Hello,
The VCC and VCCIO supplies should power on and off together and so should be supplied from the same overall source (even if VCCIO is derived from a regulator, this regulator should have the same source as VCC).
For example, you could
- Power VCC, VCCIO, 3v3out from a 3v3 rail in your device (self powered config)
- Power VCC from USB 5V and power VCCIO from 3v3OUT on the FT230X (bus powered config)
- Power VCC from USB 5V and power VCCIO from a regulator sourced from USB 5V (bus powered config) giving 3v3 or 1v8 VCCIO.
If you use the first option, you can use the VBUS Sense CBUS line to sense USB Vbus through a voltage divider to put the device in a low power suspend state when no USB host is present.
The second and third options will use power from the USB port and so won't use as much battery power. You can feed USB Vbus through a divider into an input line on your MCU and your code can use this signal to tristate it's connections to the RxD and CTS lines of the FT230X (since these are both outputs from the MCU) so that you don't have leakage from the MCU to the FT230X in their idle high state.
Best Regards, FTDI Community