Part Number: TXB0108
Hi, I’m working with a 3rd party device which contains a TXB0108 bidirectional voltage level translator. It is used, likely inappropriately, to drive GPIO signals to components external to the 3rd party device and the outputs of the TXB0108 are directly connected to a 40 pin GPIO header. It seems like a non-ideal choice as data sheet section 8.3.3 (Output Load Considerations) specifically states that PCB traces need to be kept short, and a small cable can easily violate that. Still, we are stuck with it.We are seeing lots of cases of signal instability and oscillation which we have traced back to reflected signals returning to the TXB0108 after > 10 ns propagation delay. This is constant with the warning in data sheet section 11.1 (Layout Guidelines.). Doing things as simple as probing with a logic analyzer can send the TXB0108 into oscillation. Since we can not change the 3rd party part that uses the TXB0108, our solution is to make a daughter board that fits as close to the 3rd party board connector as possible and to place a Schmitt-Trigger buffer (SN74HCS125-Q1) as close to the connector on our board as possible. Traces are ~ 250 mm long, so the reflection should be within 2 ns of signal start and w/in the one-shot activation period. Initial testing seems to work, but our limited probing ~ 50 mm from the TXB0108 is still showing ~30% ringing back at the TXB0108 (not sure how much of this is contributed by our test equipment) and after all the experiences we've been having I'm paranoid and want to make sure I get this right. My questions are as follows please:
- Do you recommend any additional termination elements be used prior to the receiving buffer? With the limited drive strength it will not be possible to use parallel termination. I’m not sure I can get close enough to the TXB0108 to use series termination at the source. AC termination before the receiving buffer might work and if you concur I would appreciate guidance on capacitor and resistor sizing if this has been explored already. My first thought would be 25 pF and 300 kOhm.
- Are there any buffers known to have already been tested with the TXB0108? If so is there a reference design please?
- Is there another buffer you would recommend instead of the SN74HCS125-Q1? We do not need to preserve the bi-directional functionality and it is fine if a line is permanently configured as an output.
Thanks in advance for your help
The most important measure is to minimize the trace length. You already did this.
See AN-903 for how to design AC termination. This load might be too high for the TXB; no termination might be the best option.
All CMOS input have high impedance; the logic family does not matter.
We are glad that we were able to resolve this issue, and will now proceed to close this thread.
If you have further questions related to this thread, you may click "Ask a related question" below. The newly created question will be automatically linked to this question.
In reply to Clemens Ladisch:
Thanks. We'll give it a shot.
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.