twi_sensor - override register value with TX and read it with RX - nrf52840 dk

Question

Hello,

i have an issue with i2c on my nrf52840 dk. I attached the distance sensor vl53l1x to pin 27 (SCL) and 26 (SDA) and wanted to communicate via i2c with it. To test the connection, I planned to write to a register on the vl53l1x and read the value afterwards. Problem, no matter what I write via TX, I can't read the correct written value from the sensor register. It always reads a zero.

#include <stdio.h>
#include <string.h>
#include "boards.h"
#include "app_util_platform.h"
#include "app_error.h"
#include "nrf_drv_twi.h"
#include "nrf_delay.h"

#include "nrf_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.h"

/* TWI instance ID. */
#define TWI_INSTANCE_ID     0

#define VL53L1X_ADDR        0x29U //from scan

/* TWI instance. */
static const nrf_drv_twi_t m_twi = NRF_DRV_TWI_INSTANCE(TWI_INSTANCE_ID);

uint8_t buffer = 8;


__STATIC_INLINE void data_handler(uint8_t newValue)
{
    NRF_LOG_INFO("New value: %d ", newValue);
}


void twi_handler(nrf_drv_twi_evt_t const * p_event, void * p_context)
{
    NRF_LOG_INFO("handler!");
    switch (p_event->type)
    {
        case NRF_DRV_TWI_EVT_DONE:
            if (p_event->xfer_desc.type == NRF_DRV_TWI_XFER_RX)
            {
                data_handler(buffer);
            }
            break;
        default:
            break;
    }
}


void twi_init (void)
{
    ret_code_t err_code;

    const nrf_drv_twi_config_t twi_vl53l1x_config = {
       .scl                = ARDUINO_SCL_PIN,
       .sda                = ARDUINO_SDA_PIN,
       .frequency          = NRF_DRV_TWI_FREQ_100K,
       .interrupt_priority = APP_IRQ_PRIORITY_HIGH,
       .clear_bus_init     = true
    };

    err_code = nrf_drv_twi_init(&m_twi, &twi_vl53l1x_config, twi_handler, NULL);
    APP_ERROR_CHECK(err_code);

    nrf_drv_twi_enable(&m_twi);
}


void sensorInit(){

    uint8_t Addr = 0x30;

	uint8_t reg[2] = {Addr, 0x10};
	ret_code_t err_code;
	
	err_code = nrf_drv_twi_tx(&m_twi, VL53L1X_ADDR, &reg, sizeof(reg), false);
    APP_ERROR_CHECK(err_code);
}

void sensorRead(){

    uint8_t regAddr = 0x30;
	ret_code_t err_code;

    err_code = nrf_drv_twi_tx(&m_twi, VL53L1X_ADDR, regAddr, 1, false);
	
	err_code = nrf_drv_twi_rx(&m_twi, VL53L1X_ADDR, &buffer, sizeof(buffer));
	APP_ERROR_CHECK(err_code);
}


int main(void)
{
    APP_ERROR_CHECK(NRF_LOG_INIT(NULL));
    NRF_LOG_DEFAULT_BACKENDS_INIT();

    NRF_LOG_INFO("\r\nTWI sensor example started.");
    NRF_LOG_FLUSH();
    twi_init();  
    sensorInit();

    nrf_delay_ms(500);   

    while (true)
    {
        nrf_delay_ms(500);

        sensorRead();
     
        NRF_LOG_FLUSH();
        
    }
}

/** @} */

How I understand the i2c drv:

I can send via tx an array like reg[2] = {Addr, value} with a new value to the sensor register.
If I want to read a register I need to point to it, by sending just the address via tx. Then read the sensor via rx and write the read value to a buffer.

Is this correct?

So it overrides the initialized value (8) of the buffer, therefore i think it is reading something but not the correct (written) value. I must have made a mistake somewhere?!

Ps: I use Linux, SDK17, nrf52840 DK, VL53L1X

Jared · Accepted Answer

Hi, 
 I suggest that you take a look at an earlier answer of mine in a another thread regarding how the I2C/TWI protocol works. The answer contains a link to a good read that is helpful to understand the protocol. 
 Regarding your issue that you only read a 0: 
 The first step is to actually check what is sent by the slave, and verify if this is the same as you receive. Use a Oscilloscope or a logic analyzer, and check if the clock on the SCL is correct and if correct data is sent and received on the SDA line. Is the Slave sending the data that you expected it to send? 
 The next step is to verify if you're writing and configuring the device correctly. I couldn't find information regarding the register layout in the datasheet of the slave, it seems that you have to register to download the driver on their website. But, I find it hard to believe that you're initializing and reading the correct register. Because you write and read to the same register during the initialization and during the read transaction. I would expect you to write to a configuration register during the initialization and then read from a data register during the read transaction. 
 You can start with the first step of checking if any data is sent by the slave and if it's correct by probing SDA 
 regards 
 Jared

Maboo · Answer

Hi Jared, 
 I finally have a working program for the vl53l1x and the nrf52840dk, its published on gitlab . 
 Thanks again for your help. 
 Maboo

twi_sensor - override register value with TX and read it with RX - nrf52840 dk

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