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I have a bluetooth module (rn4020) talking via UART to a stm32f103c8t6 MCU in a custom PCB, I also have an OLED screen and some pin headers for listening what either module receives (as seen in the right top of the schematic).

The thing is that without connecting a FTDI chip in the pin headers, communication between stm32 and RN4020 does not work, I know this because the OLED screen outputs a message whenever a message from the bluetooth is received by the microcontroller (which is the first event to happen) and the message is not being shown in the OLED screen. I am not connecting ground from the FTDI chip in the pin headers and the communication proceeds normally which leds me to believe 3.8 volts injection is the thing making UART communication work (in contrast with 3.3 volts communication without connecting anything in pin headers).

I've also seen that boards that pretend to achieve a similar functionality like Arduino Uno have some resistors for switching the UC communicates, though I know this is not the case I am looking for and makes me believe the problem I have can be noise related

  • \$\begingroup\$ I'm surprised that this works with the FTDI chip connected, because that would be shorting 2 TX's together. First of all I would always connect the FTDI gnd to your circuit, otherwise you can expect weird effects due to the protection diodes. Second, try connecting only the FTDI RX to either line and see what you get. A logic analyzer or scope connected to those lines might also help. \$\endgroup\$ – Drew Oct 31 '19 at 23:10
  • \$\begingroup\$ UART is still not working If only FTDI's RX is connected, both pins are require (TX and RX), I will work with scope soon. What would be a better way of achieving what I want without shorting to TX's? \$\endgroup\$ – Paul Max Avalos Aguilar Oct 31 '19 at 23:54
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    \$\begingroup\$ You forgot to connect boot0. Also your stacking of power pins on top of each other in the schematic is dubious - are you sure they really both get connected? \$\endgroup\$ – Chris Stratton Nov 1 '19 at 0:13
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    \$\begingroup\$ If you want help, you need to post the actual schematic, not something that is "incomplete" and shows errors you are only going to claim have been corrected. \$\endgroup\$ – Chris Stratton Nov 1 '19 at 4:58
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    \$\begingroup\$ Are Vbat and nRST also disconnected or connected somewhere? Maybe configuration error on firmware, like UART TX pins accidentally open-drain? As this is a 3.3V system, do not give it 5V signals from FTDI to rise the pin voltage to 3.8, this means current is injected to the system via internal protection diodes of those pins. \$\endgroup\$ – Justme Nov 1 '19 at 6:18

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