Updated to patch bit shifting problem. See issue #2.

.gitignore

@@ -9,6 +9,7 @@ __pycache__/
 # Distribution / packaging
 .Python
 env/
+env_py3/
 build/
 develop-eggs/
 dist/

Adafruit_MAX31856/max31856.py

@@ -124,44 +124,73 @@ def __init__(self, tc_type=MAX31856_T_TYPE, avgsel=0x0, clk=None, cs=None, do=No
         self._write_register(self.MAX31856_REG_WRITE_CR0, self.MAX31856_CR0_READ_CONT)
         self._write_register(self.MAX31856_REG_WRITE_CR1, self.CR1)
 
+    def _cjTempFromBytes(MSB, LSB):
+        """Takes in the MSB and LSB from a Cold Junction (CJ) temperature reading and converts it into a decimal value.
+        
+        This function was removed from readInternalTempC() and moved to its own method to allow for easier testing with standard values.
+        
+        Args:
+            MSB (hex): Most significant byte of CJ temperature
+            LSB (hex): Least significant byte of a CJ temperature
+        
+        """
+        #            ( ((MSB w/o +/-) shifted by number of 1 byte above LSB)
+        #                                  + val_low_byte ) 
+        #                                          >> shifted back by # of dead bits
+        temp_bytes = ( ((MSB & 0x7F) << 8) + LSB ) >> 2
+
+        if MSB & 0x80:
+            # Negative Value.  Scale back by number of bits
+            temp_bytes -= 2**(MAX31856.MAX31856_CONST_CJ_BITS -1)
+
+        #        temp_bytes*value of LSB
+        temp_C = temp_bytes*MAX31856.MAX31856_CONST_CJ_LSB
+
+        return temp_C
 
     def readInternalTempC(self):
         """Return internal temperature value in degrees celsius."""
         val_low_byte = self._read_register(self.MAX31856_REG_READ_CJTL)
         val_high_byte = self._read_register(self.MAX31856_REG_READ_CJTH)
 
-        #        ( ((val_high_byte w/o +/-) shifted by number of bits above LSB)
-        #                                            + val_low_byte )
-        temp_bytes = ( ((val_high_byte & 0x7F) << 6) + val_low_byte )
+        temp_C = MAX31856._cjTempFromBytes(val_high_byte, val_low_byte)
+        self._logger.debug("Cold Junction Temperature {0} deg. C".format(temp_C))
 
-        if val_high_byte & 0x80:
-            # Negative Value.  Scale back by number of bits
-            temp_bytes -= 2**(self.MAX31856_CONST_CJ_BITS -1)
+        return temp_C
+
+    def _thermocoupleTempFromBytes(byte0, byte1, byte2):
+        """Converts the thermocouple byte values to a decimal value.
+        
+        This function was removed from readInternalTempC() and moved to its own method to allow for easier testing with standard values.
+        
+        Args:
+            byte2 (hex): Most significant byte of thermocouple temperature
+            byte1 (hex): Middle byte of thermocouple temperature
+            byte0 (hex): Least significant byte of a thermocouple temperature
+        
+        """
+        #            ( ((val_high_byte w/o +/-) shifted by 2 bytes above LSB)
+        #                 + (val_mid_byte shifted by number 1 byte above LSB)
+        #                                             + val_low_byte )
+        #                              >> back shift by number of dead bits 
+        temp_bytes = ( ((byte2 & 0x7F) << 16) + (byte1 << 8) + byte0 ) 
+        temp_bytes = temp_bytes >> 5
+
+        if byte2 & 0x80:
+            temp_bytes -= 2**(MAX31856.MAX31856_CONST_THERM_BITS -1)
 
         #        temp_bytes*value of LSB
-        temp_C = temp_bytes*self.MAX31856_CONST_CJ_LSB
-        self._logger.debug("Cold Junction Temperature {0} deg. C".format(temp_C))
+        temp_C = temp_bytes*MAX31856.MAX31856_CONST_THERM_LSB
 
         return temp_C
 
-
     def readTempC(self):
         """Return the thermocouple temperature value in degrees celsius."""
         val_low_byte = self._read_register(self.MAX31856_REG_READ_LTCBL)
         val_mid_byte = self._read_register(self.MAX31856_REG_READ_LTCBM)
         val_high_byte = self._read_register(self.MAX31856_REG_READ_LTCBH)
 
-        #            ( ((val_high_byte w/o +/-) shifted by number of bits above LSB)
-        #                                             + (val_mid_byte shifted by number of bits above LSB)
-        #                                                                   + val_low_byte )
-        temp_bytes = ( ((val_high_byte & 0x7F) << 11) + (val_mid_byte << 3) + val_low_byte )
-
-        if val_high_byte & 0x80:
-            # Negative Value.  Scale back by number of bits
-            temp_bytes -= 2**(self.MAX31856_CONST_THERM_BITS -1)
-
-        #        temp_bytes*value of LSB
-        temp_C = temp_bytes*self.MAX31856_CONST_THERM_LSB
+        temp_C = MAX31856._thermocoupleTempFromBytes(val_low_byte, val_mid_byte, val_high_byte)
 
         self._logger.debug("Thermocouple Temperature {0} deg. C".format(temp_C))
 

Adafruit_MAX31856/test_MAX31856.py

@@ -100,6 +100,61 @@ def test_get_internal_temperaure_reading(self):
             self.assertTrue(True)
         else:
             self.assertTrue(False)
+
+    def test_temperature_byte_conversions(self):
+        '''Checks the byte conversion for various known temperature byte values.
+        '''
+        _logger.debug('test_temperature_byte_conversions()')
+
+        #-------------------------------------------#
+        # Test Thermocouple Temperature Conversions #
+        byte2 = 0x01; byte1 = 0x70; byte0 = 0x20;  
+        decimal_temp = MAX31856._thermocoupleTempFromBytes(byte0, byte1, byte2)
+        self.assertEqual(decimal_temp, 23.0078125)
+
+        # Check a couple values from the datasheet
+        byte2 = 0b00000001; byte1 = 0b10010000; byte0 = 0b00000000;
+        decimal_temp = MAX31856._thermocoupleTempFromBytes(byte0, byte1, byte2)
+        self.assertEqual(decimal_temp, 25.0)
+
+        byte2 = 0b00000000; byte1 = 0b00000000; byte0 = 0b00000000;
+        decimal_temp = MAX31856._thermocoupleTempFromBytes(byte0, byte1, byte2)
+        self.assertEqual(decimal_temp, 0.0)
+
+        byte2 = 0b11111111; byte1 = 0b11110000; byte0 = 0b00000000;
+        decimal_temp = MAX31856._thermocoupleTempFromBytes(byte0, byte1, byte2)
+        self.assertEqual(decimal_temp, -1.0)
+
+        byte2 = 0b11110000; byte1 = 0b01100000; byte0 = 0b00000000;
+        decimal_temp = MAX31856._thermocoupleTempFromBytes(byte0, byte1, byte2)
+        self.assertEqual(decimal_temp, -250.0)
+
+        #---------------------------------#
+        # Test CJ Temperature Conversions #
+        MSB = 0x1C; LSB = 0x64;
+        decimal_CJ_temp = MAX31856._cjTempFromBytes(MSB, LSB)
+        self.assertEqual(decimal_CJ_temp, 28.390625)
+
+        # Check a couple values from the datasheet
+        MSB = 0b01111111; LSB = 0b11111100;
+        decimal_CJ_temp = MAX31856._cjTempFromBytes(MSB, LSB)
+        self.assertEqual(decimal_CJ_temp, 127.984375)
+
+        MSB = 0b00011001; LSB = 0b00000000;
+        decimal_CJ_temp = MAX31856._cjTempFromBytes(MSB, LSB)
+        self.assertEqual(decimal_CJ_temp, 25)
+
+        MSB = 0b00000000; LSB = 0b00000000;
+        decimal_CJ_temp = MAX31856._cjTempFromBytes(MSB, LSB)
+        self.assertEqual(decimal_CJ_temp, 0)
+
+        MSB = 0b11100111; LSB = 0b00000000;
+        decimal_CJ_temp = MAX31856._cjTempFromBytes(MSB, LSB)
+        self.assertEqual(decimal_CJ_temp, -25)
+
+        MSB = 0b11001001; LSB = 0b00000000;
+        decimal_CJ_temp = MAX31856._cjTempFromBytes(MSB, LSB)
+        self.assertEqual(decimal_CJ_temp, -55)
 
 
 if __name__ == "__main__":

requirements.txt

@@ -1,3 +1,2 @@
 rpi.gpio
-
-
+Adafruit_GPIO

setup.py

@@ -9,7 +9,7 @@
 from setuptools import setup, find_packages
 
 setup(name              = 'Adafruit_MAX31856',
-      version           = '0.0.3',
+      version           = '0.0.4',
       author            = 'John Robinson',
       author_email      = '[email protected]',
       description       = 'Library for accessing the MAX31856 thermocouple temperature sensor on a Raspberry Pi.',