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TXB0108: This part is not working with the Bluetooth module RN4020 for VCCA=3.3V and VCCB=5V

Ajmal T
Intellectual 420 points
Part Number: TXB0108
Other Parts Discussed in Thread: SN74LV1T34, SN74AXC4T245

When I'm trying to connect TXB0108 for translating 5V system UART signal and GPIOs to 3.3V Bluetooth module RN4020. The GPIOs and TX line are getting shifted to 3.3V level but the BT module is not responding, means it is not sendinf Rx signal. Tried with 3.3V system with and without level shifter to check whether the issue is with level shifter or not, and it is found as without level shifter it is working fine.

Can some one please help me on this?  

  • 0
    TI__Guru* 97046 points
    Hi Ajmal and welcome to the forums!

    To be of any help, we really need to see a schematic including anything connected to the TXB0108, as well as scope shots of the input and output for the signal that is problematic (measured at the pins of the TXB0108 if possible).

    I will ask our translation expert to monitor this thread for your response.
  • 0 in reply to Emrys Maier
    Intellectual 420 points

    Hi,

    Thanks for the reply. Please find below schematic portion and interface device information, TX waveform at output of TXB0108

  • 0 in reply to Ajmal T
    Intellectual 420 points
    Please note that, the level shifter part# used in design is TXB0108. We tried with TXS0108 as well.
  • 0
    TI__Guru 62320 points

    Ajmal,

    Your schematic shows both the Vcca and Vccb connected to 3.3V, but your post mentions that the 5V UART signal and Vccb =5V. 

    You mentioned that the system works fine without level shifter, so I don't understand why do you have the TXB/ TXS translator device with both the supplies at the same voltage level?

    The scope-shot of the TXB output looks fine to me, and I would think something needs to be checked on the Bluetooth side if its still not responding? 

  • 0 in reply to ShreyasRao
    Intellectual 420 points

    The current diagram is the setup we tested with MCU 3.3V system. Our plan is to use with the MCU 5V as well.  Fortunately , yesterday we identified that, the system is working if we bypass the Buffer SN74LV1T34. I think the level  shifter is not able to drive the buffer. Could you please suggest any level shifter to use with the series buffer as shown in the block diagram? Actually the buffer is on the base board and we dont need to change the base board. BT module is on daughter card. We need to use the daughter card with 5V and 3.3V system. We will use a setup on the daughter card to give 5V to VCCB of TXB0108 when 5V MCU system is connecting.

  • 0 in reply to Ajmal T
    TI__Guru 62320 points
    Hi Ajmal,

    The SN74LV1T34 is unidirectional buffer. It can drive from A to B only and cannot work if the output has an incoming signal.
    The buffers are high input impedance similiar to the MCU inputs.
    Honestly, I would either recommend to remove the TXB0108 and replace it with SN74LV1T34( x 4) or the SN74AXC4T245 which supports UART.
    Watch the video here:
    training.ti.com/voltage-level-translation-uart-interface

    Else, I would recommend removing the LV1T34 and only have the TXB0108 if it works. 2 level shifters are probably an overkill.
  • 0 in reply to ShreyasRao
    Intellectual 420 points

    Hi Sreyas,


    Thanks for the suggestion. I hope this works good!

    Regards,

    Ajmal T