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In my project I have an MCU that I want to communicate with via 5V TTL serial. I have two methods of doing this in my circuit, USB via an FT232R (5V) and Bluetooth via an HC-05 module and level shifter (TXS0102, 3.3V <-> 5V).

In order to switch between the two sources, I have implemented a MAX4525 analogue switch IC which is operated using the 5V supply from the USB on the logic switch but supplied from a different 5V source. Is this a valid design?

MAX4525 TTL source switch

EDIT: Relevant parts of the schematic...

USB and Bluetooth TTL signals

Edit: A different approach. Power the FT232R from the internal 5V supply. Use the same 3.3V supplied to the reset pin for switching the MAX4525 chip. I believe this way the MAX4525L will always be within spec? Self powered FT232R

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  • \$\begingroup\$ Show the whole schematic... \$\endgroup\$
    – Voltage Spike ♦
    Commented Jun 12, 2019 at 15:15
  • \$\begingroup\$ @laptop2d - wider schematic uploaded as requested. Thanks \$\endgroup\$
    – Joe
    Commented Jun 12, 2019 at 18:44
  • \$\begingroup\$ Technically the MAX4524 like many ICs is not rated to have signals which exceed its supply, so if your supplies are not coordinated you may want to think about that. \$\endgroup\$
    – Chris Stratton
    Commented Jun 13, 2019 at 18:58
  • \$\begingroup\$ @ChrisStratton - Good point. Maybe it would be better to supply it from the 12V source. \$\endgroup\$
    – Joe
    Commented Jun 13, 2019 at 19:02
  • \$\begingroup\$ @JoeMann - the 12v source isn't necessarily any more coordinated in the time of its presence with the USB supply than the 5v derived from the 12v would be. \$\endgroup\$
    – Chris Stratton
    Commented Jun 13, 2019 at 19:05

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