basic alarm thing

 Create

// Trigger the alarm event immediately.
// You can change this value to make the cannon start with a delay if you want.
alarm[0] = 1

 Alarm 0

// Create a new cannon ball and make it move upwards
var new_cannonball;
new_cannonball = instance_create(objCannonBall, somex, somey)
new_cannonball.direction = 90
new_cannonball.speed = 3

// Re-set the alarm so that this event will trigger again
alarm[0] = 100