I have a question regarding the USB-to-UART bridge CP2102.
We used this device in Bus-Powered mode without using the 3.3V output to supply other components. When the main supply is not applied to the board and the USB cable is connected, there is a small voltage (~1.2V) on the Rx pin that sourcing some components. It sometimes give us trouble when we power on the board.
Do you have any recomemdation to fix that problem?
I did not fully understand your issue, Could you help to clarify:
1. I guess CP2102 is a component of this board, the CP2102 is bus powered. The board also have other component.
2. Here you mention 3.3V output is not used to supply other component, do you mean the 3.3V is from regulator output of the CP2102?
3. Here you mention main supply is not applied to the board, which component will use this mainly supply?
Do you mean you see a 1.2V voltage on Rx (USB) pin even you don't apply the main supply to the USB host.
4. When the main power supply is not applied to the USB host, how about the UART (RX/TX and other signal) voltage you applied on CP2102? Do you apply any voltage on some pin of UART puerperal?
5. Do you applied some voltage on other pin of CP2102?
6. If possible maybe you could upload your schematic (pdf preferred) and maybe someone could help review it.
: My views are my own and do not necessarily represent the views of Silicon Labs
This is a phenomenon called ghost-power or back-power. What happens is that the I/O pins are powering the otherwise unpowered microcontroller. Each pin normally has a diode transition to the bulk of the silicon which blocks under normal operating conditions, but conducts when the bulk is unpowered.
Please find below answers to your questions:
1- Yes, the CP2102 is part of my board and it's connected to a processor via Rx and Tx signals.
2- Yes, the 3.3V from regulator output of the CP2102 is only used to supply a 10k pull-up. All the other 3.3V components are powered with a DC-DC converter.
3- I see a 1.2V voltage on Tx (CP2102 or Rx processor) when the USB cable is connected (+5V on VBUS). All the other components on the board are not powered.
4- I'm not sure to understand the question.
5- No, the only thing I do is to connect the USB cable.
6- You will find attached the schematic.
Do you mean you see a 1.2V voltage on TX (UART peripheral) of CP2102 right after you plug in the USB cable (power on the CP2102). If this is the case how long the 1.2V voltage level last, could the CP2102 work normally any more?
For question 4, if you apply a voltage higher than 3.6V on a GPIO when CP2102 is not powered up (VDD = 0V), then you may see the back-power phenomenon as @vanmierlo commented. You could refer to table 2 on page 6 of the datasheet.