I2C: How do I specify a register address to read from when calling nrf_drv_twi_rx()?

Question

I'm using nrf_drv_twi to talk to an accelerometer from an nRF51822 on a custom board. I'm on SDK 10.0.0. I'm following the example from the SDK, peripheral/twi_sensor. And I'm using blocking mode, with no handler. 
 In the calls to nrf_drv_twi_tx(), I see the example code is using a two element array of uint8_t where the first element is the register address and the second is the value. 
 register_value_pair[0] = KXTJ2_CTRL_REG1;
register_value_pair[1] = 0b00000000;
err_code = nrf_drv_twi_tx(&m_twi_kionix, KIONIX_SLAVE_ADDRESS, register_value_pair,
 sizeof(register_value_pair), false);
APP_ERROR_CHECK(err_code);
 
 So far so good. But when it comes to the RX, the third param is just data, with no register address. How can the slave device or the driver possibly know which register I'm reading?

monpetit · Accepted Answer

Hi Eliot, 
 I have not used the KXTJ2 accelerometer, but can find output registers in datasheet of it. Here is an example to read accelerometer values. 
 #define KXTJ2_XOUT_L 0x06
#define KXTJ2_XOUT_H 0x07
#define KXTJ2_YOUT_L 0x08
#define KXTJ2_YOUT_H 0x09
#define KXTJ2_ZOUT_L 0x0A
#define KXTJ2_ZOUT_H 0x0B 

ret_code_t err_code;
uint8_t accelvalue[6];
uint8_t addr8 = KXTJ2_XOUT_L;

err_code = nrf_drv_twi_tx(&m_twi_kionix, KIONIX_SLAVE_ADDRESS, &addr8, 1, true);
if (NRF_SUCCESS == err_code)
 err_code = nrf_drv_twi_rx(&m_twi_kionix, KIONIX_SLAVE_ADDRESS, accelvalue, 6, false);
APP_ERROR_CHECK(err_code);
 
 If you want read the CTRL_REG1 register, 
 ret_code_t err_code;
uint8_t value;
uint8_t addr8 = KXTJ2_CTRL_REG1;

err_code = nrf_drv_twi_tx(&m_twi_kionix, KIONIX_SLAVE_ADDRESS, &addr8, 1, true);
if (NRF_SUCCESS == err_code)
 err_code = nrf_drv_twi_rx(&m_twi_kionix, KIONIX_SLAVE_ADDRESS, &value, 1, false);
APP_ERROR_CHECK(err_code);
 
 Merry Christmas. :)

I2C: How do I specify a register address to read from when calling nrf_drv_twi_rx()?

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