I use FT232RL to do USB->UART converter - you can see the relevant part of the schematic below. VBUS is a 5V pin in a USB socket and is only used for FT232. The PCB has another connector for 5V supply.

The FT232RL is only used to reprogram MCU and it's usually not plugged into USB during normal operation.
The problem I have is that, after the power is connected, the LEDs (D1, D2) are constantly turned ON. If I then connect a USB cable the LEDs will switch OFF and after I unplug the cable, they (correctly) remain OFF.

However, I would like the LEDs to be initially OFF. What's wrong or how to achieve it?

FT232RL schematic


The power supplies are incorrectly applied so when USB cable is disconnected or local 3.3V supply is off, half of the chip is unpowered and this results in erratic operation.

The correct methods to power the chip either as bus powered or self powered are in the datasheet. Based on your description, you want self-powered configuration.


I guess, when you don't connect USB, the VCC input also doesn't connect to any power supply. Sometimes the core of IC can feeds by voltage from VCCIO or even IO-buffers. Thus LEDs can switch ON. But it is a very wrong situation. You may use something like a diode to power the FT232RL with either +5V USB or on-board +3.3V

  • \$\begingroup\$ This will violate the USB protocol as powering the VCC pin mans the USB chip will attempt to talk over the USB wires, but this computer may not even be turned on, and it can cause latch up situations in the hosts USB chips. Make sure to follow the datasheets on external power sources, as it requires you to use its reset pins \$\endgroup\$
    – Ferrybig
    Aug 30 '21 at 13:37
  • \$\begingroup\$ Also, using the internal oscillator requires that VCC is at least 4.0V, so powering VCC from 3.3V is out of specs and will most likely not work. \$\endgroup\$
    – Justme
    Aug 30 '21 at 13:48

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