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  • TI Thinks Resolved

TMP102: Inquire FAST MODE and High-Speed (HS) Mode setting

Part Number: TMP102

Hi Sir,

May I learn from you that if the SCL, SDA and ALERT pin pull-up resistor will impact the operating frequency?

Pull-up voltage: 1.8V, Pull-up resistor: 6kohm, is this pull-up resistor will impact FAST MODE and High-Speed (HS) Mode setting? (we need to support 2.85MHz)

What pull-up resistor value would you suggest us for support 400kHz, 1.2MHz and 2.85MHz?

Thank you for your kindly help.

Best regards,

Edward Chen

  • Hello Edward,

    High Speed mode does not require pull up as the devices use Push-Pull to transmit/receive during High Speed. For Fast mode of 400 kHz, I would suggest using 2.2K and size the resistors accordingly.

    Regards,

    Amit Ashara

  • In reply to Amit Ashara:

    Hi Amit,

    Sorry for causing confusing, what I mean is if I pull-up SCL, SDA and ALERT these three pins to 1.8V, what pull-up resistor value should I use if I need support 2.85MHz? How to calculate pull-up resistor based on different pull-up voltage for Fast mode and High Speed mode?

    Based on you reply, do you mean we don't need pull-up resistor for SCL, SDA and ALERT these three pins in High Speed mode?

    Thank you.

    Best regards,

    Edward Chen
  • In reply to user4773172:

    Hello Edward,

    If you look at the I2C specification, the pull up are meant for Standard, Fast and Fast-plus mode.

    For High Speed mode, the I2C master sends the High Speed Master code which causes the master and slave device to drive 1 and 0 instead of pull to 1 and drive 0 to achieve the high speed operation.

    The calculation for Fast Mode pull up is as per the electrical chart in the I2C specification which provides guidelines for sizing the pull up resistor as per current drawn, voltage and bus capacitance. The specification provides for a min-max range for the pull up and it is up to the customer to select the right value based on the range.

    Regards,

    Amit Ashara

  • In reply to Amit Ashara:

    Hi Amit,

    So based on your knowledge, if we used 1.8V pull-up voltage, what pull-up resistor and bus capacitance that you will suggest to use in  Fast Mode?

    If we use below typical circuit, do we need to remove pull-up resistor if we want to enter High Speed mode?

    Thank you.

    Best regards,

    Edward Chen

  • In reply to user4773172:

    Hello Edward

    Please refer to the following document for the Pull Up resistor sizing (Section 7)

    www.nxp.com/.../UM10204.pdf

    As for the High Speed mode, you "must" not remove the resistor. The HS mode is negotiated by use of the HS code over standard/fast bus mode which require the pull up. The effect of the pull up will cease once the HS mode is entered.

    Regards,

    Amit Ashara

  • In reply to user4773172:

    Hello Edward,

    After discussing it with my colleague, I will redact my last post. As per the I2C specification for such high speed mode of operation and using the information in the specification, the Rp must not be less than 480 ohms, requirements of 1.8V operation. Since the Bus load is not known assuming a 100 pF bus load and 40ns rise time requirement (worst case of SCL and SDA) the Rp max comes out to be ~472 ohms. If the Bus load is lesser and/or rise time requirement is relaxed, the Rp can be increased. So using 480 ohms as the starting point the customer must size the appropriate resistor for their system.

    The equations provided for Sizing the pull up still hold good as per the I2C specification.

    Regards,

    Amit Ashara

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