Microcontroller powered from the battery gives reverse voltage through UART to FT232RL chip

[venkat9]

Hi,
i have powered my microcontroller from the battery. and have connected the Microcontroller UART connected to FT232RL converter.
Now the problem is, when the MCU is powered, the UART voltages come in to the FT232RL resulting in the damage of FT232RL chip.
can you please advise on how to fix this issue?

 

[Les Jones]

You will have to post a schematic of how the microcontroller is connected to the FT232RL for us to be able to come up with a possible explanation. I have never had any problem connecting a microcontroller to an FT232RL.

Les.

 

[venkat9]

Here is the schematic

 

[ChrisP58]

We need to see all of the circuit, not just the UART.

 

[Les Jones]

You do not show any ground connection between the FT232RL and the microcontroller. Do you have the grounds connected together ? I was expecting the full schematic rather than just part of it as this would show ground and power supply connections. Are you sure that you have the RX pin on the microcontroller configured as an input. If you have it configured as an output it could be trying to drive the line high when the FT232RL was trying to drive it low and vice versa. This may or may not be a problem depending on the value of R39 which is not shown.

Les.

 

[venkat9]

You do not show any ground connection between the FT232RL and the microcontroller. Do you have the grounds connected together ? I was expecting the full schematic rather than just part of it as this would show ground and power supply connections. Are you sure that you have the RX pin on the microcontroller configured as an input. If you have it configured as an output it could be trying to drive the line high when the FT232RL was trying to drive it low and vice versa. This may or may not be a problem depending on the value of R39 which is not shown.

Les.

Sir you have understood my problem very precisely .yes both grounds are connected together. Full circuit is in multiple pages that's the reason I just took USB section from it. RX pin on mcu I must check if it's configured as input.the value of R39 is zero ohm. Any suggested resistance value assuming the pin is configured as output?

 

[Diver300]

The Tx on the microcontroller is connected to the RX on the FT232RL. In the idle state when nothing is being transmitted, the Tx on the microcontroller will be high. The maximum voltage for input on the FT232RL is -0.5 to + (VCC +0.5) (https://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf) so you should not enable the Tx output on the microcontroller when the FT232RL is powered up.

You may be able to deal with it in software, if you disable the Tx on the microcontroller before the FT232RL is turned off, and only enable the Tx on the microcontroller when the FT232RL is powered up.

If not, add a schottky diode in place of R44, with the cathode connected to the microcontroller Tx and the anode connected to the FT232RL Rx. You will also need a pull up resistor, maybe 1 kOhm, between FT232RL Rx and FT232RL Vccio

 

[rjenkinsgb]

It may work OK if you just replace the links at R39 and R44 with reasonably high value resistors.

I'd try 100K and see if it functions OK at that with normal power.
You may be able to go down to 10K or less without damage, but I'd stay a bit above that if possible.

Limiting the current through the input to well under what the input protection can handle should avoid problems & with a high impedance input should have no significant effect on a relatively slow signal such as serial data.