TXB0104: TXB0104 GPIO voltage drop

Scott%20Chien

Part Number: TXB0104
Other Parts Discussed in Thread: SN74AXC4T774, TXS0104E, SN74AXC2T245, TXS0101, SN74AXC2T45, SN74AUP1G17

Hi TI,

We fould that if we use TXB0104, the GPIO voltage will drop from 2.1 to 1.6V like below.

After we checking MCU datasheet, the GPIO we use seems has a 13Kohm internal pull-down resister that we couldn't remove it.

Also TXB0104 datasheet refer that VOL = VCCx × 4.5 kΩ / (RPU + 4.5 kΩ).

From my understanding,in our case

VOH =2.1x × 13 kΩ / (13 kΩ+ 4.5 kΩ)=1.56V ≈1.6V(in datasheet also suggest pull-up/down resister shoud be larger than 50 kΩ)

Did I get that right?Is there any solution with that?

Thank you!

Best Regards,

Scott

over 3 years ago

0 Clemens Ladisch over 3 years ago

Guru 244230 points

You are correct; the TXB does not work with such a pulldown.

What is the actual problem you're trying to solve? I suspect that the SN74AXC4T774 would be a better solution.

0 Scott%20Chien over 3 years ago in reply to Clemens Ladisch

Prodigy 230 points

Hi Clemens,

Thank you for reply.Actually I would like to apply between the MCU and J-link like below.

In addition, I think if we go with auto bi-direction level shifter, it couldn't use SN74AXC4T774 because of the dir pin must to set, we wouldn't consider that pin in our design. Do you have any further recommend?Thank you!

Best Regards,

Scott

0 Clemens Ladisch over 3 years ago in reply to Scott%20Chien

Guru 244230 points

The TXB should not be used with pull-ups or pull-downs.

The TXS0104E works only with pull-ups.

Which if these lines actually are bidirectional? Which ones have pull-ups or pull-downs?

0 Scott%20Chien over 3 years ago in reply to Clemens Ladisch

Prodigy 230 points

Hi Clemens,

The SWDIO is for bi-direction IO as well as has a 13k internal pull-up resister;SWDCLK is for output as well as has a 13k internal pull-down resister.Define in below:

Also I would be appreciate that if let TI technical staff join to discuss this issue together.

Thank you!

Best Regards,

Scott

0 Clemens Ladisch over 3 years ago in reply to Scott%20Chien

Guru 244230 points

There is no single level shifter that can handle these three signals.

The SWDIO line can be handled with a TXS0101; the two unidirectional lines can be handled with a SN74AXC2T45 or SN74AXC2T245.

0 Dylan Hubbard over 3 years ago in reply to Clemens Ladisch

TI__Mastermind 42670 points

Hey Scott,

Clemens provided some viable device suggestions. The pull-up/pull-down on those lines will cause difficulty with all of our auto bidirectional translators.

0 Scott%20Chien over 3 years ago in reply to Clemens Ladisch

Prodigy 230 points

Hi Clemens,

Thank you for reply. In SWDIO I would try to use TXS0101,but if SWDCLK still have some doubt.Recently I also found there also has similar question.

Schmitt-Trigger Buffer SN74AUP1G17 as the solution

e2e.ti.com/.../647739

Is that also suitable for our application in SWDCLK?

Thank you!

Best Regards,

Scott

0 Clemens Ladisch over 3 years ago in reply to Scott%20Chien

Guru 244230 points

Yes; if a signal always goes from the 3 V device to the 2.1 V device, then it can also be handled by a buffer with an overvoltage-tolerant input.

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TXB0104: TXB0104 GPIO voltage drop