Ability to write to Serial3 TX port---doesn't seem to work

[KrisKasprzak]

All,

I have a project where I'm using a GPS sensor connected to Teensy 3.2 Serial3 port--the GPS sensor is mounted on a small PCB board and a USB cable between it and my Teensy 3.2 (which is mounted on a custom PCB board as well).

I'm getting data from the GPS sensor (sent from GPS Tx port and read on Teensy Rx port, pin 7), all is well. I was hoping to be able to illuminate a small LED on the GPS sensor via digitalWrite(8, HIGH) and send through Serial3 Tx port as I'm not using the Tx port on the teensy.

The LED stays high. If I comment out my MyGPSSerial.begin(9600); line i can write to the LED through the Port. See my cheezy drawing. Note PCB boards are printed, etc. so i CAN just use a different digital pin.

I'm guessing i can't manually send a high/low signal through Serial3 Tx port. Any way to force teensy to let me write to Tx pin (pin 7)?

Thanks

 

[PaulStoffregen]

Any way to force teensy to let me write to Tx pin (pin 7)?

Yes.

pinMode(7, OUTPUT);

On Teensy 3.2, every pin has a mux which allows 1 of 8 things to control the pin.

Serial3.begin() writes to the mux to cause both the pins to become controlled by the serial port.

pinMode() writes to the mux to cause a pin to be controlled by GPIO.

 

[KrisKasprzak]

Yep, that was it. I had pinMode(8, OUTPUT); but that statement was before the GPSSerial.begin(9600);

Once i moved the pinMode after the .begin(), my system worked as expected.

Thanks a million!