--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/si7021.py	Thu Feb 09 00:13:22 2017 +0100
@@ -0,0 +1,137 @@
+from micropython import const
+import ustruct
+import sys
+
+
+_HUMID_NOHOLD = const(0xf5)
+_TEMP_NOHOLD = const(0xf3)
+_RESET = const(0xfe)
+_READ_USER1 = const(0xe7)
+_USER1_VAL = const(0x3a)
+
+
+def _crc(data):
+    crc = 0
+    for byte in data:
+        crc ^= byte
+        for i in range(8):
+            if crc & 0x80:
+                crc <<= 1
+                crc ^= 0x131
+            else:
+                crc <<= 1
+    return crc
+
+
+class SI7021:
+    """
+    A driver for the SI7021 temperature and humidity sensor.
+
+    MicroPython example::
+
+        import si7021
+        from machine import I2C, Pin
+
+        i2c = I2C(-1, Pin(5), Pin(4))
+        s = si7021.SI7021(i2c)
+        print(s.temperature())
+        print(s.humidity())
+
+    """
+
+    def __init__(self, i2c, address=0x40):
+        self.i2c = i2c
+        self.address = address
+        self.init()
+        self._measurement = 0
+
+    def init(self):
+        self.reset()
+        # Make sure the USER1 settings are correct.
+        while True:
+            # While restarting, the sensor doesn't respond to reads or writes.
+            try:
+                value = self.i2c.readfrom_mem(self.address, _READ_USER1, 1)[0]
+            except OSError as e:
+                if e.args[0] != 19: # errno 19 ENODEV
+                    raise
+            else:
+                break
+        if value != _USER1_VAL:
+            raise RuntimeError("bad USER1 register (%x!=%x)" % (
+                value, _USER1_VAL))
+
+    def _command(self, command):
+        self.i2c.writeto(self.address, ustruct.pack('B', command))
+
+    def _data(self):
+        data = bytearray(3)
+        data[0] = 0xff
+        while True:
+            # While busy, the sensor doesn't respond to reads.
+            try:
+                self.i2c.readfrom_into(self.address, data)
+            except OSError as e:
+                if e.args[0] != 19: # errno 19 ENODEV
+                    raise
+            else:
+                if data[0] != 0xff: # Check if read succeeded.
+                    break
+        value, checksum = ustruct.unpack('>HB', data)
+        if checksum != _crc(data[:2]):
+            raise ValueError("CRC mismatch")
+        return value
+
+    def reset(self):
+        self._command(_RESET)
+
+    def humidity(self, raw=False, block=True):
+        """
+        Start a humidity measurement.
+
+        If ``block`` is ``True``, block until it is ready and return the
+        measured value. If it's ``False``, return None immediately, and the
+        value can be read later with a blocking call.
+
+        If ``raw`` is ``True``, return the measured value as 16-bit integer,
+        otherwise convert it into percentage of relative humidity and return it
+        as a floating point number.
+        """
+
+        if not self._measurement:
+            self._command(_HUMID_NOHOLD)
+        elif self._measurement != _HUMID_NOHOLD:
+            raise RuntimeError("other measurement in progress")
+        if not block:
+            self._measurement = _HUMID_NOHOLD
+            return None
+        self._measurement = 0
+        value = self._data()
+        if raw:
+            return value
+        return value * 125 / 65536 - 6
+
+    def temperature(self, raw=False, block=True):
+        """
+        Start a temperature measurement.
+
+        If ``block`` is ``True``, block until it is ready and return the
+        measured value. If it's ``False``, return None immediately, and the
+        value can be read later with a blocking call.
+
+        If ``raw`` is ``True``, return the measured value as 16-bit integer,
+        otherwise convert it into Celsius degrees and return as a floating
+        point number.
+        """
+        if not self._measurement:
+            self._command(_TEMP_NOHOLD)
+        elif self._measurement != _TEMP_NOHOLD:
+            raise RuntimeError("other measurement in progress")
+        if not block:
+            self._measurement = _TEMP_NOHOLD
+            return None
+        self._measurement = 0
+        value = self._data()
+        if raw:
+            return value
+        return value * 175.72 / 65536 - 46.85