esp8266/modules: Split onewire.py into OneWire and DS18X20 driver.

The OneWire class is now in its own onewire.py module, and the temperature
sensor class is in its own ds18x20.py module.  The latter is renamed to
DS18X20 to reflect the fact that it will support both the "S" and "B"
variants of the device.

These files are moved to the modules/ subdirectory to take advantage of
frozen bytecode.

esp8266/modules/ds18x20.py

@@ -0,0 +1,37 @@
+# DS18x20 temperature sensor driver for MicroPython.
+# MIT license; Copyright (c) 2016 Damien P. George
+
+_CONVERT = const(0x44)
+_RD_SCRATCH = const(0xbe)
+_WR_SCRATCH = const(0x4e)
+
+class DS18X20:
+    def __init__(self, onewire):
+        self.ow = onewire
+
+    def scan(self):
+        return [rom for rom in self.ow.scan() if rom[0] == 0x28]
+
+    def convert_temp(self):
+        self.ow.reset(True)
+        self.ow.writebyte(self.ow.SKIP_ROM)
+        self.ow.writebyte(_CONVERT)
+
+    def read_scratch(self, rom):
+        self.ow.reset(True)
+        self.ow.select_rom(rom)
+        self.ow.writebyte(_RD_SCRATCH)
+        buf = self.ow.read(9)
+        if self.ow.crc8(buf):
+            raise Exception('CRC error')
+        return buf
+
+    def write_scratch(self, rom, buf):
+        self.ow.reset(True)
+        self.ow.select_rom(rom)
+        self.ow.writebyte(_WR_SCRATCH)
+        self.ow.write(buf)
+
+    def read_temp(self, rom):
+        buf = self.read_scratch(rom)
+        return (buf[1] << 8 | buf[0]) / 16

esp8266/modules/onewire.py

@@ -15,8 +15,11 @@ class OneWire:
         self.pin = pin
         self.pin.init(pin.OPEN_DRAIN)
 
-    def reset(self):
-        return _ow.reset(self.pin)
+    def reset(self, required=False):
+        reset = _ow.reset(self.pin)
+        if required and not reset:
+            raise OneWireError
+        return reset
 
     def readbit(self):
         return _ow.readbit(self.pin)
@@ -87,41 +90,3 @@ class OneWire:
 
     def crc8(self, data):
         return _ow.crc8(data)
-
-class DS18B20:
-    CONVERT = const(0x44)
-    RD_SCRATCH = const(0xbe)
-    WR_SCRATCH = const(0x4e)
-
-    def __init__(self, onewire):
-        self.ow = onewire
-
-    def scan(self):
-        return [rom for rom in self.ow.scan() if rom[0] == 0x28]
-
-    def convert_temp(self):
-        if not self.ow.reset():
-            raise OneWireError
-        self.ow.writebyte(SKIP_ROM)
-        self.ow.writebyte(CONVERT)
-
-    def read_scratch(self, rom):
-        if not self.ow.reset():
-            raise OneWireError
-        self.ow.select_rom(rom)
-        self.ow.writebyte(RD_SCRATCH)
-        buf = self.ow.read(9)
-        if self.ow.crc8(buf):
-            raise OneWireError
-        return buf
-
-    def write_scratch(self, rom, buf):
-        if not self.ow.reset():
-            raise OneWireError
-        self.ow.select_rom(rom)
-        self.ow.writebyte(WR_SCRATCH)
-        self.ow.write(buf)
-
-    def read_temp(self, rom):
-        buf = self.read_scratch(rom)
-        return (buf[1] << 8 | buf[0]) / 16