TXB0104: half level signal problem

user6164777

Part Number: TXB0104
Other Parts Discussed in Thread: SN74AXC4T774, LSF0204

We used TXB0104 as the level shift between MCU and EG25 module four line UART interface. MCU’s UART level is 3.3V and the module UART level is 1.8V. The circuit is as following picture.

When doing a lot of data communication, there will be a half level on the signal line, as the following picture shows. What’s the reason? Could you help to analysis?

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 318050 points

The images did not attach correctly. Please edit the question and try again.

0 user6164777 over 5 years ago in reply to Clemens Ladisch

Prodigy 50 points

0 user6164777 over 5 years ago in reply to user6164777

Prodigy 50 points

The picture as upter picture

0 Clemens Ladisch over 5 years ago in reply to user6164777

Guru 318050 points

The schematic is incomplete. What is connected to the MCU signals? Are there any pull-ups/-downs configured at the MCU/EG25 pins? How long are the traces on the board?

0 user6164777 over 5 years ago in reply to Clemens Ladisch

Prodigy 50 points

MCU UART signals connect to the B side directly, and EG25 UART signals connect to the A side directly.

MCU TXD_3V3 and RTS_3V3 is configured as output mode, no pull up, pull down, EG25 RXD_1V8 is configure as input pull up mode, and RTS_1V8 is configured as input pull down mode.

0 Clemens Ladisch over 5 years ago in reply to user6164777

Guru 318050 points

You must not use pull-up/-down resistors with the TXB (see the datasheet).

If all your signals are unidirectional, consider using a direction-controlled translator like the SN74AXC4T774 instead.

0 user6164777 over 5 years ago in reply to Clemens Ladisch

Prodigy 50 points

1. The pull-up/down resistor is not external, but configured chip internal.

2. We've been using it like this without any problems.

3. The probalitity of the customer's problem is 3/1000.

So I think this is not the reason. Do you have any ideas?

0 ShreyasRao over 5 years ago in reply to user6164777

TI__Guru 62550 points

please read the app note on the effects of pullup and pulldown resistors.

http://www.ti.com/lit/an/scea054a/scea054a.pdf

The probability of 3/1000 translates to 3000ppm which is not ideal. It could work on some devices but is not guaranteed to work on all devices.

What are the internal pullup value? Is there a way to disable the internal pullup resistors? You could also consider using the LSF0204 which requires external pullup resistors anyway, and this internal pullup will be ideal. Let me know what you consider.

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TXB0104: half level signal problem