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TPS650832 I2C fall time

Antony Lin
Genius 13480 points
Other Parts Discussed in Thread: TPS650832

Hi,

In our spec we specify the I2C fall time as 20x(Vdd/5.5V)=20x(3.3/5.5)=12ns.  Customer got result of 11ns on their board.  Do you think there's an impact or not?

They tried to change external pull-up resistor value, but not helpful.

Thanks!

Antony

  • 0
    TI__Genius 17380 points

    Hi Antony,

    Those are official specifications from I2C specification. They should consult the official specification. It appears to be for backwards compatibility with various I2C speeds. It will depend what else is on the bus, but our device should be fine. If they want to increase the fall time, they can add extra capacitance.

  • 0 in reply to Kevin LaRosa
    TI__Genius 13480 points

    Hi Kevin,

    When you said backwards compatible, do you mean we can actually just refer to the spec under "standard-mode" in our datasheet?  (It only defines max value @ 300ns but no min value.)

    What do you mean "our device should be fine"?  Do you mean 11ns should be ok?

    Antony

  • 0 in reply to Antony Lin
    TI__Genius 17380 points

    Hi Antony,

    I'm referring to the I2C specification: UM10204 I 2C-bus specification and user manual

    The table is found on page 47. In the footnote, it says "Necessary to be backwards compatible with Fast-mode", which I take to mean that if customer wants their Fast-mode (or Fast-mode plus) I2C bus to be backwards compatible with a Standard-mode only device, then you need to have a fall time of > 20 × (VDD / 5.5 V). A device which only uses Standard-mode would not have required the same speed as a Fast-mode or above and can't be guaranteed to detect a falling edge in < 20 × (VDD / 5.5 V).

    Since the TPS650832 can support Fast-mode, this specification is only important if:

    1. They are running the I2C bus in Standard mode AND
    2. They have ICs on the bus that do not support Fast-mode or higher I2C speeds

    If neither of these are true, then they can ignore this specification. It is not a requirement for the TPS650832, we will detect the falling edge because we are Fast mode plus compliant.