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Using TXB0108 in a uart

Edgardo Vaz
Prodigy 50 points
Other Parts Discussed in Thread: TXB0108, TXS4555, TXS0108E
Hello.

I'm designing a contact card reader with SBC-BeagleBoard (Rev C4).
The contact card is connected to one of the UART BeagleBoard through a fullduplex to halfduplex converter. Attached the circuit schematic.
This circuit connects directly to the BeagleBoard with 1V8 voltage (without smart card) and works perfectly (I can see the echo in Rx).
As the contact cards handle voltages of 5V and the BeagleBoard only 1V8  :(  I used a TXB0108 Voltage Level Traslator to convert voltages.
Shorting TXB0108 pins associated with Tx and Rx (5V side) also receives the echo of the transmission.

The problem is when I connect two circuits (card reader and SBC), in this case stop receiving the
echo Rx. 

Will this problem be associated with pull up or pull down resistance that
mention the datasheet?
Changing resistance R9 1KOhm for 100KOhms but it didn't work either.

I used these converters on SPI port, LCD16x2 displays, LEDs and buzzer applications to the Beagle and never had any problems.

If someone can help me, I appreciate it.

Thank you very much.
  • 0
    TI__Genius 9085 points

    Hi,

    So you are having trouble with signal propagation when you connect two circuits? What is the combined capacitance of this connection? The TXB can only handly about 70pF per line. Do you have a scope capture of what occurs on the line when you are trying to communicate?

    Another device with an identical pinout is the TXS0108E. This may give you better performance. Another option when you are working with SIM cards is the TXS4555. 

  • 0 in reply to Hattie
    Prodigy 50 points

    Hi,
    thanks for your answer.
    Ther is only one circuit connected to TXB0108, that is the fullduplex to halfduplex converter, I don't know what is
    capacitance of this connection.
    I will take a capture and I'll attache it.

    In my application the smart card is directly supplied with 5V, can't use the TXS4555 but I'll keep that in mind
    for others applications.

    I Knew the TXS0108E for applications with I2C ports. Is it possible to use it for applications with UART ports?


    Thank you very much.
    Regards,
    Edgardo.

  • 0 in reply to Edgardo Vaz
    Prodigy 50 points

    Hi.

    I solved the problem with the fullduplex to halfduplex converter connected to TXB0108.
    The problem was the resistance R9 (see schematic), I changed it for a 180 Ohms value (it also works with resistances in the range between 90 and 180 Ohms) .
    I attached the image captured for the osciloscope with the shape of pulses in Tx(channel 2) and Rx(channel 1).


    It may be useful for someone else.
    Regards,
    Edgardo.