Hello,
We have a circuit where we use a SN74LVC2G125 as a buffer between the UART signals of an FTDI FT232H device and our own device. One use case involves the power to the SN74LVC2G125 being cut off while the FTDI chip remains powered. Expectation would have been that VCC, 1Y and 2A pins of the SN74LVC2G125 are 0V in this case. However, I measure the following voltage levels:
Disconnecting the TXD pin connection does not have much effect. Disconnecting the RXD pin, however, has a significant effect and the voltage levels on all other pins of interest (VCC, 1Y, 2A) immediately goes towards 0. Although I do not know the value of the internal pull-up of the FTDI chip, I suspect it is really weak, the floating RXD pin only measures some 2.4V when not connected to the SN74LVC2G125.
I was under the impression that the statement in the datasheet regarding back-drive protection refers exactly to such a scenario:
Ioff Supports Live Insertion, Partial-Power-Down Mode, and Back-Drive Protection
Is this situation harmful for the SN74LVC2G125? Is there something I could do against it?
Regards,
Cristian
Edit: in the meanwhile I have also found out the value of the pull-up on the RxD line: it is between 40 and 190kOhm, with a typical value of 75kOhm. So quite weak.
