add morse code background example

examples/pioasm_background_morse.py

@@ -0,0 +1,104 @@
+# SPDX-FileCopyrightText: 2022 Jeff Epler, written for Adafruit Industries
+#
+# SPDX-License-Identifier: MIT
+
+"""Demonstrate background writing, including loop writing, with morse code.
+
+On any rp2040 board with board.LED, this will alternately send 'SOS' and 'TEST'
+via the LED, demonstrating that Python code continues to run while the morse
+code data is transmitted. Alternately, change one line below to make it send
+'TEST' forever in a loop, again while Python code continues to run.
+
+The combination of "LED status" and duration is sent to the PIO as 16-bit number:
+The top bit is 1 if the LED is turned on and 0 otherwise.  The other 15 bits form a delay
+value from 1 to 32767. A subset of the morse code 'alphabit' is created, with everthing
+based on the 'DIT duration' of about 128ms (1MHz / 32 / 4000).
+
+https://en.wikipedia.org/wiki/Morse_code
+"""
+
+import time
+import array
+from board import LED
+from rp2pio import StateMachine
+from adafruit_pioasm import Program
+
+# This program turns the LED on or off depending on the first bit of the value,
+# then delays a length of time given by the next 15 bits of the value.
+# By correctly choosing the durations, a message in morse code can be sent.
+pio_code = Program(
+    """
+        out x, 1
+        mov pins, x
+        out x, 15
+    busy_wait:
+        jmp x--, busy_wait [31]
+        """
+)
+
+
+# The top bit of the command is the LED value, on or off
+LED_ON = 0x8000
+LED_OFF = 0x0000
+
+# The other 15 bits are a delay duration.
+# It must be the case that 4 * DIT_DURATION < 32768
+DIT_DURATION = 4000
+DAH_DURATION = 3 * DIT_DURATION
+
+# Build up some elements of morse code, based on the wikipedia article.
+DIT = array.array("H", [LED_ON | DIT_DURATION, LED_OFF | DIT_DURATION])
+DAH = array.array("H", [LED_ON | DAH_DURATION, LED_OFF | DIT_DURATION])
+# That is, two more DAH-length gaps for a total of three
+LETTER_SPACE = array.array("H", [LED_OFF | (2 * DAH_DURATION)])
+# That is, four more DAH-length gaps (after a letter space) for a total of seven
+WORD_SPACE = array.array("H", [LED_OFF | (4 * DIT_DURATION)])
+
+# Letters and words can be created by concatenating ("+") the elements
+E = DAH + LETTER_SPACE
+O = DAH + DAH + DAH + LETTER_SPACE
+S = DIT + DIT + DIT + LETTER_SPACE
+T = DIT + LETTER_SPACE
+SOS = S + O + S + WORD_SPACE
+TEST = T + E + S + T + WORD_SPACE
+
+# 8 slots of the shortest possible led-off time
+# A background write is 'complete' as soon as all data has been placed
+# in the StateMachine's FIFO.  This FIFO has either 4 or 8 entries.
+# By adding 8 very short "led off" times, we can have our 'sm.writing' test
+# tell us when the actual letter data has all been
+FILLER = array.array("H", [LED_OFF | 1]) * 8
+
+sm = StateMachine(
+    pio_code.assembled,
+    frequency=1_000_000,
+    first_out_pin=LED,
+    pull_threshold=16,
+    auto_pull=True,
+    out_shift_right=False,
+)
+
+# To simply repeat 'TEST' forever, change to 'if True':
+if False:  # pylint: disable=using-constant-test
+    print("Sending out TEST forever", end="")
+    sm.background_write(loop=TEST)
+    while True:
+        print(end=".")
+        time.sleep(0.1)
+
+# But instead, let's alternate SOS and TEST, forever:
+
+while True:
+    for plain, morse in (
+        ("SOS", SOS),
+        ("TEST", TEST),
+    ):
+        print(f"Sending out {plain}", end="")
+        sm.background_write(morse + FILLER)
+        while sm.writing:
+            print(end=".")
+            time.sleep(0.1)
+        print()
+        print("Message all sent to StateMachine")
+        time.sleep(1)
+        print()