Add Trigger/Program relationship to Readme and example

pio/pio_servo/README.adoc

@@ -3,6 +3,17 @@
 
 Drive a servo motor by using two PIO statemachines for pulse output.
 
+Each output needs a statemachine running servo_trigger and one running servo_prog.
+Due to the IRQ assignments, the statemachines need to be in ordered to have the servo_trigger program first, e.g.:
+
+```
+trig = Servo_Trigger(0)                 # Statemachine 0
+s = Servo(1, 16) # phys IO on pin 16    # Statemachine 1, since the trigger is on SM0
+
+trig2 = Servo_Trigger(2)                # Statemachine 2
+s2 = Servo(3, 25) # Builtin LED         # Statemachine 3, since the trigger is on SM2
+```
+
 == Wiring information
 
 See <<servo-wiring-diagram>> for wiring instructions.

pio/pio_servo/pio_servo.py

@@ -76,11 +76,11 @@ class Servo:
         self.sm.put(int(self.free_pulse*n))
 
 # Trigger needs to be the sm before the servo, so the IRQs set by rel(n) match
-trig = Servo_Trigger(2)
-s = Servo(3, 16) # phys IO on pin 16
+trig = Servo_Trigger(0)
+s = Servo(1, 16) # phys IO on pin 16
 
-trig2 = Servo_Trigger(0)
-s2 = Servo(1, 25) # Builtin LED
+trig2 = Servo_Trigger(2)
+s2 = Servo(3, 25) # Builtin LED
 
 for _ in range(2):
     for p in range(10+1):