TXB0108: logic level on the output side is sometimes 1.7V and sometimes 3.3V

Oseihie Eigbobo

Part Number: TXB0108
Other Parts Discussed in Thread: TXS0108E

Hello TI Forum,

I am using the TXB0108 to shift the logic level of a microcontroller from 1.8V to 3.3V. The system which uses the 3.3V logic is an e-paper display and it uses SPI for communication and this is not working, so I have been trying to figure out the problem and I found out the following:

On the VCCA side, I measure 1.8V coming from the µC with no problem. It is stable. But on the VCCB side, the voltage is sometimes 3.3V and other times 1.5V. I expect it to be 3.3V as long as its input pin on VCCA side has 1.8V coming in. Or shouldn't this be the case? Also what could be the problem?

I have attached a snippet of my schematic, the part that shows the TXB0108 and its connection to the µC and Connector for the e-paper. J7 is the TXB0108, X1 is the connector for the e-paper and J1 is the µC

Any help / suggestion will be greatly appreciated. Thank you.

over 3 years ago

0 Dylan Hubbard over 3 years ago

TI__Mastermind 42670 points

Hey Oseihie,

What's all connected to the output side? I need to understand the loading of the output since that will be what's impacting the output voltage. Also, do you have any example scope shots of the issue you are experiencing?

0 Oseihie Eigbobo over 3 years ago in reply to Dylan Hubbard

Prodigy 40 points

Hi Dylan,

Thanks for your reply.

An e-paper module is connected to the output side of the TXB0108 via a XH connector ( made up of VCC, GND, MOSI, CLK, CS, DC, RST, BUSY Pins) . This is the link to the e-paper https://www.waveshare.com/wiki/1.54inch_e-Paper_Module_(B)#About_the_codes. If it is of any importance, the e-paper module has the TXS0108E level shifter on it.

I do not have example shots and unfortunately at the moment do not have access to an oscilloscope.

0 Clemens Ladisch over 3 years ago in reply to Oseihie Eigbobo

Guru 243980 points

This circuit cannot work with the TXB; see sections 8.3.3 and 8.3.5 of the datasheet.

Replace the TXB with the TXS0108E.

0 Oseihie Eigbobo over 3 years ago in reply to Clemens Ladisch

Prodigy 40 points

Hi Clemens,

Yes, I read what is in these sections and the only reason why I believe it isn't working is because of the connectors I am using which add to the load capacitance. Is there any other reason why this circuit isn't suitable for the TXB0108?

And what makes the TXS0108E any different apart from the fact it works for both open drain and push-pull?

Thanks

0 Clemens Ladisch over 3 years ago in reply to Oseihie Eigbobo

Guru 243980 points

The pull-up resistors would require the TXB to drive more than 20 µA.

While the TXB has weak output drivers, the TXS is a passive switch and relies on the drive strength of the other devices.

0 Oseihie Eigbobo over 3 years ago in reply to Clemens Ladisch

Prodigy 40 points

Okay. I will get the TXS and try it out. Thank you

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TXB0108: logic level on the output side is sometimes 1.7V and sometimes 3.3V