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I am interfacing a MSP432 microcontroller with a SHT21 sensor. The I2C bus contains only these two devices and has 2k2 pull-ups.

The problem is that the sensor continually generates impulses on SDA line and because of this I cannot communicate correctly with the sensor. I've tried changing the sensor with another one, but it behaves the same.

This is how the pulses look like:

enter image description here

The datasheet and Internet forums do not say anything about this behavior. It is weird that both sensors behaved the same. When I remove the sensor, the SDA line is OK.

The sensor is not PWM type because the frequency is much higher than that described in the datasheet.

enter image description here

Has anybody encountered this? Could it be a hardware-related problem?

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  • \$\begingroup\$ What is the SCL line doing while this is happening? \$\endgroup\$
    – evildemonic
    Nov 15 '17 at 16:32
  • \$\begingroup\$ It is high. Even if a I generate a start or stop, the impulses are still generated by the sensor. \$\endgroup\$
    – Cristian M
    Nov 15 '17 at 16:59
  • \$\begingroup\$ What is the power supply voltage of the sensor? Is there a bulk capacitor close? This may be a sensor SHT21P with PWM output. \$\endgroup\$
    – AltAir
    Nov 15 '17 at 17:13
  • \$\begingroup\$ The power supply is 3.3V. There is a 100nF capacitor on the microcontroller board, but not on the sensor board which is connected to the first by 10 cm wires. I think you are right, it may be a SHT21P version (I didn't even knew that there is another version of the sensor). I ordered the SHT21 version from Farnell, but I wouldn't be surprised if they gave me the wrong chip (it wouldn't be the first time). I will verify and return with an update. Thank you! \$\endgroup\$
    – Cristian M
    Nov 15 '17 at 19:34
  • \$\begingroup\$ @AltAir I does not seem to be PWM since the frequency is much higher than that described in the datasheet. Please see my updated answer. \$\endgroup\$
    – Cristian M
    Nov 15 '17 at 20:15
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I found out what is the problem. The marking on the sensor is S7WPH, which means it has SDM interface and is manufactured in 2017, according to the datasheet. I received a different sensor from Farnell than that which I ordered.

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