Reading magnetometer LIS2MDL via SPI

[Varun]

Hi,

using segger v5.66, SDK 17.1

My hardware has 4 wire SPI lines running to magnetometer, Initially I was unable to read the WHO_AM_I(4F)  register. After going through the datasheet I came across this

so I was suspecting this was actual problem because of which I was unable to read register

The information given in datasheet I don't understand completely what to be done, Here is my current code

#define ADD_REG_WHO_AM_I          0x4F
#define WHO_AM_I_DEFAULTvalue     0x40  //reserved register

#define SPI_BUFFER    8
#define SPI_INSTANCE  0

#define SET_READ_SINGLE_CMD(x)  (x | 0x80)

uint8_t spi_tx_buf[SPI_BUFFER];
uint8_t spi_rx_buf[SPI_BUFFER];

static volatile bool spi_xfer_done;

//create an instance
static const nrf_drv_spi_t m_spi = NRF_DRV_SPI_INSTANCE(SPI_INSTANCE);

void spi_event_handler(nrf_drv_spi_evt_t const * p_event, void * p_context)
{
    spi_xfer_done = true;
}

void spi_init(void)
{
    nrf_drv_spi_config_t spi_config = NRF_DRV_SPI_DEFAULT_CONFIG;

    spi_config.ss_pin     = 27;
    spi_config.miso_pin   = 05;
    spi_config.mosi_pin   = 17;
    spi_config.sck_pin    = 26;
    spi_config.frequency  = NRF_DRV_SPI_FREQ_4M;

    APP_ERROR_CHECK(nrf_drv_spi_init(&m_spi, &spi_config, spi_event_handler, NULL));

}

int lsm2_read_reg(char reg)
{
    spi_tx_buf[0] = SET_READ_SINGLE_CMD(reg); //creat read comd
    spi_xfer_done = false;

    APP_ERROR_CHECK(nrf_drv_spi_transfer(&m_spi, spi_tx_buf, 2, spi_rx_buf, 2));

    while(spi_xfer_done == false){}

    return spi_rx_buf[1];
}



int main(void)
{

    int interRegValue;
    APP_ERROR_CHECK(NRF_LOG_INIT(NULL));
    NRF_LOG_DEFAULT_BACKENDS_INIT();

    spi_init();

    NRF_LOG_INFO("Spi application started");

    interRegValue = lsm2_read_reg(ADD_REG_WHO_AM_I);

    if(interRegValue == WHO_AM_I_DEFAULTvalue)
    {
        NRF_LOG_INFO("WHO AM I value is correct");
    }

    while (1)
    {
     
    }
}

So now, I dont understand how do we interface this
this is what I though initially

1. initialize, config, read the register

btw, any help from community shall help, Thanks

[Jørgen Holmefjord]

Hi,

That is correct, the nRF52 series chips does not support 3-wire SPI.

If I understand the documentation of your device correctly, you need to write the value "1" to register 0x62 before you can read the WHO_AM_I register using 4 wires. Have you written a "lsm2_write_reg" function?

Best regards,
Jørgen

[Jørgen Holmefjord]

It looks like you are hardcoding the second byte to zero in your write function:

spi_tx_buf[1] = 0;

This should be set to data if your code should work correctly and write 1 to the register.

Do the datasheet tell you to mask the register with SET_WRITE_SINGLE_CMD? This will mask out the MSB of 0x62, setting the register address to 0x22.

[Varun]

Yes, I realised it was setting the second byte to zero; I updated the code and removed the masking, as nothing was mentioned on the data sheet regarding the same.

spi_tx_buf[0] = reg;
spi_tx_buf[1] = data;

but still I'm unable to read register

the above snip is taken when comparing WHO_AM_I reg in if condition..
I shall get back once I have an update regarding the same

[Jørgen Holmefjord]

Varun said:

the above snip is taken when comparing WHO_AM_I reg in if condition..

Did you first write the 0x62 register to 1?

Have you checked the SPI lines with a logic analyzer/scope to see if the input and output data match what you expect?

[Varun]

1. Yes, I first write the register to 1 before reading
2. No, wasn't able to check

I wanted to know one more thing that (let me know if im correct/wrong), even if magneto is 3 wire I should at least be able to read the register via SPI line (disable MISO) initially,
If im unable to read this then I wont be able to switch to 4W-SPI

[Jørgen Holmefjord]

As long as you have the MOSI pin connected to the SDI/O pin of the sensor, you should be able to write registers, not read registers (as a read operation typically requires both writing the register address that you want to read, and then read back the result).

I would highly recommend you to check the SPI lines with a logic analyzer, to verify that the input and output data is correct.

[Varun]

Yes, I will check and update here

Thank you.

[Varun]

Currently this is on hold, have switched to I2C