sensor/mhz19: Add driver for MH-Z19 (CO2 sensor).

Originally by Fredrik Strupe, updated for micropython-lib.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

micropython/drivers/sensor/mhz19/manifest.py

@@ -0,0 +1,3 @@
+metadata(description="Driver for MH-Z19 CO2 sensor.", version="0.1.0")
+
+module("mhz19.py", opt=3)

micropython/drivers/sensor/mhz19/mhz19.py

@@ -0,0 +1,48 @@
+# MH-Z19 CO2 sensor driver for MicroPython.
+# MIT license; Copyright (c) 2018 Fredrik Strupe
+
+import machine
+import utime
+
+
+class TimeoutError(Exception):
+    pass
+
+
+class MHZ19:
+    """MH-Z19 CO2 sensor driver"""
+
+    def __init__(self, pin, max_value=5000):
+        """
+        Args:
+            pin: the pin that the PWM pin on the MH-Z19 is connected to.
+            max_value: upper bound of measuring range. usually 2000 or 5000.
+        """
+        self.pin = pin
+        self.max_value = max_value
+
+    def _wait_on_condition(self, cond, timeout=5000):
+        start = utime.ticks_ms()
+        while not cond():
+            if utime.ticks_diff(utime.ticks_ms(), start) > timeout:
+                raise TimeoutError
+
+    def pwm_read(self):
+        """Read CO2 value via PWM pin.
+
+        Reading usually takes 1-2 seconds.
+
+        Returns:
+            CO2 value in ppm (parts per million), with an accuracy of
+            ±(50 + result * 0.05) ppm.
+        Raises:
+            TimeoutError: if the reading takes more than 5 seconds.
+        """
+        # Wait until a new cycle starts
+        self._wait_on_condition(lambda: self.pin.value() == 0)
+
+        # Measure high and low duration during cycle
+        t_h = machine.time_pulse_us(self.pin, 1, 1500000)
+        t_l = machine.time_pulse_us(self.pin, 0, 1500000)
+
+        return self.max_value * (t_h - 2000) / (t_h + t_l - 4000)