How to find a truly bi-directional level shifter?

Question

I'm trying to create an interface PCB to a 32 channel USB GPIO device which has 3.3v IO, and I want the flexibility to use 1.8, 3.3 or 5v - across all pins, or at most in 2x banks of 16 - I don't need different voltages on a per-pin basis.

Speed-wise I'd be looking for at most 1MHz switching frequency, but as low as 200kHz would probably be fine.

When searching through Farnell (UK version) I've found lots of 16-channel level shifters that are technically bi-directional, but only with a switchable direction using a DIR pin handling an 8 channel bank. I need something that will give true bi-directionality on each pin without having to set the direction.

As an example, what I currently use for level shifting is this SparkFun board: SparkFun Logic Level Converter - Bi-Directional but this requires a 'low' side and a 'high' side. I suppose I could shift everything to 5v and then back down to 3.3v, but I'd prefer a simpler packaged solution.

Is there some terminology I'm missing that would help me narrow down my search to find what I need? If it even exists, that is.

Answer 1

There are bidirectional level shifters with automatic direction sensing, but that ability requires compromises in other electrical parameters.

There are two widespread architectures for automatic direction sensing:

• TI's TXB010x (and equivalent chips from other manufacturers) have the outputs in both directions active at all times:

  
  _{source: A Guide to Voltage Translation With TXB-Type Translators}

  The output drivers have a high impedance to allow other devices to override the voltage level. This means that those other devices need to have enough drive strength (at least 2 mA), and that the TXB's outputs have a very low drive strength (only 20 µA guaranteed).

• TI's TXS010x (and equivalent chips from other manufacturers) use passive switches to pass low voltage levels, and rely on pull-up resistors to generate high voltage levels:

  
  _{source: A Guide to Voltage Translation With TXS-Type Translators}

  For a low signal, other devices must have enough drive strength to sink the current from the pull-up resistors and to drive the input(s) on the other side of the translator. A high signal is generated by the pullups.

  There are level translators based on discrete MOSFETs; their architecture is essentially the same (without the one-shot edge accelerators).

TXB-type translators are appropriate only when connecting directly to CMOS outputs and inputs; pull-up/-down resistors will not work (except when extremely weak), neither will any load, such as a LED.
TXS-type translators are appropriate only when an open-drain signal is possible.

In the general case, you should use unidirectional or direction-controlled (with DIR) translators, if possible. (And for the same reasons, most GPIO pins are designed so that they must be explicitly configured as input or output.)

Answer 2

The TXB0102 is a good bidirectional level shifter that can take multiple voltages and automatically determines the direction. (the Vcc determines the switching and the B side needs to be higher voltage than the A side if I remember right).