DDC and HDCP problems related to DDC I2C Bus cable capacitance

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  • japlytic
    Badcaps Legend
    • Oct 2005
    • 2086
    • Australia
    #1

    DDC and HDCP problems related to DDC I2C Bus cable capacitance

    I happened to find something about HDCP problems due to excessive cable capacitance (especially with long cables) on the DDC I2C Bus, causing unacceptable rise and fall times and therefore, the HDCP message exchanges are delayed for too long.

    The I2C Bus specification allows up to 400pF of capacitance (including stray) on the bus; however, some long HDMI cables can have up to or at least 700pF, causing HDCP problems due to undesirable rise/fall time on the DDC I2C Bus.

    Rise/fall constant on I2C Bus related to pullup resistor (on chip or external) value on one side of 400pF/700pF cable capacitance (as calculated by ElectroDroid) is typically:

    4K7 pullup: 1.88uS/3.29uS
    3K3 pullup: 1.32uS/2.31uS
    2K2 pullup: 0.88uS/1.54uS
    1K0 pullup: 0.50uS/0.70uS
    470R pullup: 0.235uS/0.329uS

    Rise/fall constants will be half these figures if we assume pullup resistors (again, on chip or external) are the same on each end of the DDC I2C Bus.

    According to the I2C Bus Specification:
    * For bus loads between 200pF and 400pF, the pullup device can be a current source (3mA max) or a switched resistor network.
    * Standard Mode (100 kHz) parts have a maximum rise time of 1uS for both the SDA and SCL signals.
    My first choice in quality Japanese electrolytics is Nippon Chemi-Con, which has been in business since 1931... the quality of electronics is dependent on the quality of the electrolytics.
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