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TXS0108E-Q1: push pull and open drain

Elec Cheng
Expert 7055 points
Part Number: TXS0108E-Q1

Hi Expert,

I have some questions about TXS0108E-Q1 push pull and open drain.

  • As you can see as below in datasheet, does this data rate apply to both TXS0108E-q1 input port and output port? If the input signal is push pull/open drain, TXS0108E-q1 can accept up to 100Mbps/1.2Mbps. Likewise,  If the output signal is push pull/open drain, TXS0108E-q1 can transmits up to 100Mbps/1.2Mbps. Is it right? And I also don't understand why there are two different data rate. What limits the rate? The interface's own feature?

  • I see that TXS0108E-q1 has internal pull up resistor about 4k ohm. If the signal path is MCU1(i2C)-->TXS0108E-q1-->MCU2(i2c), do I need pull up external resistor for both side? From my understanding, I don't need since we have internal pull up resistor.

Thanks a lot for help!

BR,

Elec Cheng

  • 0
    Guru 243220 points

    When the external device uses push/pull drivers, the TXS can work with up to 110 Mbps. When the external device uses open-drain drivers, the TXS can work with up to 1.2 Mbps. (This limit is determined by how fast the voltage at the TXS input reaches the switching threshold.)

    The TXS is a passive switch with open-drain outputs, but it has edge accelerators that work like push/pull outputs for a short time when switching.

    The TXS has interal pull-up resistors on both sides; you should not need external ones.

  • 0 in reply to Clemens Ladisch
    TI__Expert 7055 points

    Hi Clemens,

    Thanks for your quick reply. 

    Clemens Ladisch said:
    This limit is determined by how fast the voltage at the TXS input reaches the switching threshold

    But I still have some confusion about the data rate. I take Vcca= 1.8V, Vccb= 3.3 V as example. We can get data rate parameter and rise time parameter. Is there a relationship between pulse duration and rise time? How could you get corresponding data rate from different rise time? Thanks a lot for your help!

    BR,

    Elec Cheng

  • 0 in reply to Elec Cheng
    Guru 243220 points

    The pulse duration is the same as the data rate: 1 / 45 Mbps = 22.2 ns/bit, 1 / 65 Mbps = 15.3 ns/bit, 1 / 70 Mbps = 14.3 ns/bit.

    The recommended operating conditions table specifies the maximum input transition rise/fall rate for which the "push/pull" characteristics are guaranteed. If you rise/fall rate is slower, only the "open-drain" characteristics are guaranteed.

  • 0 in reply to Clemens Ladisch
    TI__Expert 7055 points

    Hi Clemens,

    What's the  "push/pull" characteristic and what's the "open-drain" characteristic? The driving ability ? Thanks.

    BR,

    Elec Cheng

  • 0 in reply to Elec Cheng
    Guru 243220 points

    These are labels for the entries in the tables shown above. For practical purposes, they are fast edges and slow edges.

  • +1 in reply to Clemens Ladisch
    TI__Mastermind 42600 points

    Hey Elec,

    These labels refer to the type of driver interfaced with this device. Open-drain is going to have slower overall data rate due to the reliance on a pull-up to drive High vs a pFET.