I feel embarrassed to ask for such a stupid issue:
I cannot get a simple button attached to G26 (pressed: G26->GND) generate an interrupt and light up the on board LED. With digitalRead it works. I want to use the interrupt version to read pwm signals eventually.
I am using Arduino-IDE.

Here is my code:

#include <M5StickCPlus.h>
/* pin that is attached to interrupt */
uint8_t interruptPin = 26;

void pressed() {
digitalWrite(10, LOW); // LED on
delay(100);
digitalWrite(10,HIGH); // LED off
}

void setup() {
// when pin G26 goes low, call the pressed function
pinMode(interruptPin,INPUT_PULLUP);
attachInterrupt(digitalPinToInterrupt(interruptPin), pressed, LOW);
pinMode(10,OUTPUT);
digitalWrite(10, HIGH); // LED off to start with
}

void loop() {
// put your main code here, to run repeatedly:
}