SPI (Serial Peripheral Interface) is a synchronous 4-wire serial interface commonly used to connect components in embedded systems.
For a detailed description of SPI, see Wikipedia.
The Mynewt HAL interface supports the SPI master functionality with both blocking and non-blocking interface. SPI slave functionality is supported in non-blocking mode.
SPI is called a 4-wire interface because of the 4 signals, MISO, MOSI, CLK, and SS. The SS signal (slave select) is an active low signal that activates a SPI slave device. This is how a master “addresses” a particular slave device. Often this signal is also referred to as “chip select” as it selects particular slave device for communications.
The Mynewt SPI HAL has blocking and non-blocking transfers. Blocking means that the API call to transfer a byte will wait until the byte completes transmissions before the function returns. Blocking interface can be used for only the master slave SPI type. Non-blocking means he function returns control to the execution environment immediately after the API call and a callback function is executed at the completion of the transmission. Non-blocking interface can be used for both master and slave SPI types.
The hal_spi_config method in the API above allows the SPI to be configured with appropriate settings for master or slave. It Must be called after the spi is initialized (i.e. after hal_spi_init is called) and when the spi is disabled (i.e. user must call hal_spi_disable if the spi has been enabled through hal_spi_enable prior to calling this function). It can also be used to reconfigure an initialized SPI (assuming it is disabled as described previously).
int hal_spi_config(int spi_num, struct hal_spi_settings *psettings);
The SPI settings consist of the following:
struct hal_spi_settings { uint8_t data_mode; uint8_t data_order; uint8_t word_size; uint32_t baudrate; /* baudrate in kHz */ };
The Mynewt SPI HAL does not include built-in SS (Slave Select) signaling. It's up to the hal_spi user to control their own SS pins. Typically applications will do this with GPIO.