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TXB0108: TXB, TXS, LSF for High-Z / Push-Pull signals?

AP
Intellectual 390 points
Part Number: TXB0108
Other Parts Discussed in Thread: TXS0108E, SN74LVC8T245

Hello,

TXB0108 (weak buffer) and TXS0108e (switch with intergaretd smart pullups) seem to rely on a low-to-high or high-to-low edge on the input,
for typical/fast output = input tracking and the datasheets are often referring to either (plain) push-pull and/or open drain signals for the input port.

Please assume an 8 bit databus between a 3.3V device and a 5V device, where the signals are not plainly push-pull or open drain, but rather High-Impedance (floating) or Active (Push-Pull) over time.
For example, the 3.3V device ist the local bus master/host/controller and starts a write accesss to a legacy 5V I/O controller.
Select, direction control, address and strobe signals may be plain push-pull, but the data bus signals may just change from high-Z to say high (or low) without a clear rail-rail edge.

Would any of the TXB, TXS or LSF 8 bit/channel family be suitable for this (with propagation delay for the valid level of say < 20 ns)?

Or does a simple 74LVC245A device seem to be a better solution (with an additional direction control signal and the 5V I/O controller being TTL input level capable)?

Thank you.

  • 0
    Guru 317080 points

    Whan all drivers are in the Hi-Z state, the bus is floating. But the TXB/TXS always weakly drive their outputs (TXB to the last value, TXS high), so the bus will never be floating.

    If the traces are short and you do not have any pull-up/pull-down resistors, then the TXB is suitable.
    If the traces are not too long and all lines are allowed to be pulled high, then the TXS is suitable.

    If you already have a direction control signal, then you can use a direction-controlled shifter with proper buffers like the SN74LVC8T245 (but you still need weak pullups to prevent the bus from floating).