RE: SPI communication on Thingy:91 board

bojan — Tue, 25 Feb 2020 08:05:18 GMT

OK, guys, I think I figured out how to send/receive data over SPIM. It is indeed necessary to have a single transmit buffer (tx_buf) that will contain command byte(s), address byte(s) as well as data to write (if there are some) as well as a single receive buffer (rx_buf) that will contain data read from SPI device. Here is how I accessed single register of ADXL362 device:

int read_adxl362(struct device *spi, const struct spi_config *spi_cfg,
                 u8_t addr, u8_t *data, u32_t num_bytes){
    
    int err = 0;

    /* read cmd */
    err = adxl362_access(spi, spi_cfg,
                           ADXL362_READ_CMD, addr, data, num_bytes);
    if (err) {
            printk("Error during SPI read\n");
            return -EIO;
    }

    return 0;       
}

int write_adxl362(struct device *spi, const struct spi_config *spi_cfg,
                  u8_t addr, u8_t *data, u32_t num_bytes){

    int err = 0;
    /* write cmd */
    err = adxl362_access(spi, spi_cfg,
                           ADXL362_WRITE_CMD, addr, data, num_bytes);
    if (err) {
            printk("Error during SPI write\n");
            return -ENXIO;
    }

    return 0;
}

int adxl362_access(struct device *spi, const struct spi_config *spi_cfg,
			    u8_t cmd, u8_t addr, void *data, size_t len)
{

	int err;
	static u8_t tx_buffer[2];
	//static u8_t rx_buffer[1];
        tx_buffer[0] = cmd;
        tx_buffer[1] = addr;

	const struct spi_buf tx_buf = {
		.buf = tx_buffer,
		.len = sizeof(tx_buffer)
	};
	const struct spi_buf_set tx = {
		.buffers = &tx_buf,
		.count = 1
	};

	struct spi_buf rx_buf = {
		.buf = data,
		.len = len + 2,
	};
	const struct spi_buf_set rx = {
		.buffers = &rx_buf,
		.count = 1
	};

	err = spi_transceive(spi_dev, spi_cfg, &tx, &rx);
	if (err) {
		printk("SPI error: %d\n", err);
	}

        return err;
}

According to Nordic's documentation, SPI master is a synchronous interface and for every byte that is sent, a different byte will be received at the same time. That is why we should look the content of the register read into rx_buf[2]. rx_buf[0] and rx_buf[1] store some data received while sending command and address bytes (tx_buf[0] and tx_buf[1], respectively).

Cheers!

Bojan.