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Original question:

TXB0104: Trying to adapt voltage levels for an SDI-12 bus with a 3.3V MCU

SN74LVC1G17: SN74AHCT1G125 + SN74LVC1G17 used as SDI-12 Voltage Level Translator

Lim Phang Moh
Intellectual 270 points
Part Number: SN74LVC1G17
Other Parts Discussed in Thread: SN74AHCT1G125, SN74AHCT1G126, SN74LVC1G99, SN74LXC1T45

Hi guys,

As suggested on this thread by TI engineers: https://e2e.ti.com/support/logic-group/logic/f/logic-forum/887817/txb0104-trying-to-adapt-voltage-levels-for-an-sdi-12-bus-with-a-3-3v-mcu

to use a combination of SN74AHCT1G125 + SN74LVC1G17 to implement the voltage translation from SDI-12 bus to 3.3V MCU.

But, with the present of the SN74LVC1G17 which shared a single pin used both TX and RX mode, the output of the SN74AHCT1G125 will not work (goes flat). When toggling the pin (IO33 in schematic) at 3.3V, the measured input of SN74AHCT1G125 about 0.5V only and output of SDI bus voltage level (output of SN74AHCT1G125) is 0V. If I were to remove the SN74LVC1G17 chip, then the SDI bus voltage level (output of SN74AHCT1G125) goes to 5.0V as expected. And input of SN74AHCT1G125 goes to 3.3V as expected driven by the MCU.

This is how the connection looks like:

  • +1
    Guru 243820 points

    When I suggested the SN74LVC1G17, I assumed that the MCU would use separate I/Os for input and output. In this circuit, the output of the SN74LVC1G17 is always active, so it conflicts with the signal output by the MCU. (Apparently, the LVC output is six times stronger than the MCU's.)

    You could insert a resistor (e.g., 4.7 kΩ) between the SN74LVC1G17's output and IO33 to ensure that the MCU's output has priority.

    Alternatively, replace the '17 with a three-state buffer, e.g., SN74LVC1G99. (If you replace the SN74AHCT1G125 (active-low /OE) with the SN74AHCT1G126 (active-high OE), you can control both buffers with a single signal.)

    Alternatively, replace both buffers with the new SN74LXC1T45.

  • 0 in reply to Clemens Ladisch
    Intellectual 270 points

    Hi Clemens,

    That is a neat solution. I will try the resistor fix but will revert to the 3-state output buffer and SN74AHCT1G126. Will report back. Thank you.

  • 0 in reply to Lim Phang Moh
    Intellectual 270 points

    Hi Clemens,

    Just want to let you know the resistor fix works very well.

    That SN74LXC1T45 is pretty new which is a better solution for low power approach. I will design 2 circuit as you have suggested and see how they fare.

    Do you want me to close the thread or want to wait for the new design feedback? I'm cool with both.

    Thank you very much.

  • 0 in reply to Lim Phang Moh
    Guru 243820 points

    Sounds as if the question is solved.

  • 0 in reply to Lim Phang Moh
    TI__Guru* 96591 points

    Hi Lim,

    If your question is resolved, it would benefit others if you can click the green "This resolved my issue" button on Clemens's post.

    Have a great day!