Added shake detection (#13)

Author: kattni

adafruit_lis3dh/lis3dh.py

@@ -9,6 +9,8 @@
     import ustruct as struct
 
 from micropython import const
+import time
+import math
 
 # Register addresses:
 REG_OUTADC1_L   = const(0x08)
@@ -113,6 +115,33 @@ def acceleration(self):
 
         return (x / divider * 9.806, y / divider * 9.806, z / divider * 9.806)
 
+    def shake(self, shake_threshold=30, avg_count=10, total_delay=0.1):
+        """Detect when the accelerometer is shaken. Optional parameters:
+         shake_threshold - Increase or decrease to change shake sensitivity. This
+                           requires a minimum value of 10. 10 is the total
+                           acceleration if the board is not moving, therefore
+                           anything less than 10 will erroneously report a constant
+                           shake detected. (Default 30)
+         avg_count - The number of readings taken and used for the average
+                     acceleration. (Default 10)
+         total_delay - The total time in seconds it takes to obtain avg_count
+                       readings from acceleration. (Default 0.1)
+         """
+        shake_accel = (0, 0, 0)
+        for i in range(avg_count):
+            # shake_accel creates a list of tuples from acceleration data.
+            # zip takes multiple tuples and zips them together, as in:
+            # In : zip([-0.2, 0.0, 9.5], [37.9, 13.5, -72.8])
+            # Out: [(-0.2, 37.9), (0.0, 13.5), (9.5, -72.8)]
+            # map applies sum to each member of this tuple, resulting in a
+            # 3-member list. tuple converts this list into a tuple which is
+            # used as shake_accel.
+            shake_accel = tuple(map(sum, zip(shake_accel, self.acceleration)))
+            time.sleep(total_delay / avg_count)
+        avg = tuple(value / avg_count for value in shake_accel)
+        total_accel = math.sqrt(sum(map(lambda x: x * x, avg)))
+        return total_accel > shake_threshold
+
     def read_adc_raw(self, adc):
         """Retrieve the raw analog to digital converter value.  ADC must be a
         value 1, 2, or 3.