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HDC1080: HDC1080 I2C pull up resistor value

Iven Xu
Intellectual 2090 points
Part Number: HDC1080

Dear AE team,

We are meeting an application issue of HDC1080, different pull up resistors of SDA/SCL will result of different result.

We tested the pull up resistor lower than 2.2Kohm, the communication is error with HDC1080.

When changed to 10Kohm, the communication is correct with HDC1080.

Also we have added an pull down capacitor 10pF or 22pF on SDA, and keep the 2.2Kohm pull up resistors on SDA/SDA, then the communication is correct.

You can check the attached figure for the connection between MCU(GPIO to simulate I2C) and HDC1080.

vcc=3.3V

Figure 1#: I2C communication error

Figure 2# work

Figure 3# work

My questions:

what is the pull up resistors value on the SDA/SCL between MCU and HDC1080?

how to calculate the value of these pull resistors? based on which theory?

Appreciated your comments.

Best Regards

Iven

  • 0
    TI__Intellectual 1960 points
    Dear Iven

    In the I2C communication there also other parameter that we have to consider:
    - Power supply noise
    - How many device are connected on the I2C bus
    - distance between the ICs

    Based on your feedback both solution (10Kohm and 2.2Kohm+10pF) work.
    The capacitor most probably filter out a little bit the noise on the power supply.

    Both are Ok also from our side.

    Regards
    Massimo
  • 0 in reply to Massimo Mascotto
    TI__Intellectual 2090 points

    Hi Massimo,

    thanks for your feedback.

    In customer's application, there is only one HDC1080 in the I2C bus between MCU(GPIO I2C) and HDC1080 (I2C).

    Distance from MCU to HDC1080 is very short, within 5cm.

    Question:

    1, Customer asked us how to caclulate the pull up resistors on SDA/SCL, especially for SDA?

    2, Customer asked why we put the 4.99Kohm pull up resistors on SDA/SCL in our HDC1080EVM? why put 4.99Kohm? how do product line get the 4.99Kohm?

    Best Regards

    Iven

     

  • 0 in reply to Iven Xu
    TI__Intellectual 1960 points
    HI Iven

    Several parameter can affect the selection of the pull up resistor:
    - Power supply noise
    - How many device are connected on the I2C bus
    - distance between the ICs

    The impact is on the rise time, fall time and phase delay of SCL and SDA.

    For the HDC1080EVM considering the IC present on the board and the PCB layout we identify the 4.9kohm resistor as good solution.

    If the customer already identify a resistor value that works on their application, I suggest to keep that value.

    Regards
    Massimo
  • 0 in reply to Massimo Mascotto
    TI__Intellectual 2090 points
    Hi Massimo

    I am sorry that I still cannot understand your explannation.
    Based on your reply, I still cannot understand how did you get the 4.9K on the EVM, I am looking for a theroy on how to fix the 4.9kohm.


    Best Regards
    Iven