If I have a device specified for 3.3V operation and I connect a 5V level TTL signal to it ? Will it's UART ports be damaged or not ?
I have a WiFi module and I want to know if its UART ports will have been damaged by applying 5 Volt signal levels during an experiment or if there is still hope that it has not been damaged ?
While there is no yes/no answer to whether your particular device has a damaged UART port many devices that are specified for 3.3V operation can have their I/O lines damaged by applying 5V logic levels. Best practice is to check the datasheet and confirm the particular device is 5V tolerant and if not mentioned in the specifications either assume it's not and use 3.3V logic or contact the manufacturer for clarification.
As mentioned by other, you have to check the datasheet to see if the Rx pin is 5V tolerant. If not, there are many level shifter ICs you can use to shift 5v to 3.3 and vise versa.
Whenever you are stuck with any of these questions, it is best to refer to the datasheet for your specific module. I have worked with modules which run at 3.3(V) but can tolerate 5(V) inputs to the serial port. That being said, this is very device specific and really depends on if the manufacturer has added the ability to tolerate 5(V) line.
As for your UART issue where "...." being seen, you should check whether this is happening if you have none of the TX and RX connected. If you still get that without any serial connection, it might just be something the WiFi module is sending by default (possible bad WiFi module firmware or maybe malfunction). Like mentioned, however, that would be the topic of another question, which you should post if you want the proper answer to that.
A solution to your issue is to just you a logic level shifter. Either that or you can choose to run everything at 3.3(V) if you don't need the higher 5(V) rail which usually can be done.
Hope that helps. Happy Engineering!!!
