. Of course,
using these two nodes requires two different open(2) calls.
The SPI_IOC_MESSAGE(n) macro is used in the ioctl(2) call to perform one or
more SPI I/O operations. The macro is unusual because it requires an argument n .
(Perhaps someone will take it upon themselves someday to clean this interface up to
work like I2C.) This specifies how many I/O transfers you would like to perform.
An array of spi_ioc_transfer structures is declared and configured for each transfer
required, as shown in the next example:

struct spi_ioc_transfer io[3]; / Define 3 transfers /

∗ ∗

int rc;

io[0].tx_buf = . . . ; / Configure I/O /

∗ ∗

...
io[2].bits_per_word = 8;

rc = ioctl(fd,SPI_IOC_MESSAGE(3),& io[0]);

The preceding example will perform three I/O transfers. Since the application
never gets to perform any GPIO manipulation between these I/Os, this applies to
communicating with one particular slave device.
The following example code brings all of the concepts together, to demonstrate one
I/O. The spi_ioc_transfer structure is initialized so that 32 bytes are transmitted and
simultaneously 32 are received.

uint8_t tx[32], rx[32];
struct spi_ioc_transfer tr;
int rc;

tr.tx_buf = (unsigned long) tx;
tr.rx_buf = (unsigned long) rx;
tr.len = 32;
tr.delay_usecs = delay;
tr.speed_hz = speed;
tr.bits_per_word = bits;