docs: Add DHT to ESP8266 Quick Ref and Tutorial

docs/esp8266/quickref.rst

@@ -291,6 +291,24 @@ For low-level driving of an APA102::
     import esp
     esp.apa102_write(clock_pin, data_pin, rgbi_buf)
 
+DHT driver
+----------
+
+The DHT driver is implemented in software and works on all pins::
+
+    import dht
+    import machine
+
+    d = dht.DHT11(machine.Pin(4))
+    d.measure()
+    d.temperature() # eg. 23 (°C)
+    d.humidity()    # eg. 41 (% RH)
+
+    d = dht.DHT22(machine.Pin(4))
+    d.measure()
+    d.temperature() # eg. 23.6 (°C)
+    d.humidity()    # eg. 41.3 (% RH)
+
 WebREPL (web browser interactive prompt)
 ----------------------------------------
 

docs/esp8266/tutorial/dht.rst

@@ -0,0 +1,65 @@
+Temperature and Humidity
+========================
+
+DHT (Digital Humidity & Temperature) sensors are low cost digital sensors with
+capacitive humidity sensors and thermistors to measure the surrounding air.
+They feature a chip that handles analog to digital conversion and provide a
+1-wire interface. Newer sensors additionally provide an I2C interface.
+
+The DHT11 (blue) and DHT22 (white) sensors provide the same 1-wire interface,
+however, the DHT22 requires a separate object as it has more complex
+calculation. DHT22 have 1 decimal place resolution for both humidity and
+temperature readings. DHT11 have whole number for both.
+
+A custom 1-wire protocol, which is different to Dallas 1-wire, is used to get
+the measurements from the sensor. The payload consists of a humidity value,
+a temperature value and a checksum.
+
+To use the 1-wire interface, construct the objects referring to their data pin::
+
+    >>> import dht
+    >>> import machine
+    >>> d = dht.DHT11(machine.Pin(4))
+
+    >>> import dht
+    >>> import machine
+    >>> d = dht.DHT22(machine.Pin(4))
+
+Then measure and read their values with::
+
+    >>> d.measure()
+    >>> d.temperature()
+    >>> d.humidity()
+
+Values returned from ``temperature()`` are in degrees Celsius and values
+returned from ``humidity()`` are a percentage of relative humidity.
+
+The DHT11 can be called no more than once per second and the DHT22 once every
+two seconds for most accurate results. Sensor accuracy will degrade over time.
+Each sensor supports a different operating range. Refer to the product
+datasheets for specifics.
+
+In 1-wire mode, only three of the four pins are used and in I2C mode, all four
+pins are used. Older sensors may still have 4 pins even though they do not
+support I2C. The 3rd pin is simply not connected.
+
+Pin configurations:
+
+Sensor without I2C in 1-wire mode (eg. DHT11, DHT22, AM2301, AM2302):
+
+    1=VDD, 2=Data, 3=NC, 4=GND
+
+Sensor with I2C in 1-wire mode (eg. DHT12, AM2320, AM2321, AM2322):
+
+    1=VDD, 2=Data, 3=GND, 4=GND
+
+Sensor with I2C in I2C mode (eg. DHT12, AM2320, AM2321, AM2322):
+
+    1=VDD, 2=SDA, 3=GND, 4=SCL
+
+You should use pull-up resistors for the Data, SDA and SCL pins.
+
+To make newer I2C sensors work in backwards compatible 1-wire mode, you must
+connect both pins 3 and 4 to GND. This disables the I2C interface.
+
+DHT22 sensors are now sold under the name AM2302 and are otherwise identical.

docs/esp8266/tutorial/index.rst

@@ -29,4 +29,5 @@ to `<https://www.python.org>`__.
    powerctrl.rst
    onewire.rst
    neopixel.rst
+   dht.rst
    nextsteps.rst