add calibrate_zero() and set_auto_calibrate_zero()

README.md

@@ -30,6 +30,7 @@ __License MIT__
 
 Some hints taken from:
 https://github.com/nara256/mhz19_uart/blob/master/src/MHZ19_uart.cpp
+https://emariete.com/en/sensor-co2-mh-z19b/#El_sensor_de_CO2_MH-Z19C
 
 
 
@@ -49,3 +50,13 @@ print('ppm:',    sensor.ppm)
 print('temp:',   sensor.temp)
 print('status:', sensor.co2status)
 ```
+
+CALIBRATE IF NEEDED:
+```
+#perform a manual zero point calibration (let it sit at 400pm for 20mins first)
+sensor.calibrate_zero()
+
+#enable/disable automatic zero point calibration, which depends on a daily drop
+#to 400ppm to work
+sensor.set_auto_calibrate_zero(True)
+```

mhz19.py

@@ -69,6 +69,17 @@ def get_data(self):
             self.co2status = ord(chr(s[5]))
             return 1
 
+    def set_auto_calibrate_zero(self, enable):
+        if enable:
+            msg = b'\xff\x01\x79\xa0\x00\x00\x00\x00\xe6'
+        else:
+            msg = b'\xff\x01\x79\x00\x00\x00\x00\x00\x86'
+        self.uart.write(msg)
+
+    def calibrate_zero(self):
+        msg = b'\xff\x01\x87\x00\x00\x00\x00\x00\x78'
+        self.uart.write(msg)
+
     def crc8(self, a):
         crc = 0x00
         count = 1