nRF52832 i2c communication with LIS3DH accelerometer

[augustopeterle]

Hi guys.

I'm working with a nRF52832 custom board.

In my case my board communicates with the LIS3DH through i2c protocol. In the other hand based on LIS3DH datasheet the maximum frequency of its FIFO updating is around 5KHz.

I am trying to read the LIS3DH data in the maximum frequency, but I checked how many cpu cycles was used to read the i2c register.

For read any register from LIS3DH I spend around 10500 CPU cycles. But I need to read 2 registers, OUT_X_L and OUT_X_H. In this case I spend around 21000 cpu cycles.

Analysing the time needed is 215000*1/64000000 = 0,0003359375 seconds.

In frequency terms I have the maximum 3 KHz, this math is just for one axis, there are other two axis: Y and Z.

So, I don't known why my i2c read is too slowly. Does anyone have this problem ? Could anyone help me to improve my i2c read ?

uint8_t i2c_read(uint8_t twi_addr, uint8_t reg, uint8_t * data, uint16_t len)
{
    uint32_t err_code;
    uint32_t timeout = TWI_TIMEOUT;

    twi_tx_done = false;
    err_code = nrf_drv_twi_tx(&m_twi_instance, twi_addr, &reg, 1, false);
    if(err_code != NRF_SUCCESS)
    {
      twi_tx_done = false;
      return err_code;
    }

    while((!twi_tx_done) && --timeout);
    if(!timeout) 
    {
      return NRF_ERROR_TIMEOUT;
    }
		
    twi_rx_done = false;
    err_code = nrf_drv_twi_rx(&m_twi_instance, twi_addr, data, len);
    if(err_code != NRF_SUCCESS)
    {
      twi_rx_done = false;
      return err_code;
    }

    timeout = TWI_TIMEOUT;
    while((!twi_rx_done) && --timeout);
    
    if(!timeout) 
    {
      return NRF_ERROR_TIMEOUT;
    }
    twi_rx_done = false;
		
    return err_code;
}

[Patrick]

Are you reading the two registers independently (read x_h, read x_l) or are you requesting them in one read command (requesting two bytes during read)? 

[Nguyen Hoan Hoang]

Have tried to change the frequency ? The i2c data rate can be set up to 400kbps which is approx 40KB/s transfer rate.

You may try this driver.  It's more efficient tha the sdk.  https://github.com/IOsonata/IOsonata/blob/master/ARM/Nordic/src/i2c_nrf5x.cpp

usage example https://github.com/IOsonata/IOsonata/blob/master/exemples/i2c/i2c_master_slave.cpp

[bjorn-spockeli]

As Patrick has pointed out it will be faster to read the two registers at the same time, i.e. sending the address of the register you want to start reading from and then reading the number of bytes you need. If you are doing this, then the only thing you can adjust is the frequency i think.

[augustopeterle]

Hi,

I'm reading just one byte at time. I read OUT_X_L and after OUT_X_H.

I tried to read 2 bytes but in this case I always receive zero em both registers.

Thanks for reply.

[augustopeterle]

Yes, I'm using 400KHz.

I've never used this library, I'll spent a little time studying this.

Thanks!