Add functions to get/set baseline

Author: anlorn

adafruit_ccs811.py

@@ -44,13 +44,16 @@
 __version__ = "0.0.0-auto.0"
 __repo__ = "https://github.com/adafruit/Adafruit_CircuitPython_CCS811.git"
 
+_BASELINE_PACKING_FORMAT = "<H"
+
 _ALG_RESULT_DATA = const(0x02)
 _RAW_DATA = const(0x03)
 _ENV_DATA = const(0x05)
 _NTC = const(0x06)
 _THRESHOLDS = const(0x10)
 
-# _BASELINE = 0x11
+_BASELINE = const(0x11)
+
 # _HW_ID = 0x20
 # _HW_VERSION = 0x21
 # _FW_BOOT_VERSION = 0x23
@@ -106,7 +109,6 @@ def __init__(self, i2c_bus, address=0x5A):
             raise RuntimeError(
                 "Device ID returned is not correct! Please check your wiring."
             )
-
         # try to start the app
         buf = bytearray(1)
         buf[0] = 0xF4
@@ -156,6 +158,31 @@ def _update_data(self):
             if self.error:
                 raise RuntimeError("Error:" + str(self.error_code))
 
+    @property
+    def baseline(self):
+        """
+        The function read and return the current baseline value.
+        The returned value is packed into an integer.
+        Later the same integer can be passed `set_baseline` function
+        in order to set a new baseline.
+        """
+        buf = bytearray(3)
+        buf[0] = _BASELINE
+        with self.i2c_device as i2c:
+            i2c.write_then_readinto(buf, buf, out_end=1, in_start=1)
+        return struct.unpack(_BASELINE_PACKING_FORMAT, buf[1:])[0]
+
+    def set_baseline(self, baseline_int):
+        """
+        The function lets you set a new baseline. As an argument accepts
+        integer which represents packed baseline 2 bytes value.
+        """
+        buf = bytearray(3)
+        buf[0] = _BASELINE
+        struct.pack_into(_BASELINE_PACKING_FORMAT, buf, 1, baseline_int)
+        with self.i2c_device as i2c:
+            i2c.write(buf)
+
     @property
     def tvoc(self):  # pylint: disable=invalid-name
         """Total Volatile Organic Compound in parts per billion."""