opensteno_qmk/docs/spi_driver.md

SPI Master Driver

The SPI Master drivers used in QMK have a set of common functions to allow portability between MCUs.

AVR Configuration

No special setup is required - just connect the SS, SCK, MOSI and MISO pins of your SPI devices to the matching pins on the MCU:

MCU	SS	SCK	MOSI	MISO
ATMega16/32U2/4	B0	B1	B2	B3
AT90USB64/128/162	B0	B1	B2	B3
ATmega32A	B4	B7	B5	B6
ATmega328/P	B2	B5	B3	B4

You may use more than one slave select pin, not just the SS pin. This is useful when you have multiple devices connected and need to communicate with them individually. SPI_SS_PIN can be passed to spi_start() to refer to SS.

ChibiOS/ARM Configuration

You'll need to determine which pins can be used for SPI -- as an example, STM32 parts generally have multiple SPI peripherals, labeled SPI1, SPI2, SPI3 etc.

To enable SPI, modify your board's halconf.h to enable SPI - both HAL_USE_SPI and SPI_USE_WAIT should be TRUE, and SPI_SELECT_MODE should be SPI_SELECT_MODE_PAD. Then, modify your board's mcuconf.h to enable the SPI peripheral you've chosen -- in the case of using SPI2, modify STM32_SPI_USE_SPI2 to be TRUE.

As per the AVR configuration, you may select any other standard GPIO as a slave select pin, and can be supplied to spi_start().

Configuration-wise, you'll need to set up the peripheral as per your MCU's datasheet -- the defaults match the pins for a Proton-C, i.e. STM32F303.

config.h override	Description	Default Value
#define SPI_DRIVER	SPI peripheral to use - SPI1 => SPID1, SPI2 => SPID2 etc.	SPID2
#define SPI_SCK_PIN	The pin to use for the SCK	B13
#define SPI_SCK_PAL_MODE	The alternate function mode for the SCK pin	5
#define SPI_MOSI_PIN	The pin to use for the MOSI	B15
#define SPI_MOSI_PAL_MODE	The alternate function mode for the MOSI pin	5
#define SPI_MISO_PIN	The pin to use for the MISO	B14
#define SPI_MISO_PAL_MODE	The alternate function mode for the MISO pin	5

Functions

void spi_init(void)

Initialize the SPI driver. This function must be called only once, before any of the below functions can be called.

---

bool spi_start(pin_t slavePin, bool lsbFirst, uint8_t mode, uint16_t divisor)

Start an SPI transaction.

Arguments

• pin_t slavePin
  The QMK pin to assert as the slave select pin, eg. B4.

• bool lsbFirst
  Determines the endianness of the transmission. If true, the least significant bit of each byte is sent first.

• uint8_t mode
  The SPI mode to use:

  Mode	Clock Polarity	Clock Phase
  0	Leading edge rising	Sample on leading edge
  1	Leading edge rising	Sample on trailing edge
  2	Leading edge falling	Sample on leading edge
  3	Leading edge falling	Sample on trailing edge

• uint16_t divisor
  The SPI clock divisor, will be rounded up to the nearest power of two. This number can be calculated by dividing the MCU's clock speed by the desired SPI clock speed. For example, an MCU running at 8 MHz wanting to talk to an SPI device at 4 MHz would set the divisor to 2.

Return Value

false if the supplied parameters are invalid or the SPI peripheral is already in use, or true.

---

spi_status_t spi_write(uint8_t data)

Write a byte to the selected SPI device.

Arguments

• uint8_t data
  The byte to write.

Return Value

SPI_STATUS_TIMEOUT if the timeout period elapses, or SPI_STATUS_SUCCESS.

---

spi_status_t spi_read(void)

Read a byte from the selected SPI device.

Return Value

SPI_STATUS_TIMEOUT if the timeout period elapses, or the byte read from the device.

---

spi_status_t spi_transmit(const uint8_t *data, uint16_t length)

Send multiple bytes to the selected SPI device.

Arguments

• const uint8_t *data
  A pointer to the data to write from.
• uint16_t length
  The number of bytes to write. Take care not to overrun the length of data.

Return Value

SPI_STATUS_TIMEOUT if the timeout period elapses, SPI_STATUS_SUCCESS on success, or SPI_STATUS_ERROR otherwise.

---

spi_status_t spi_receive(uint8_t *data, uint16_t length)

Receive multiple bytes from the selected SPI device.

Arguments

• uint8_t *data
  A pointer to the buffer to read into.
• uint16_t length
  The number of bytes to read. Take care not to overrun the length of data.

Return Value

SPI_STATUS_TIMEOUT if the internal transmission timeout period elapses, SPI_STATUS_SUCCESS on success, or SPI_STATUS_ERROR otherwise.

---

void spi_stop(void)

End the current SPI transaction. This will deassert the slave select pin and reset the endianness, mode and divisor configured by spi_start().