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)? 

[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.

[Patrick]

When requesting multiple bits from the LIS3DH you need to make sure you are requesting the correct sub-address. See this excerpt from the datasheet:

In order to read multiple bytes, it is necessary to assert the most significant bit of the subaddress
field. In other words, SUB(7) must be equal to 1 while SUB(6-0) represents the
address of first register to be read.

If you want to read registers starting at OUT_X_L you need to set bit 7 to 1, ie, 0x28 |= 0x40.

[augustopeterle]

I had success at reading with this function. I read two registers of OUT_X, but I still spent 13500 CPU cycles to receive those 2 registers data.

With 13500 cpu cycles, the maximum frequency is 4.7KHz.

I don't know why my function is too slow. Do you have any suggest ?

Thanks for reply.

uint8_t dado[] = {0x0, 0x0};
uint32_t c_start = DWT->CYCCNT;
i2c_read(0x19,LIS3DH_OUT_X_L|0x80,dado,2);
uint32_t c_stop = DWT->CYCCNT;

[Patrick]

What compiler optimization level are you using? Release or debug mode? Playing with those settings may be able to help some but I have no idea by how much.

[augustopeterle]

I tried now Debug mode and Release mode, but nothing change.

Thanks for reply!