TXS0108E: I2C signal abnormal

Howard Zou

Part Number: TXS0108E
Other Parts Discussed in Thread: LSF0108

Hi,

my customer use this device for 1.8V to 3.3V I2C voltage translation, the circuit is as below: I2C signal on 3.3V side will connect to a touch screen whose communication interface is I2C.

When the touch screen is not connected, the signal of I2C1_SCL_3V3 is as below: very clean

when we connect the touch screen, the signal amplitude decrease and it's not clean:

We've connect the touch screen to other 3.3V MCU's I2C, and the signal is good.

Do you know what may cause the problem?

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 284820 points

Is there any additional DC load, such as pull-up resistors?

Is there any capacitive or inductive load, such as cables or long traces?

0 Howard Zou over 5 years ago in reply to Clemens Ladisch

TI__Genius 12365 points

No matter we add external 10k pull-up resistors or not, the waveform is the same.

How long traces and cables is recommended?

0 Clemens Ladisch over 5 years ago in reply to Howard Zou

Guru 284820 points

For the output load, see section 8.3.3 of the datasheet, and secion 4 of SCEA044.

I suspect that signal edges on those traces/cables have ringing that is large enough to trigger the edge accelerators in the opposite direction, which results in oscillations.

It should be possible to use the LSF0108 instead (which does not have edge accelerators).

0 Dylan Hubbard over 5 years ago in reply to Clemens Ladisch

TI__Mastermind 44775 points

Hi Howard,

It seems you are experiencing the effects described in this app note: http://www.ti.com/lit/an/scea054a/scea054a.pdf.

As Clemens has stated, whatever load is connected on the B side is most likely causing this. Unfortunately, I can't see anything connected to the B port in your schematic.

Logic

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TXS0108E: I2C signal abnormal