TXB0108 Level shifter - usage questions - Page 1
Forum section: electronics / projects

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: January 20, 2020, 08:33:26 pm
content: Who has experience with these things?https://www.ti.com/lit/ds/symlink/txb0108.pdfAt a first glance, this sounds like a no-worries bridge between two devices of different I/O voltage levels, like 3V vs. 5V.And one doesn't even need to care about directions.So I got one device with a couple of outputs and inputs, and another one with corresponding inverse function I/Os to be connected to, so some of the "translation channels" on the same level shifter IC would go in one direction, the rest in the other.And it just works.Right?Or are there caveats?How does this auto-direction-sensing work? Can it be "confused" by suboptimal conditions at a corresponding pair of A/B pins?Say I have a design where the B device is not always connected, like a PCB test rig. The A device (tester) has inputs which should not be floating when nothing is connected, like would be the case when the device under test is not connected and therefore I pull the OE pin down, to make everything hi-Z.So I configure those inputs to have a "weak" internal pull-up.Would that confuse the auto-direction sensing to output something on the counter pin? (which may not be good when, after that, the device to test gets plugged in)I guess this could all be worked around by a proper sequence of applying power to stuff and toggling enable pins etc.But it would be nice to know if there even is a potential problem, I guess.

author: ataradov
author_rank: Super Contributor
timestamp: January 20, 2020, 09:06:01 pm
content: Section 8.3.1 "Architecture" of the datasheet describes in details how it works - " In a dc state, the output drivers of the TXB0108 can maintain a high or low, but are designed to be weak so that they can be overdriven by an external driver when data on the bus starts flowing the opposite direction".Basically as soon as it detects the level change on one of the ports that it did not drive itself, it assumes that this side is now driven from the outside.There are no issues with things being disconnected. But you will have an issue if both sides actively drive the line.

author: fabiodl
author_rank: Frequent Contributor
timestamp: January 21, 2020, 01:16:21 am
content: one caveat is that they are pretty weak, so they do not work for buses longer than 20 cm past few MHz

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: January 21, 2020, 07:36:24 pm
content: Quote from: fabiodl on January 21, 2020, 01:16:21 amOh!That would be bad, as there may be a 1m or so cable between things.But the highest I/O frequency would be ~ 100kHz in the particular application I'm thinking of.

author: kripton2035
author_rank: Super Contributor
timestamp: January 21, 2020, 08:04:42 pm
content: I used one ( in 4 channels version) to link between 3v and 5v on an esp8266 boardnever succeded to make it work.then I made a protoboard with 2 2n7000 and 4 resistors ( I had 2 channels to implement) and it worked well ...

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: January 22, 2020, 04:21:00 pm
content: Ok I now saw a passage in the datasheet mentioning about the shortness of traces, and direction sensing getting messed up with reflections, mkay that sounds like "no, not cable, you fool".I don't really need bidirectional per se for my current application (but I'll keep the TXB in mind for something else it may really come in handy).It was just a comfy seeming package - I need 4 ins and 4 outs, and level translation, and this seemed like some carefree block where I just connect stuff to the right side of the case according to level, all else is taken care of - too good to be trueI'll try a fixed direction device that can't get confused like: SN74LV4T125.