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

@@ -0,0 +1,92 @@
+# 1-Wire driver for MicroPython on ESP8266
+# MIT license; Copyright (c) 2016 Damien P. George
+
+import _onewire as _ow
+
+class OneWireError(Exception):
+    pass
+
+class OneWire:
+    SEARCH_ROM = const(0xf0)
+    MATCH_ROM = const(0x55)
+    SKIP_ROM = const(0xcc)
+
+    def __init__(self, pin):
+        self.pin = pin
+        self.pin.init(pin.OPEN_DRAIN)
+
+    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)
+
+    def readbyte(self):
+        return _ow.readbyte(self.pin)
+
+    def read(self, count):
+        buf = bytearray(count)
+        for i in range(count):
+            buf[i] = _ow.readbyte(self.pin)
+        return buf
+
+    def writebit(self, value):
+        return _ow.writebit(self.pin, value)
+
+    def writebyte(self, value):
+        return _ow.writebyte(self.pin, value)
+
+    def write(self, buf):
+        for b in buf:
+            _ow.writebyte(self.pin, b)
+
+    def select_rom(self, rom):
+        self.reset()
+        self.writebyte(MATCH_ROM)
+        self.write(rom)
+
+    def scan(self):
+        devices = []
+        diff = 65
+        rom = False
+        for i in range(0xff):
+            rom, diff = self._search_rom(rom, diff)
+            if rom:
+                devices += [rom]
+            if diff == 0:
+                break
+        return devices
+
+    def _search_rom(self, l_rom, diff):
+        if not self.reset():
+            return None, 0
+        self.writebyte(SEARCH_ROM)
+        if not l_rom:
+            l_rom = bytearray(8)
+        rom = bytearray(8)
+        next_diff = 0
+        i = 64
+        for byte in range(8):
+            r_b = 0
+            for bit in range(8):
+                b = self.readbit()
+                if self.readbit():
+                    if b: # there are no devices or there is an error on the bus
+                        return None, 0
+                else:
+                    if not b: # collision, two devices with different bit meaning
+                        if diff > i or ((l_rom[byte] & (1 << bit)) and diff != i):
+                            b = 1
+                            next_diff = i
+                self.writebit(b)
+                if b:
+                    r_b |= 1 << bit
+                i -= 1
+            rom[byte] = r_b
+        return rom, next_diff
+
+    def crc8(self, data):
+        return _ow.crc8(data)