docs/rp2/quickref.rst: Add section on PIO.

docs/rp2/quickref.rst

@@ -92,6 +92,37 @@ Use the :ref:`machine.Pin <machine.Pin>` class::
     p4 = Pin(4, Pin.IN, Pin.PULL_UP) # enable internal pull-up resistor
     p5 = Pin(5, Pin.OUT, value=1) # set pin high on creation
 
+Programmable IO (PIO)
+---------------------
+
+PIO is useful to build low-level IO interfaces from scratch.  See the :mod:`rp2` module
+for detailed explaination of the assembly instructions.
+
+Example using PIO to blink an LED at 1Hz::
+
+    from machine import Pin
+    import rp2
+
+    @rp2.asm_pio(set_init=rp2.PIO.OUT_LOW)
+    def blink_1hz():
+        # Cycles: 1 + 7 + 32 * (30 + 1) = 1000
+        set(pins, 1)
+        set(x, 31)                  [6]
+        label("delay_high")
+        nop()                       [29]
+        jmp(x_dec, "delay_high")
+
+        # Cycles: 1 + 7 + 32 * (30 + 1) = 1000
+        set(pins, 0)
+        set(x, 31)                  [6]
+        label("delay_low")
+        nop()                       [29]
+        jmp(x_dec, "delay_low")
+
+    # Create and start a StateMachine with blink_1hz, outputting on Pin(25)
+    sm = rp2.StateMachine(0, blink_1hz, freq=2000, set_base=Pin(25))
+    sm.active(1)
+
 UART (serial bus)
 -----------------