Add PIO example for servo control

Controls one servo at GP16 and additionally the LED (to see, if it's doing anything, and to demonstrate usage of two servo outputs)

pio/pio_servo.py

@@ -0,0 +1,88 @@
+# Example of using PIO for Servo control
+ 
+from machine import Pin
+from rp2 import PIO, StateMachine, asm_pio
+from time import sleep
+
+@asm_pio()
+def servo_trigger():
+    irq(clear, rel(1))         # Clear next relative ISR, allows servo code to run again
+    mov(y, x)             # Counter is stored in x, copy to y for use
+    label("base")
+    jmp(y_dec, "base")    # wait for programmed time
+    
+@asm_pio(sideset_init=PIO.OUT_LOW)
+def servo_prog():
+    wrap_target()
+    
+    irq(block, rel(0)) .side(0) # Wait here for IRQ to be released by trigger SM
+    
+    pull(noblock)        # pull new pulse length into fifo (pull fifo into OSR, if empty fifo copies X->OSR)
+    mov(x, osr)          # Keep most recent pull data stashed in X, for recycling by noblock (later)
+    mov(y, isr) .side(1) # ISR must be preloaded with base length
+    
+    label("base")
+    jmp(y_dec, "base")    # wait in state 1 for steps in y register (base pulse length)
+    
+    mov(y, x)
+    label("var")    
+    jmp(y_dec, "var")     # wait in state 1 for steps in x register (variable pulse length)
+    
+    wrap()
+        
+class Servo_Trigger:
+    '''
+    Run one statemachine in a loop, that clears IRQ4 every 20ms as the
+    base for the servo statemachine.
+    '''
+    def __init__(self, sm_idx):
+        # Trigger SM should output a pulse every 20ms for the servo SM to run
+        trig_target = 20 # ms
+        
+        trig_frq = 10_000 #Hz
+        sm_trig = StateMachine(sm_idx, servo_trigger, freq=trig_frq)
+        trig_ctr = (trig_frq // 1000 * trig_target) - 3 # 3 instructions to have perfect 20ms on IRQ4
+
+        sm_trig.put(trig_ctr)
+        sm_trig.exec("pull()")
+        sm_trig.exec("mov(x, osr)")
+        sm_trig.active(1)
+    
+class Servo:
+    '''
+    Accepts the servo setpoint via FIFO input.
+    It raises and waits for IRQ4 after the positive part of the pulse has been output.
+    The other statemachine should clear IRQ4 every 20ms so that a new pulse is output cyclically.
+    
+    Preload the ISR with the base duration (fixed pulse length for position 0°)
+    Send position data via FIFO into the OSR (variable pulse length for 0°..max)
+    '''
+    def __init__(self, sm_idx, pin):
+        self.baseFrq = 1_000_000 # 1MHz = 1us clock base
+        
+        self.base_pulse = 1000 # us, base width of pulse
+        self.free_pulse = 1000 # us, max. additional length set by percent
+        
+        self.sm = StateMachine(sm_idx, servo_prog, freq=self.baseFrq, sideset_base=Pin(pin))
+
+        # Use exec() to load max count into ISR
+        self.sm.put(self.base_pulse)
+        self.sm.exec("pull()")
+        self.sm.exec("mov(isr, osr)")
+        self.sm.active(1)
+        
+    def pos(self, n):
+        '''Set servo position. Range 0.0 to 1.0'''
+        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
+
+trig2 = Servo_Trigger(0)
+s2 = Servo(1, 25) # Builtin LED
+
+for _ in range(2):
+    for p in range(10+1):
+        s.pos(p/10)
+        sleep(0.5)