docs/rp2: Make LED have exactly 50% duty cycle in PIO 1Hz example.

This ensures the same number of cycles are used for LED on and LED off in
the PIO 1Hz example.  It's also possible to swap the first set() and the
irq() to avoid using an extra instruction, but this tutorial is a good
example of how to calculate the cycles.

Signed-off-by: Stig Bjørlykke <stig@bjorlykke.org>

docs/rp2/tutorial/pio.rst

@@ -76,9 +76,10 @@ and state machines. Below is the code for reference.
         nop()                       [29]
         jmp(x_dec, "delay_high")
 
-        # Cycles: 1 + 7 + 32 * (30 + 1) = 1000
+        # Cycles: 1 + 1 + 6 + 32 * (30 + 1) = 1000
+        nop()
         set(pins, 0)
-        set(x, 31)                  [6]
+        set(x, 31)                  [5]
         label("delay_low")
         nop()                       [29]
         jmp(x_dec, "delay_low")
@@ -113,6 +114,8 @@ the following:
   X starts with the value 31 this jump will happen 31 times, so the ``nop() [29]``
   runs 32 times in total (note there is also one instruction cycle taken by the
   ``jmp`` for each of these 32 loops).
+- The single ``nop()`` correlates with the cycle used for IRQ raise, and ensures
+  the same number of cycles are used for LED on and LED off.
 - ``set(pins, 0)`` will turn the LED off by setting pin 25 low.
 - Another 32 loops of ``nop() [29]`` and ``jmp(...)`` will execute.
 - Because ``wrap_target()`` and ``wrap()`` are not specified, their default will

examples/rp2/pio_1hz.py

@@ -16,9 +16,10 @@ def blink_1hz():
     nop()                       [29]
     jmp(x_dec, "delay_high")
 
-    # Cycles: 1 + 7 + 32 * (30 + 1) = 1000
+    # Cycles: 1 + 1 + 6 + 32 * (30 + 1) = 1000
+    nop()
     set(pins, 0)
-    set(x, 31)                  [6]
+    set(x, 31)                  [5]
     label("delay_low")
     nop()                       [29]
     jmp(x_dec, "delay_low")