TXB0102: Oscillations using TXB0102 and Arduino Nano

Francesco Ladogana

Part Number: TXB0102
Other Parts Discussed in Thread: TXB0106, TXS0102

Hi,
I'm using Arduino Nano with a Bluetooth Module (RN42) connected on the hardware UART with the TX and RX pins.
I'm using the TXB0102 bidirectional level shifters from Texas Instruments to pass from 5V logic levels to 3.3V logic levels. I'm using a dedicated PCB.
While I'm having no issues with the TX pin, I'm noticing a weird behaviour on RX.
Basically whenever the BT module tries to pull down the voltage at RX, oscillations occur.
This is the picture of what I'm getting when I'm sending the ASCII character 'n':
https://imgur.com/NkxLwFD

and this shows the oscillations I'm getting in greater detail:
https://imgur.com/P4j6hzR

Instead of using the hardware UART, I've then tried to implement a serial port using software serial library on common digital pins and I'm NOT having particular issues:
https://imgur.com/Oe6NiTs

(here I've disabled the internal pull-up but it works even with pullup enabled although the low logic level is higher than 0V but this is explained in the TXB0102 datasheet).

So the problem is on the RX pin used an input. I've read Arduino schematic here:
www.arduino.cc/.../Arduino_Nano-Rev3.2-SCH.pdf
and basically the hardware difference on the RX and TX pins compared to the normal digital inputs is that they're also internally connected to the FT232 USB UART chip and there is a 1K series resistor.
Could you please help me in understanding if this is pure hardware issue and I should have used an other type of level shifter or if it's a software issue in the way the serial library of Arduino is initializing the RX pin? In this case I might try to get some help on the software side on the Arduino message boards.

I've tried to add the pinMode(0,INPUT) line in my code after initializing the serial port (this should also disable the interal pullup resistor) but it doesn't fix the issue.

Thanks for your help!

over 7 years ago

0 Dylan Hubbard over 7 years ago

TI__Mastermind 46555 points

Hi Francesco,

Could I get a schematic of your set up? I believe it may be a hardware issue. The TXB device doesn't really like excessive loading as it doesn't provide much driver strength. It also doesn't like strong pull-ups ore pull-downs as those cause issues as well. My assumption is the pull-up causing the issue as the device oscillates when driving low in which case the pull-up will create a voltage divider with the internal 4k resistor in the TXB device.

Also the link for the Arduino schematic didn't work for me.

0 Francesco Ladogana over 7 years ago in reply to Dylan Hubbard

Prodigy 150 points

Hi, thanks for your reply.
This is the Arduino Nano schematic that I try to link again:
[url]www.arduino.cc/.../url]

This is the part of my schematic relative to Arduino Nano:
[url]imgur.com/.../url]

I'm using two TXB0102 chips to connect either a Bluetooth or a Xbee Module to the Arduino Nano UART through a digital selector with a switch. I'm also using a TXB0106 but that is used with the SPI bus to drive an ADC and a SDcard and it works well.
Like you said I think that the RX pin is different from other digital inputs and it's being pulled up either internally by the UART in the microcontroller or through the FT232 chip that is still connected even if I'm not using the on-board USB port. In this latter case the pullup would be through the 1k resistor in series with the pullup equivalent resistor of the FT232.
Indeed I've just tried using a different voltage translator that is meant for I2C bus and it seems to be able to drive the RX line low without issues:
[url]learn.sparkfun.com/.../url]

So I guess that the TXB0102 is not the right voltage translator to use with the Arduino Nano UART :/
Do you think that the TXS0102 from Texas could work instead?

0 Francesco Ladogana over 7 years ago in reply to Francesco Ladogana

Prodigy 150 points

The links don't work well and I can't edit the post:

My schematic:
https://imgur.com/VivGeUh

Arduino schematic:
www.arduino.cc/.../Arduino_Nano-Rev3.2-SCH.pdf

Other level shifter I've tried:
learn.sparkfun.com/.../all

0 Clemens Ladisch over 7 years ago in reply to Francesco Ladogana

Guru 318740 points

As mentioned by Dylan, you cannot use pull-up/-down or series resistors with the TXB.

If you have pull-up resistors, you can use the TXB0102 instead.

0 Francesco Ladogana over 7 years ago in reply to Clemens Ladisch

Prodigy 150 points

Clemens Ladisch said:
As mentioned by Dylan, you cannot use pull-up/-down or series resistors with the TXB.

If you have pull-up resistors, you can use the TXB0102instead.

Thank you, but you mean the TXS0102 right?

TXB is what I'm using right now.

0 Clemens Ladisch over 7 years ago in reply to Francesco Ladogana

Guru 318740 points

Oops, sorry. I indeed meant the TXS.

0 Francesco Ladogana over 7 years ago in reply to Clemens Ladisch

Prodigy 150 points

OK thanks, I'm going to use the other voltage translator.

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TXB0102: Oscillations using TXB0102 and Arduino Nano