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I have a circuit with two voltage domains that need to talk to each other. One side is always 3.3v, but the other side can either be 3.3v or 5v depending on what physical hardware is connected. The voltage level is set by a physical switch. There are three 8 bit buses that need to communicate across the domains.

I have 3.3v <-> 5v working fine using a TXS0108E bidirectional logic level shifters. Data flows in both directions, shifting up or down as appropriate.

For simplicities sake, I was hoping that 3.3v <-> 3.3v logic could still go via the TXS0108E chip, but with the low and high voltage reference pins both set to 3.3v. This predictably doesn't work.

A simple solution to the 3.3v <-> 3.3v would be to use some T74FST3390 multiplexer bus chips to bypass the logic level shifters when in 3.3v mode, this could easily be hooked up to the physical voltage switch.

Is there a better solution that is more compact and cost effective? As stated, I was hoping that I could do all this through the same chip, but logic level shifters with a pass-through option don't seem to exist.

Thanks.

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  • \$\begingroup\$ I had a similar situation. see if it helps: electronics.stackexchange.com/questions/525785/… \$\endgroup\$ – Tirdad Sadri Nejad Nov 9 '20 at 11:26
  • \$\begingroup\$ Thanks @TirdadSadriNejad I am not understanding the accepted answer, can you explain it a little more? \$\endgroup\$ – Jay8ee Nov 9 '20 at 12:09
  • \$\begingroup\$ The datasheet states that Vcca should be <= Vccb at all times, which at least suggests it should work with 3.3V on both sides. electronics.stackexchange.com/questions/429927/… even shows an implementation of this on the Pixhawk 2 board, which presumably works. \$\endgroup\$ – ocrdu Nov 9 '20 at 12:18
  • \$\begingroup\$ @ocrdu thanks for pointing that out. I did read that in the datasheet, but in my testing, I couldn't get the hi side to drive the low side when the reference voltages were the same. Perhaps I have something not hooked up correctly. \$\endgroup\$ – Jay8ee Nov 9 '20 at 14:06
  • \$\begingroup\$ Or the voltages differ just enough to make Vcca > Vccb. \$\endgroup\$ – ocrdu Nov 9 '20 at 14:14

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