You'll need to find the pin from the datasheet, then look at the T4.0 schematic and see where it's connected to the teensy's header (assuming it is, and I wouldn't guarantee it), and then you'll need to figure out how to tell the processor to use that clock.
That being said, you might find it less faff to use a hardware timer. Try timer3's built-in pwm function. I personally cant get it to work with my T3.2 forr some reason, but I also note that it doesn't seem to like very long perdoids >0.15 seconds, presumably becuase it takes it's input in micro seconds so there's probably an issue with the varible size. A quick and dirty way of doing it
Code:
#include <TimerThree.h>
const int led = LED_BUILTIN; // the pin with a LED
void setup(void)
{
pinMode(led, OUTPUT);
Timer3.initialize(10000); // set base as 0.01 seconds
Timer3.attachInterrupt(myPwm);
}
// The interrupt will blink the LED, and keep
// track of how many times it has blinked.
int ledState = LOW;
volatile int value = 75; // set at 0-100
volatile int count =0;
void myPwm(void)
{
if (count == 100)
{
count =0;
}
if (count < value) {
ledState = HIGH;
} else {
ledState = LOW;
}
digitalWrite(led, ledState);
count++;
}
void loop(void)
{
}
gives me a 75% duty, 1Hz wave. It's not the best practice to just use a global volatile varible, but hopefully that's enough to get you started with a software solution.