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ISO1640: Pull up value, HDMI CEC possible?

Johnson John
Intellectual 640 points
Part Number: ISO1640

Hello TI team,

Can you please clarify following questions:

1.  Is there any requirement for minimum recommended pull up resistor value? can we use pull up resistor of 27k, is it fine?

2.  We are using this device for DP++, HDMI CEC signal interface isolation, Is it fine?

3.  As we require only one section (only SCL is used for HDMI CEC), another section (SDA) pin is open, Is this fine or do we need to connect any resistor for the unused section in and out pins?

4. Can we use this device for HDMI DDC signal isolation, do you see any challenge?

thank you for the support, appreciated

Johnson John

  • +1
    TI__Mastermind 29745 points

    Hello Johnson, 

    Thank you for reaching out. Please see responses below:

    1. Is there any requirement for minimum recommended pull up resistor value? can we use pull up resistor of 27k, is it fine?
      1. 27k should be a sufficient pull-up for the application. This is outlined in 9.2.2 Detailed Design Procedure.
      2. "The minimum pullup resistors on the input lines, SDA1 and SCL1 to VCC1 must be selected in such a way that input current drawn is ≤ 3.5 mA. The minimum pullup resistors on the input lines, SDA2 and SCL2, to VCC2 must be selected in such a way that output current drawn is ≤ 50 mA. The maximum pullup resistors on the bus lines (SDA1 and SCL1) to VCC1 and on bus lines (SDA2 and SCL2) to VCC2, depends on the load and rise time requirements on the respective lines to comply with I2C protocols. For more information, see I2C Bus Pullup Resistor Calculation."
    2. We are using this device for DP++, HDMI CEC signal interface isolation, Is it fine?
      1. This depends on the role of ISO1640 in your system. ISO1640 is an I2C isolator with a maximum data rate of 50MBit/s. 
    3. As we require only one section (only SCL is used for HDMI CEC), another section (SDA) pin is open, is this fine or do we need to connect any resistor for the unused section in and out pins?
      1. The unused inputs can be left floating or connected to VCC. 
    4. Can we use this device for HDMI DDC signal isolation, do you see any challenge?
      1. From what I can tell, HDMI DDC is normally implemented with standard I2C bus. ISO1640 would be able to provide isolation in this case. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Intellectual 640 points

    Thank you, Andrew

  • 0 in reply to Johnson John
    TI__Mastermind 29745 points

    Hi Johnson, 

    I wanted to provide a quick correction. A colleague pointed out that 27kΩ is too high a value and it restricts the data rate too much. The pull-up should be <3kΩ as instructed in the app note. 

    Also, the maximum speed supported I2C channels of ISO1640 is 1.7MHz. The data rate of 50Mbps is the data rate for the integrated GPIO channels on other devices in the ISO1640 family. 

    Sorry for any confusion.
    Best,
    Andrew

  • 0 in reply to Andrew J
    Intellectual 640 points

    Hi Andrew,

    No confusion, you replied clearly, i understand, thank you for the update.

    Yes, agreed, the speed depends on the RC values as you have already described and as also in the app note.

    As HDMI CEC  runs at very low data rate <500bits/sec, we are using 27K pull up value.

    For DDC lines, we are using low value resistors to meet the high speed.

    regards,

    Johnson John

  • 0 in reply to Johnson John
    TI__Mastermind 29745 points

    Ok, glad I could help. Let me know if you need anything else. 

    Best,

    Andrew