Issue with I2C communication in with multiple I2C sensors

[frax84]

Hello,

i'm going crazy trying to solve an issue involving I2C communication. I have the following setup: 

- ncs 2.4.2 (can't upload to latest because i have an issue with that...i have an issue open on DevZone)

- NRF5340DK

- I2C sensor A (TCS3400)

- I2C sensor B (MCP9808)

- I2C sensor C (LSM6DSOX)

- I2C sensor D (FDC1004)

I developed all the libraries to communicate with these sensors (yes, i know that many of them are supported by Zephyr but drivers were lacking many features such as interrupt, etc.). All the libraries have been tested arranging the breakouts of the sensors on a breadboard.

Now, this is the problem: when i try to read data from the FDC1004 (using the required sequence of commands: trigger measure -> check if measure is done -> read measure) AND the sensor A is connected, the "MEASURE_DONE" bit is NEVER set. Please note that the I2C function that reads the register is working (i can read the register content and all the other bits are correctly set). If i unplug the SDA or SCL or VDD for the breakout of sensor A the MEAS_DONE bit is instantly set.

Please note that:

- Connecting or disconnecting sensor B and C is totally irrelevant. If they are connected they work fine, if they are disconnected i still have the exact same problem with A and D. This exclude any problem about the loading of the SDA/SCL paths. Moreover, as i said, ALL the functions with ALL the sensors are working, with the exception of the function "isMeasDone()";

- The only function with this strange behaviour is "isMeasDone()". All the other functions set and read all the correct values in all the registers of every sensor;

- Each sensor by itself works with the code i wrote. This means that there are no issues with the addresses and the write/read functions;

- Sensors do not have the same I2C address. I double-checked it and, moreover, i highlight again that i have this issue ONLY with that specific function;

- The TCS3400 breakout is the only one without pull-up resistors for SDA and SCL on the board, but because of the fact that the other sensor breakouts have it, this is not needed. I measured the overall pull-up values (the parallel of all the pull-up of the several breakouts) is 5.2k, so this should be fine;

- Logging is active in the project. This allows me to track any error on the comunication...and there is not.

Unfortunately i cannot share the project because it has tens of source files and would be very unpractical. Here below you can find the definition of the of the function that is giving me trouble. Again, this function works perfectly if sensor A is not connected, so there should be no issue with the code....however...here you go:

bool fdc1004_isMeasDone(fdc1004_measCh_t measNum) { 

	//Read actual register value
	uint16_t regData = 0;
	if (fdc1004_read16(FDC1004_REG_FDC_CONF, &regData) != 0)
		return false;

	switch (measNum) {
		case FDC1004_MEAS1:
		return (regData & (1 << 3));
		break;
		case FDC1004_MEAS2:
		return (regData & (1 << 2));
		break;
		case FDC1004_MEAS3:
		return (regData & (1 << 1));
		break;
		case FDC1004_MEAS4:
		return (regData & (1 << 0));
		break;
	}	

	return false;
}

int fdc1004_read16(uint8_t regAddr, uint16_t* data) {
	
  // Initialize read and write buffers
  const uint8_t wbuffer = regAddr;
  uint8_t rbuffer[2];
	
  // Read register
  int ret = i2c_write_read_dt(fdc1004_dev,&wbuffer,1,rbuffer,2);
  if (ret != 0) {
	  LOG_ERR("%s error: Failed to read register 0x%0x", __func__, regAddr);
	  return SENSOR_ERROR_READ_FAILED;
    }

  // Prepare data to return
  *data = rbuffer[0]<<8|rbuffer[1];
  
  return SENSOR_SUCCESS;
}

Just for the sake of completeness, here below is the section of code that is generating my headache:

    // Perform measurement on channel 1
    fdc1004_triggerMeasure(FDC1004_MEAS1, true);
    while (!fdc1004_isMeasDone(FDC1004_MEAS1))      //ISSUE: THIS NEVER BECOMES TRUE WHEN SENSOR A IS CONNECTED!
    {
        k_busy_wait(100000);
    }
    fdc1004_getCapacitance(FDC1004_MEAS1, &capLevel);

I'm not able to get out of this on my own. I am not even able to understand if this is an hardware or a software issue. I hope your experience can provide me some hints to solve it.

Thank you,
Frax

[hmolesworth]

This may be a simple bus loading issue; 5k2 is insufficient pull-up for 400kHz and only sufficient for a single device at 100kHz; for multiple devices 1k0 would be a better choice. The loading is more related to capacitive load than current load, since the capacitive load causes the slow rise times on I2C. Simple to test, make the pull-ups 1k0 and see if it starts working.

A second issue may be the TCS3400 colour sensor; the threshold should match the application I2C voltage to minimise loading:

TCS34001FNM 0x39 I²C VBUS = VDD Interface
TCS34003FN  0x39 I²C bus = 1.8V Interface

The last item to verify is that the I2C SCL and SDA ports are using H0D1 not S0D1. Indeed, SCL can use H0H1 if none of the I2C devices does clock-stretching or acts as a master.

[frax84]

Hi hmolesworth,

thank you for your comments. Unfortunately what you suggested seems not to apply to my case. Let's go in order:

1) I don't agree about the loading issue. In any case, i tried to check what i could and a) I2C speed is set to 100kHz and b) if i put 1k Ohm as pull-up. The result was that not a single sensor was able to communicate

2) The TCS3400 that i'm using is the TCS34001FNM. As i said in the first post, i'm able to comunicate with all the sensors taken as single

3) I still need to verify the H0D1/S0D1 element, but still i don't understand why because just 1 bit of tons of register comunication should be related with that :).

I'm planning to use a logic analyzer soon. I'll keep the post updated. In the meantime, if anyone has any good idea is more than welcome

[frax84]

I'll test everything on Monday, when I go back to work. I'll even provide a picture of the breadboard. By the way, it's a really simple configuration where I use 4 horizontal long lines to have VDD, GND, SCL and SDA and jumper wires to connect every sensor breakout to the 4 lines. The only extra connection is for two resistors that goes between VDD and SCL/SDA lines that I use to modulate the total value of the pull-up resistors. This is the list of the breakouts I am using:

1) TCS3400 (of course only the sensor part, not the AMS interface)

2) MCP9808

3) LSM6DSOX

4) FDC1004 (in this case I broke the board in three parts and soldered some pin stripe on the one with the sensor)

As you can see, some of the breakout has already pull-up resistors on it, some has not. I evaluate the total just using a multimeter between SCL/SDA line and VDD, being all of them in parallel

Again, thank you 

[hmolesworth]

As an aside "4 horizontal long lines" in general means serious capacitive loading which in turn means high-drive becomes more of an issue.

I raised this issue in a separate ticket to get a Nordic response:

nrf5340-conflicting-documentation-on-i2c-aka-twi-twim

[frax84]

Hello,

i tried the drive setting you suggested (EOE1), without any change on the final result.

I even acquired the I2C plots that you can see here below. The plots are obtained with the following settings:

- I2C speed = 100kHz (i never specified but it was always 100kHz in ALL the experiments)

- Normal drive (no H or E settings on the pins as you suggested because i was interested in verifying the shape of the I2C SDA line).

The plots show the behaviour of the FDC1004 during the I2C transaction related to the "isMeasDone()" function.

As you can see, aside from the fact that unplugging the TCS3400 (bottom plot) the "guilty bit" triggers, there are no other remarkable difference in the quality of the shape (no loading effect).

I'm confident in saying that, whatever the issue is, it's not related to loading effect. My head hurts...

Frax

[Michal]

Hello Frax,

What about the SCL line?

Could you measure both at the same time?

Best regards,

Michal

[hmolesworth]

Yes, without the SCL to mark the bits it's hard to decode; assuming only Meas 1 is enabled and complete there should be a '1' for only meas 1 and not 2, 3 and 4 but I'm not seeing that in the traces above (al least not without SCL as a guide). Does the 'scope have an I2C decode function? That would obviously help.

With the TCS3400 in place, does shorting the capacitive sense CIN or guard SHLD pin to Gnd have the same adverse effect as removing the TSC3400? Maybe something is stopping the measure clock which in turn defeats the completion of measurement.