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I designed a circuit based on FT232XS (USB bus powered configuration). I placed red Led on vbus, when I disconnect the usb cable the led doesn't completely turn off and (tx, rx) leds start slightly blinking. when I reconnect the USB cable (Tx,Rx) leds stop blinking and the intensity of Vbus's led increases.

Does anyone face the same problem. Any suggestion to fix this. PS: USB to UART works just fine? enter image description here

PCB-ABBS
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  • Side note: I'd recommend NOT connecting the GND to shield, if you have the option to do so. Shield goes to Earth and keeps the noise away from your circuit, including your GND – Christianidis Vasileios Aug 26 '21 at 11:29
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    @ChristianidisVasileios The shield connection on device side depends on multiple factors, so there is no one true answer what should be done with it, because it depends. On the PC side, shield and GND are connected anyway together at the connector. It may or may not be earthed, depending on PC, obviously. – Justme Aug 26 '21 at 11:36

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You say it is designed for self-powered configuration, but the schematics do not match a self-powered configuration.

The chip is in USB bus powered configuration. It makes it's own supply voltages from USB 5V. But when USB 5V is not present, the chip has no power supplies. Yet it has TXD and RXD connection to the main MCU. The TXD of the MCU is pushing high the FT230XS RXD input and it gets power via that IO pin.

The FT230 datasheet has an example how it should be properly connected when using the self-powered configuration.

Justme
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  • I made a mistake I meant bus powered configuration – PCB-ABBS Aug 26 '21 at 11:34
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    @walid24 OK fine, but connecting powered and unpowered chips together so that the powered chip accidentally powers the unpowred chip via data pins is still what happens. You might not want bus powered but self powered configuration. – Justme Aug 26 '21 at 11:38
  • the USb port is used to program wipy 3.0, when I removed the wipy Vbus led turns off and the other leds stop blinking. May you explain to me how a powered IC can power an unpowered IC in details. – PCB-ABBS Aug 26 '21 at 12:50
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    @walid24 MCU holds TXD pin at 3.3V. FTDI RXD pin has internal protection diodes from RXD to VCC. Thus FTDI VCC gets powered from MCU TXD pin via RXD pin protection diode to FTDI VCC. It will load down the MCU TXD pin and might draw too much current from MCU TXD pin. The protection diodes are also not intended to withstand constant misuse like that. – Justme Aug 26 '21 at 13:15
  • Thank you very much, I got it now – PCB-ABBS Aug 26 '21 at 13:29
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You are phantom powering the FTDI through the clamping diodes and leaking via Vccio.

Refer again to the datasheet and observe in self-power configuration you're to be using "vbus_sense" and not Vcc.

Jeroen3
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