SPI_WRITE AND SPI_READ FUNCTIONS

[gianmarcod]

Hi everybody,

I would like to make my nrf52840dk communicating with an external ADC. As the ADC communicates only using SPI, I need to use spi_write and spi_read functions (actually spi_transceive). Communication must be performed using byte (8-bit) transactions. 

The spi_write function is defined in this way:

spi_write ( const struct device *dev, const struct spi_config *config, const struct spi_buf_set *tx_bufs)

Suppose I would like to transmit 1 byte of data, (the address of a register) I would write something like:

uint8_t reg_address;

const struct spi_buf tx_buf = {

.buf = &reg_address,

.len = 1

};

const struct spi_buf_set tx {

.buffers = &tx_buf,

.count = 1

};

ret = spi_write (spi_dev, spi_cfg, &tx);

Does it make sense? I set tx_buf->len to 1 as myData is 1 byte long, and tx->count to 1 as I want to transmit 1 byte only.

As for the spi_transceive function, suppose I would like to read 1 byte of data from a register located at reg_address. Recycling the code snippet written above, I would write something like:

uint8_t adc_data;

const struct spi_buf rx_buf {

.buf = &adc_data,

.len = 1

};

const struct spi_buf_set rx {

.buffers = &rx_buf,

.count = 1

};

ret = spi_transceive (spi_dev, spi_cfg, &tx, &rx);

Is it correct?

Finally, I did not understand why spi_buf can be turned into an array, and spi_buf_set can point to an array of spi_buf... Is it useful in case of multiple consecutive transactions (maybe coiled inside a for loop)? Could you please provide me with an example of multiple-consecutive 1-byte write/read operations?

Thanks in advance.

Gianmarco

[Kenneth]

I suggest to search through the examples for spi_transceive(), and you can find many example that use spi to transfer 1 or several bytes of data. There is also spi_loopback example you can try. Also look at the api documentation. 

spi loopback example: zephyr\tests\drivers\spi\spi_loopback\src\spi.c  
spi api documentation: https://developer.nordicsemi.com/nRF_Connect_SDK/doc/latest/zephyr/hardware/peripherals/spi.html#c.spi_transceive 

One thing I would mention, have in mind that the two buffers (rx and tx) run in parallell here, since there is a MOSI and MISO pin in each direction. So if you want to read something from a sensor, you typically need to make the rx buffer larger than the pure data you plan to receive (e.g. the rx buf length need to account for instance for the amount of data written on the tx to ensure you can read the value in the first place).

Kenneth