Add software watchdog timer.

README.md

@@ -22,6 +22,13 @@ program to access shared data in a manner which ensures data integrity.
 
 Access the simpler of the Pyboard's watchdog timers.
 
+# software watchdog (soft_wdt)
+
+A software watchdog timer with a fixed or variable timeout. Supports temporary
+suspension and permanent cancellation. The latter can be useful when debugging
+code to prevent a machine reboot when the application fails, terminates or is
+interrupted with ctrl-c. See code and comments in swdt_tests.py.
+
 # reverse
 
 Fast reverse a bytearray in Arm Thumb assembler.  

soft_wdt/soft_wdt.py

@@ -0,0 +1,38 @@
+# soft_wdt.py A software watchdog timer
+# Supports fixed or variable time period.
+# Supports temporary suspension and permanent cancellation.
+
+# Copyright (c) Peter Hinch 2019
+# Released under the MIT licence.
+
+from machine import Timer, reset
+from micropython import const
+WDT_SUSPEND = const(-1)
+WDT_CANCEL = const(-2)
+WDT_CB = const(-3)
+
+def wdt(secs=0):
+    timer = Timer(-1)
+    timer.init(period=1000, mode=Timer.PERIODIC, callback=lambda t:wdt_feed())
+    cnt = secs
+    run = False  # Disable until 1st feed
+    def inner(feed=WDT_CB):
+        nonlocal cnt, run, timer
+        if feed > 0:  # Call with variable timeout
+            cnt = feed
+            run = True
+        elif feed == 0:  # Fixed timeout
+            cnt = secs
+            run = True
+        elif feed < 0:  # WDT control/callback
+            if feed == WDT_SUSPEND:
+                run = False  # Temporary suspension
+            elif feed == WDT_CANCEL:
+                timer.deinit()  # Permanent cancellation
+            elif feed == WDT_CB and run:  # Timer callback and is running.
+                cnt -= 1
+                if cnt <= 0:
+                    reset()
+    return inner
+
+wdt_feed = wdt(2)  # Modify this for preferred default period (secs)

soft_wdt/swdt_tests.py

@@ -0,0 +1,75 @@
+# swdt_tests Test/demo scripts for soft_wdt
+
+# Copyright (c) Peter Hinch 2019
+# Released under the MIT licence.
+import utime
+from soft_wdt import wdt_feed, WDT_CANCEL, WDT_SUSPEND
+
+# Exception trapping and cancellation are invaluable when debugging code: put
+# cancellation in the finally block of a try statement so that the hardware
+# doesn't reset when code terminates either naturally or in response to an
+# error or ctrl-c interrupt.
+
+# Normal operation. Illustrates exception trapping. You can interrupt this with
+# ctrl-c
+def normal():
+    try:
+        for x in range(10, 0, -1):
+            print('nunning', x)
+            utime.sleep(0.5)
+            wdt_feed(5)  # Hold off for 5s
+
+        print('Should reset in 5s')
+        utime.sleep(10)
+    except KeyboardInterrupt:
+        pass
+    finally:
+        wdt_feed(WDT_CANCEL)  # Should never execute
+
+# Suspend and resume
+
+def suspend():
+    for x in range(10, 0, -1):
+        print('nunning', x)
+        utime.sleep(0.5)
+        wdt_feed(5)  # Hold off for 5s
+
+    wdt_feed(WDT_SUSPEND)
+    for x in range(5, 0, -1):
+        print('suspended', x)
+        utime.sleep(0.5)
+
+    for x in range(5, 0, -1):
+        print('nunning', x)
+        utime.sleep(0.5)
+        wdt_feed(5)  # Hold off for 5s
+
+    print('Should reset in 5s')
+    utime.sleep(10)
+    wdt_feed(WDT_CANCEL)  # Should never execute
+
+# Default period
+
+def default():
+    for x in range(10, 0, -1):
+        print('nunning', x)
+        utime.sleep(0.5)
+        wdt_feed(5)  # Hold off for 5s
+
+    wdt_feed(0)  # Use default period
+    print('Should reset in 2s')
+    utime.sleep(10)
+    wdt_feed(WDT_CANCEL)  # Should never execute
+
+# Cancellation
+def cancel():
+    for x in range(10, 0, -1):
+        print('nunning', x)
+        utime.sleep(0.5)
+        wdt_feed(5)  # Hold off for 5s
+
+    wdt_feed(WDT_CANCEL)
+
+    print('Pause 10s: should not reset in 5s')
+    utime.sleep(10)
+    print('WDT is permanently cancelled.')