I2C Load Capacitance ISSUE-Urgent

Discussion in 'The Projects Forum' started by ssundar.shan, Sep 5, 2013.

  1. ssundar.shan

    Thread Starter New Member

    Aug 28, 2013
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    Hi,
    I am interfacing my FPGA with DS2782 ("Stand-Alone Fuel Gauge IC") for I2C communication. And i have the doubt of AC Loading .
    For FPGA, the output load capacitance of each pin is given as 10pF and for DS2782, the input capacitance is given as 60 pF. It seems to be Loading problem.
    Will my I2C communication be proper? or Should i have to use any Fan Out buffers?

    Thanks in advance.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    I'm not sure buffers are useful in I2C. You can buffer the clock, SCL, because it is unidirectional. Buffering the bidirectional SDA is hard because you need logic to turn the buffer around for ACK bits and data reads. Many have tired and few have succeeded in this endeavor. If capacitance is a problem you can adjust the baudrate down to account for additional rise and fall times.
     
  3. ssundar.shan

    Thread Starter New Member

    Aug 28, 2013
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    Hi Papabravo,
    Thanks for your intrest.
    All i want to know is, Is my connection between the two devices is correct of wrong?
    My FPGA Out put capacitance is 10 pF and DS2782 input capacitance is 60 pF.
    Does it cause any problem? Any have i will be working in Standard mode (in kbps),
    Can please explain me AC Loading Analysis between these two devices?
     
  4. Papabravo

    Expert

    Feb 24, 2006
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    Assuming the chips are on the same board, so you are not connecting them with a long cable, you can add the two capacitances together for a total of 70 pf. Using the value of your pullup resister you can compute the RC time constant. This time constant needs to be small with respect to the bit rate that you contemplate using.

    Example: If the pullup is 1K Ohms the time constant is 70 nanoseconds. Maximum bitrate in I2C standard mode is 100 K bits per second for a period of 10 microseconds per bit.
    Code ( (Unknown Language)):
    1.  
    2. 10 μsec / 70 nsec ≈ 143
    3.  
    I don't foresee a problem unless there is something missing from the information you have provided.
     
    ssundar.shan likes this.
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    There should be no problems connecting these two devices as I suspect you have misread the fpga data sheet.

    Additionally, there are many I2C buffers on the market, just drop them in and off they go: they sense the data direction and configure themselves as need be. I've used them when I needed level translation between 5V and 3.3V domains.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    Do you have a link to a datasheet for one of those I2C buffers?
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Just one? You pick the one. Here's the first three hits off Google.

    TI

    LT

    NXP
     
  8. ssundar.shan

    Thread Starter New Member

    Aug 28, 2013
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    HI ErnieM,
    Thanks for your interest.
    And you can get the datasheet form google for "DS2782".
    I have also used those philips product "PCA9517". Now i need to confirm should i have to use the buffer or not?
    Please clarify my doubt, Does this Load Capacitor (CL) represents internal capacitor or the maximum capacitance allowable to connect with the particular pin?
     
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Without looking I would be 99% sure you do not need a buffer.

    However, to be 100% sure and to exercise due diligence I would need a link to the mysterious unnamed FPGA you are using, along with a helpful pointer from you as to where this 10pF spec you are concerned with.
     
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