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TXB0104: Direct Connection of Level translators

Jejomar Ildefonso
Expert 8590 points
Part Number: TXB0104
Other Parts Discussed in Thread: TXB0106, TXS0104E, TXS0108E, SN74AHC125, SN74AXC4T774, SN74AHCT1G125, SN74AHCT125, SN74AHC1G125, SN74LVC2T45, SN74LVC1T45

Hi Team,

Our customer is currently using the TXB0104 in a first prototype. Now they're having a 2nd PCB with the same level-shifter in obtaining the required voltage levels. The current requirement of the input driver is being met and as per their testing it works fine. Is it okay to directly connect the two devices? What other considerations should be taken?

Thanks in advance!


Kind Regards,

Jejomar

  • 0
    TI__Expert 6680 points

    Hi Jejomar,

    Could you please post a schematic of what this intended circuit will look like?

    Thanks!

    Chad Crosby

  • 0 in reply to Chad Crosby
    TI__Expert 8590 points

    Hello Chad,

    Thanks for looking into this. Attached is the schematic connection of the two PCBs. One correction: the device used in the first PCB is TXB0104 and not TXB0106.

    TXB_connection.pdf


    Kind Regards,

    Jejomar

  • 0 in reply to Jejomar Ildefonso
    Guru 244020 points

    It is not possible to connect two TXBs to each other; a TXB input requires a drive strength of at least 2 mA, while a TXB output is not rated for more than 20 µA.

    If might be possible to use TXS0104E or TXS0108E translators, if there are not any pull-down resistors.

    But SPI signals are not bidirectional. You could use transceivers with proper buffers (SN74AXC4T774) if the signals are allowed to be 3.3 V. There is no 774 device that can handle 5 V; you would need two devices on each board for uptranslating and downtranslating (SN74AHCT1G125 and SN74AHCT125 accept 3.3 V inputs; SN74AHC1G125 and SN74AHC125 running at 3.3 V accept 5 V inputs).

  • 0 in reply to Clemens Ladisch
    TI__Expert 8590 points

    Hello Clemens,

    Thank you for the inputs. However this somehow conflicts with our customer's set-up. The hardware works fine without problems. It has been tested in a lot of pcbs and the pulses on oscilloscope are appropriate. 

    When the microcontroller from either the first or second pcb gives 3.3V, in header connection between two pcbs we measure 5V steady and the same goes with 0V. We don't have any drifts in voltage levels. Microcontrollers read the packets appropriate without any lost, we check in debuggers the data send and received and all is ok. We posted here to make sure that we got everything accounted. Are you certain that the outputs can't drive that input driver of the 2nd TXB? Can you advise on some other factors that we should be looking?

    Really appreciate the help.


    Kind Regards,

    Jejomar

  • 0 in reply to Jejomar Ildefonso
    Guru 244020 points

    Sorry, I forgot the edge accelerators. They can supply enough current, but only for about 10 ns. So it will work if the other TXB has switched in this time; you will get problems only if the total capacitance of the traces, the connectors, and the I/Os exceeds 70 pF.

  • 0 in reply to Jejomar Ildefonso
    TI__Expert 6680 points

    Hi Jejomar,

    Clemens is right: There is a possibility that the edge accelerators of the TXB (Figure 8) could drive the input of the other TXB for the short time they're active; however, these edge accelerators' specifications are not characterized in the Datasheet. In short: Sure this might work, but it'd require a lot of additional edge case testing on the customer's part to see what conditions these boards would fail in.

    So, if in testing this configuration is working, then I would think it's ok to keep it; just keep in mind the above caveat.

    Thanks!

    Chad Crosby

  • 0 in reply to Clemens Ladisch
    TI__Guru 56100 points

    The requirement for the TXB is to have a driver with DC drive strength of at least 2mA if not higher.

    The TXB provides 20uA of DC drive strength, but as Clemens suggested the oneshot provides higher drive. However, we do not have any specs on the OS edge acceleration circuitry in the datasheet ( unfortunate) and hence it cannot be guaranteed to always reliably switch.

    For SPI, you could consider using the SN74LVC2T45 device and 2 *SN74LVC1T45 as well as overvoltage tolerant devices for down translation.

  • 0 in reply to ShreyasRao
    TI__Expert 8590 points

    Hi Team,

    Many thanks to your collective inputs. Apparently the current given by the accelerators are sufficient for our customer's system to work. The total capacitance of the traces, headers and input ports were only measured at 25pF, which gives enough leeway to the threshold. Attach is a test report done in their set-up. These were done across a number boards and none of them seem to fail.

    We're certainly aware of the critical precautions to be mindful in this configuration. Again, thank you very much.

    TXB_report.pdf


    Kind Regards,

    Jejomar