Merge pull request #39 from jepler/signed-bits

RWBits: Add support for signed registers

adafruit_register/i2c_bits.py

@@ -45,10 +45,18 @@ class RWBits:
     :param type lowest_bit: The lowest bits index within the byte at ``register_address``
     :param int register_width: The number of bytes in the register. Defaults to 1.
     :param bool lsb_first: Is the first byte we read from I2C the LSB? Defaults to true
+    :param bool signed: If True, the value is a "two's complement" signed value.
+                        If False, it is unsigned.
     """
 
     def __init__(  # pylint: disable=too-many-arguments
-        self, num_bits, register_address, lowest_bit, register_width=1, lsb_first=True,
+        self,
+        num_bits,
+        register_address,
+        lowest_bit,
+        register_width=1,
+        lsb_first=True,
+        signed=False,
     ):
         self.bit_mask = ((1 << num_bits) - 1) << lowest_bit
         # print("bitmask: ",hex(self.bit_mask))
@@ -58,6 +66,7 @@ def __init__(  # pylint: disable=too-many-arguments
         self.buffer = bytearray(1 + register_width)
         self.buffer[0] = register_address
         self.lsb_first = lsb_first
+        self.sign_bit = (1 << (num_bits - 1)) if signed else 0
 
     def __get__(self, obj, objtype=None):
         with obj.i2c_device as i2c:
@@ -69,7 +78,11 @@ def __get__(self, obj, objtype=None):
             order = reversed(order)
         for i in order:
             reg = (reg << 8) | self.buffer[i]
-        return (reg & self.bit_mask) >> self.lowest_bit
+        reg = (reg & self.bit_mask) >> self.lowest_bit
+        # If the value is signed and negative, convert it
+        if reg & self.sign_bit:
+            reg -= 2 * self.sign_bit
+        return reg
 
     def __set__(self, obj, value):
         value <<= self.lowest_bit  # shift the value over to the right spot