SoftwareSerial brings pins HIGH

Using Arduino Programming Questions
#1

Hello. Im trying to use SoftwareSerial but i encountered a really weird bug. This is my code:

#include <SoftwareSerial.h>
SoftwareSerial BTSerial(7, 8);

void setup() {
  BTSerial.begin(9600);
}

void loop() {
  
}

This is supposed to turn pin 7 and 8 into serial pins, but instead it brings them both to constant 5V. Why is this happening?

#2

Why is this happening?

Because it is supposed to. Why is this a problem?

#3

Because im not communicating anything over the pins. I could understand if TX would be high because its trying to send some information, but RX should be low if its supposed to receive data.

#4

but RX should be low if its supposed to receive data.

The other end pulls RX low to make it receive data. The Arduino pulls TX low to send data.

#5

What is hooked to the input pin? If it is floating it could be anything, though it is possible that the internal pull-ups are active.

#6

Oh, i didn't know that, thank you. Its confusing, because i was using this design to communicate with a Bluetooth module that can only handle 3.3V and none of the documentation on the internet suggested to also do a voltage divider on RX from the arduino. Because i thought it was the BT module that was sending signals on the serial RX(to arduino) pin i assumed that there wasnt any risk that that pin could get over 3.3V, and the module is now destroyed.

#7

KeithRB: If it is floating it could be anything, though it is possible that the internal pull-ups are active.

For normal polarity, SoftwareSerial activates the internal pull-up for RX. If it didn't do this an unconnected input could generate frequent interrupts. It sets the TX line high since that's the idle state.

#8

(I wouldn't think that a pin with the internal pullup would burn out a "3.3v-only" input on a connected device. The pullup is a pretty large resistor, and should limit current to levels that are likely to be safe...)

#9

jammasterz:
Hello. Im trying to use SoftwareSerial but i encountered a really weird bug. This is my code:

#include <SoftwareSerial.h>

SoftwareSerial BTSerial(7, 8);

void setup() {
 BTSerial.begin(9600);
}

void loop() {
 
}



This is supposed to turn pin 7 and 8 into serial pins, but instead it brings them both to constant 5V. Why is this happening?

Instead? OMG!

SoftwareSerial is emulating a hardware serial port in software.
So what's the voltagelevel on hareware serial ports?

Perhaps use an Aruino UNO and in the setup do !"Serial.begin(9600);", then measure pin-0 and pin-1vcoltage?
!What do you expect? What is the measurement?
#10

i was using this design to communicate with a Bluetooth module that can only handle 3.3V and none of the documentation on the internet suggested to also do a voltage divider on RX from the arduino.

That’s because a voltage divider isn’t really appropriate. There are properly designed voltage reduction devices available with several channels for your purpose.

#11