Allow negative index with direct channel write

adafruit_tlc5947.py

@@ -245,6 +245,8 @@ def __getitem__(self, key):
         """Retrieve the 12-bit PWM value for the specified channel (0-max).
         max depends on the number of boards.
         """
+        if key < 0:  # allow reverse adressing with negative index
+            key = key + _CHANNELS * self._n
         assert 0 <= key < _CHANNELS * self._n
         return self._get_gs_value(key)
 
@@ -255,6 +257,8 @@ def __setitem__(self, key, val):
         immediately be updated too, otherwise you must call write to update
         the chip with the new PWM state.
         """
+        if key < 0:  # allow reverse adressing with negative index
+            key = key + _CHANNELS * self._n
         assert 0 <= key < _CHANNELS * self._n
         assert 0 <= val <= 4095
         self._set_gs_value(key, val)

examples/tlc5947_simpletest.py

@@ -57,12 +57,14 @@
 # Note if auto_write was disabled you need to call write on the parent to
 # make sure the value is written (this is not common, if disabling auto_write
 # you probably want to use the direct 12-bit raw access instead shown below).
-# tlc5947.write()
+#            tlc5947.write()
 
 # The other way to read and write channels is directly with each channel 12-bit
 # value and an item accessor syntax.  Index into the TLC5947 with the channel
 # number (0-23) and get or set its 12-bit value (0-4095).
 # For example set channel 1 to 50% duty cycle.
-# tlc5947[1] = 2048
+#tlc5947[1] = 2048
+# Or set channel 23 (first channel from the end) to 2/3 duty cycle.
+#tlc5947[-1] = 2730
 # Again be sure to call write if you disabled auto_write.
 #tlc5947.write()