Fix negative temperatures (#12)

The DHT22 supports negative temperatures which weren't read correctly.

Fixes #10.

README.md

@@ -10,7 +10,17 @@ lot of code.
 
 ## Implementation details
 
-The DHT11 and DHT22 drivers are timing sensitive, and the drivers are therefore implemented using the RMT controller which allows for precise timings.
+The DHT11 and DHT22 drivers are timing sensitive, and the drivers are therefore implemented
+using the RMT controller which allows for precise timings.
+
+## Compatibility
+
+The driver has been tested on DHT11 and DHT22 sensors.
+
+It should work with other sensors that use the same protocol:
+DHT12, KY-015, DHT33, AM2320, AM2321, or AM2322.
+
+If you have one of these sensors, please let us know if it works.
 
 ## Features and bugs
 

package.yaml

@@ -1,4 +1,6 @@
 name: dhtxx
-description: Drivers for the DHT11 and DHT22 (aka AM2302 or RHT03) humidity and temperature sensors.
+description: |
+  Drivers for the DHT11 and DHT22 (aka AM2302 or RHT03) humidity and temperature sensors.
+  Other compatible sensors: DHT12, KY-015, DHT33, AM2320, AM2321, or AM2322.
 environment:
   sdk: ^2.0.0-alpha.1

src/dht11.toit

@@ -5,7 +5,16 @@
 import gpio
 import .driver_ as driver
 
+/**
+Driver for the DHT11 sensor.
+
+Should also work for compatible sensors like the DHT12 or KY-015.
+*/
 class Dht11 extends driver.Driver:
+  static HUMIDITY_INTEGRAL_PART_    ::= 0
+  static HUMIDITY_DECIMAL_PART_     ::= 1
+  static TEMPERATURE_INTEGRAL_PART_ ::= 2
+  static TEMPERATURE_DECIMAL_PART_  ::= 3
 
   /**
   Constructs an instance of the Dht11 driver.
@@ -19,7 +28,7 @@ class Dht11 extends driver.Driver:
     super pin --in_channel_id=in_channel_id --out_channel_id=out_channel_id --max_retries=max_retries
 
   parse_temperature_ data/ByteArray -> float:
-    return data[driver.Driver.TEMPERATURE_INTEGRAL_PART_].to_float
+    return data[TEMPERATURE_INTEGRAL_PART_].to-float + data[TEMPERATURE_DECIMAL_PART_] * 0.1
 
   parse_humidity_ data/ByteArray -> float:
-    return data[driver.Driver.HUMIDITY_INTEGRAL_PART_].to_float
+    return data[HUMIDITY_INTEGRAL_PART_].to-float + data[HUMIDITY_DECIMAL_PART_] * 0.1

src/dht22.toit

@@ -6,7 +6,14 @@ import gpio
 import io show BIG_ENDIAN
 import .driver_ as driver
 
+/**
+Driver for the DHT22 sensor.
+
+Should also work for compatible sensors like the DHT33, AM2320, AM2321, or AM2322.
+*/
 class Dht22 extends driver.Driver:
+  static HUMIDITY_INDEX_    ::= 0
+  static TEMPERATURE_INDEX_ ::= 2
 
   /**
   Constructs an instance of the Dht22 driver.
@@ -20,7 +27,24 @@ class Dht22 extends driver.Driver:
     super pin --in_channel_id=in_channel_id --out_channel_id=out_channel_id --max_retries=max_retries
 
   parse_temperature_ data/ByteArray -> float:
-    return (BIG_ENDIAN.uint16 data driver.Driver.TEMPERATURE_INTEGRAL_PART_) / 10.0
+    // The temperature is a big-endian 16-bit integer.
+    // Some sensors use the first bit to indicate the sign of the temperature; others
+    // encode the value as 2's complement.
+    // Since valid temperature values can only be in a small range, we can use the
+    // second bit to determine which approach the sensor uses.
+    // If the first two bits were 1 and the sensor wasn't using 2's complement, then
+    // the temperature would be lower than -64 degrees which is outside the supported
+    // range.
+    byte1 := data[TEMPERATURE_INDEX_]
+    temperature10/int := ?
+    if (byte1 & 0x80 == 0) or (byte1 & 0x40 == 1):
+      // The temperature is positive or the sensor uses 2's complement.
+      temperature10 = BIG-ENDIAN.int16 data TEMPERATURE_INDEX_
+    else:
+      // The temperature is negative, but the sensor uses the first bit to indicate the sign.
+      temperature10 = BIG-ENDIAN.uint16 data TEMPERATURE_INDEX_
+      temperature10 = -(temperature10 & 0x7FFF)
+    return temperature10 * 0.1
 
   parse_humidity_ data/ByteArray -> float:
-    return (BIG_ENDIAN.uint16 data driver.Driver.HUMIDITY_INTEGRAL_PART_) / 10.0
+    return (BIG_ENDIAN.uint16 data HUMIDITY_INDEX_) * 0.1

src/driver_.toit

@@ -26,12 +26,6 @@ class DhtResult:
     return "T: $(%.2f temperature), H: $(%.2f humidity)"
 
 abstract class Driver:
-  static HUMIDITY_INTEGRAL_PART_    ::= 0
-  static HUMIDITY_DECIMAL_PART_     ::= 1
-  static TEMPERATURE_INTEGRAL_PART_ ::= 2
-  static TEMPERATURE_DECIMAL_PART_  ::= 3
-  static CHECKSUM_PART_             ::= 4
-
   channel_in_    /rmt.Channel
   channel_out_   /rmt.Channel
   max_retries_   /int