Address designation method for reading data from the acceleration sensor (twi_sensor example)

[y001]

Hi,

I am using nRF_SDK_15.0.0?a53641 and writing a program with keil.

I am trying to read data from the acceleration sensor(LSM303DLHC) by communication with BLENanov2(nRF52832)by TWI and to write the data read into the SD card.

Currently, I was able to communicate twi with sensor using twi_sensor example.

in twi_sensor example,write 1byte to an address

nrf_drv_twi_rx(nrf_drv_twi_t const * p_instance,
                          uint8_t               address,
                          uint8_t *             p_data,
                          uint8_t               length);

 

while reading data by,

In this acceleration sensor,addresses are dirrerent for X,Yand Z axes, so I am troubled.

There is an argument describing the slave address in nrf_drv twi_rx , but you can not specify such as reading data from a specific address(for example,read from X axis address0x28 or from Y axis address 0x2B)etc, so I would like advice.

thank you.

[awneil]

y001 said:

There is an argument describing the slave address in nrf_drv twi_rx , but you can not specify such as reading data from a specific address

I think you are confusing the Slave Address with the internal addresses of the sensor?

As far as the I2C Master is concerned (whether it's a Nordic Master or any other), these internal "addresses" are just data.

[y001]

Thank you for reply!

After all, by using nrf_drv_twi_rx, what address is reading data from?

Is it to write 1 byte with "nrf_drv_twi_tx" just before to specify the address to be read?

Sorry I do not understand how to use twi_sensor

thank you

[awneil]

Again, the nRF TWI Master just transmits data to the specified Slave address - it neither knows nor cares how the Slave will interpret that data.

Likewise, the nRF TWI Master just reads data from the specified Slave address - it neither knows nor cares how the Slave formats that data.

It would be the same for any other I2C (or "TWI") Master - so look at the examples & notes that ST supply.

Do you understand the basics of how I2C itself works?

Again, I2C just defines how a Slave is addressed - not the content of the transfers.

The I2C specification is here: http://www.nxp.com/docs/en/user-guide/UM10204.pdf

More I2C references:

http://en.wikipedia.org/wiki/I%C2%B2C

http://learn.sparkfun.com/tutorials/i2c/all

http://www.i2c-bus.org/

http://i2c.info/

[awneil]

The red-highlighted section represents the Standard I2C operation - this is the part handled by the I2C Master:

ie, the Start Condition and Slave address up to and including the ACK return.

The rest of the operation, as far as the I2C Master is concerned, is just arbitrary data:

The I2C Master has no concept or understanding of what this represents - the interpretation is purely internal to the Slave.

So you just form this into a buffer for the Master to transmit.

[y001]

Thank you for teaching carefully!
Thanks to all the output values from the sensor are 0, it is now possible to communicate.

ret_code_t err_code = nrf_drv_twi_rx(&m_twi, LM75B_ADDR, m_sample, sizeof(m_sample));
    APP_ERROR_CHECK(err_code);
  	while(m_xfer_done == false);

Even if I try to operate in blocking mode after nrf_drv_twi_rx, the output is still 0,
If you know other solutions, please reply.
Thank you very much.

[awneil]

You need to look on the wires - with an oscilloscope, logic analyser, or I2C analyser -  to see what's actually happening .

Do you have correct pullups?