nrf52810: TWI interfacing with SHTW2 Humidity/Temperature Sensor

[Mohammed]

Hi,

I am using a custom board with the nrf52810 and trying to interface with the SHTW2 sensor. The sensor has an I2C interface and hence I use the TWI peripheral for this. However, the transfer of data does not take place. Below is my read function:

void SHTW2_read(){
    uint8_t m_sample[2];
    uint8_t writeMode[] = {0x70};
    uint8_t readMode[] = {0x71};
    uint8_t reg[2] = {0x78, 0x66};
    ret_code_t err_code;
    //while (m_xfer_done == false);
              
    m_xfer_done = false;
    err_code = nrf_drv_twi_tx(&m_twi, SHTW2_ADDR, writeMode, sizeof(writeMode), true);
    APP_ERROR_CHECK(err_code);
    while (m_xfer_done == false);
    nrf_delay_ms(5);

    m_xfer_done = false;
    err_code = nrf_drv_twi_tx(&m_twi, SHTW2_ADDR, reg, sizeof(reg), true);
    APP_ERROR_CHECK(err_code);
    while (m_xfer_done == false);
    nrf_delay_ms(5);

    m_xfer_done = false;
    err_code = nrf_drv_twi_tx(&m_twi, SHTW2_ADDR, readMode, sizeof(readMode), true);
    APP_ERROR_CHECK(err_code);
    while (m_xfer_done == false);
    m_xfer_done = false;

    err_code = nrf_drv_twi_rx(&m_twi, SHTW2_ADDR, m_sample, sizeof(m_sample));
    APP_ERROR_CHECK(err_code);
    while (m_xfer_done == false);
    temperature = m_sample[0] << 8 | m_sample[1];
}

I am using the TWI sensor example as base for this, but the code gets stuck at the while loop waiting for the handler to be called where the flag m_xfer_done is set. The handler never gets called. Can anyone point me as to where I am going wrong? May be my fundamental understanding of TWI is wrong. Thanks for the help in advance.

[Håkon Alseth]

Hi,

 

In which context are you calling the function SHTW2_read() ? If this is called from an interrupt context, your TWI handler, which sets the m_xfer_done = true, must be a higher priority.

Since it looks like you are using the TWI driver in a block fashion, you can just omit the handler, like this:

err_code = nrf_drv_twi_init(&m_my_twi_instance, &my_twi_config, NULL, NULL);

 

This way, each call will only return when the bus itself it done.

 

Kind regards,

Håkon

[Mohammed]

Hi,

I am calling the SHTW2_read() in a while loop in my main function. 

I managed to get it working using the blocking method until I try reading, then I get a NRF_DRV_TWI_EVT_ADDRESS_NACK error. Do you have any idea of what this points to? Thanks for the help in advance.

Regards,

Faizan  

[Håkon Alseth]

Could you attach the scoped signals?

[Mohammed]

Hi,

Here it is:

The four transitions denote the four transmissions I am sending but after that I should be getting data from the sensor however I don't get it. Is my understanding correct? 

Thanks,

Faizan.

[Håkon Alseth]

If you remove the delay between each TWI call, and on the oscilloscope, zoom in to show the bits and bytes going over the TWI, it would be easier to see where the sensor NACKs.

 

Best regards,

Håkon

[Mohammed]

Hi,

Sorry about the bad scope picture. Below is the zoomed up version but I always get bad looking wave-forms, this was the best one I could get. 

Looking at it it seems like we can make out some of the transmission data - like the first value I send is 0x70 which should be as marked below:

Is this correct? I'll try to get better data.

[Håkon Alseth]

Looks like your scope is giving you aliasing, as the frequency sampling (Acquisition) is set to 500 kHz. It's hard to state what is what without seeing the actual signal. Could you try either setting the scope to sample faster (in MHz range) or reduce the TWI frequency to 100 kHz?

Kind regards,

Håkon

[Mohammed]

Hi,

Thanks for the help. I do get a good waveform now with the acquisition tip you gave. Sample of the waveform is below:

I was able to check on the waveform that I get and modify the code to get the required result. Basically I was sending a write command but this is already sent when I call nrf_drv_twi_tx. I got rid of that statement and send only the command for measurement and then read.

Thanks again for the help.