esp8266/scripts/onewire.py: Simplify and improve 1-wire driver.

Changes are:
- added OneWireError exception and used where errors can occur
- renamed read/write functions to use same names as C _onewire funcs
- read_bytes is now read, write_bytes is now write
- add ability to read/write DS18B20 scratch pad
- rename start_measure to convert_temp (since that's what it does)
- rename get_temp to read_temp (consistency with other read names)
- removed test function

esp8266/scripts/onewire.py

+# 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:
-    CMD_SEARCHROM = const(0xf0)
-    CMD_READROM = const(0x33)
-    CMD_MATCHROM = const(0x55)
-    CMD_SKIPROM = const(0xcc)
+    SEARCH_ROM = const(0xf0)
+    MATCH_ROM = const(0x55)
+    SKIP_ROM = const(0xcc)
 
     def __init__(self, pin):
         self.pin = pin
@@ -13,32 +18,32 @@ class OneWire:
     def reset(self):
         return _ow.reset(self.pin)
 
-    def read_bit(self):
+    def readbit(self):
         return _ow.readbit(self.pin)
 
-    def read_byte(self):
+    def readbyte(self):
         return _ow.readbyte(self.pin)
 
-    def read_bytes(self, count):
+    def read(self, count):
         buf = bytearray(count)
         for i in range(count):
             buf[i] = _ow.readbyte(self.pin)
         return buf
 
-    def write_bit(self, value):
+    def writebit(self, value):
         return _ow.writebit(self.pin, value)
 
-    def write_byte(self, value):
+    def writebyte(self, value):
         return _ow.writebyte(self.pin, value)
 
-    def write_bytes(self, buf):
+    def write(self, buf):
         for b in buf:
             _ow.writebyte(self.pin, b)
 
     def select_rom(self, rom):
         self.reset()
-        self.write_byte(CMD_MATCHROM)
-        self.write_bytes(rom)
+        self.writebyte(MATCH_ROM)
+        self.write(rom)
 
     def scan(self):
         devices = []
@@ -55,7 +60,7 @@ class OneWire:
     def _search_rom(self, l_rom, diff):
         if not self.reset():
             return None, 0
-        self.write_byte(CMD_SEARCHROM)
+        self.writebyte(SEARCH_ROM)
         if not l_rom:
             l_rom = bytearray(8)
         rom = bytearray(8)
@@ -64,8 +69,8 @@ class OneWire:
         for byte in range(8):
             r_b = 0
             for bit in range(8):
-                b = self.read_bit()
-                if self.read_bit():
+                b = self.readbit()
+                if self.readbit():
                     if b: # there are no devices or there is an error on the bus
                         return None, 0
                 else:
@@ -73,7 +78,7 @@ class OneWire:
                         if diff > i or ((l_rom[byte] & (1 << bit)) and diff != i):
                             b = 1
                             next_diff = i
-                self.write_bit(b)
+                self.writebit(b)
                 if b:
                     r_b |= 1 << bit
                 i -= 1
@@ -84,60 +89,39 @@ class OneWire:
         return _ow.crc8(data)
 
 class DS18B20:
-    THERM_CMD_CONVERTTEMP = const(0x44)
-    THERM_CMD_RSCRATCHPAD = const(0xbe)
+    CONVERT = const(0x44)
+    RD_SCRATCH = const(0xbe)
+    WR_SCRATCH = const(0x4e)
 
     def __init__(self, onewire):
         self.ow = onewire
-        self.roms = []
 
     def scan(self):
-        self.roms = []
-        for rom in self.ow.scan():
-            if rom[0] == 0x28:
-                self.roms += [rom]
-        return self.roms
+        return [rom for rom in self.ow.scan() if rom[0] == 0x28]
 
-    def start_measure(self):
+    def convert_temp(self):
         if not self.ow.reset():
-            return False
-        self.ow.write_byte(CMD_SKIPROM)
-        self.ow.write_byte(THERM_CMD_CONVERTTEMP)
-        return True
+            raise OneWireError
+        self.ow.writebyte(SKIP_ROM)
+        self.ow.writebyte(CONVERT)
 
-    def get_temp(self, rom):
+    def read_scratch(self, rom):
         if not self.ow.reset():
-            return None
+            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.write_byte(THERM_CMD_RSCRATCHPAD)
+        self.ow.writebyte(WR_SCRATCH)
+        self.ow.write(buf)
 
-        buf = self.ow.read_bytes(9)
-        if self.ow.crc8(buf):
-            return None
-
-        return self._convert_temp(buf)
-
-    def _convert_temp(self, data):
-        temp_lsb = data[0]
-        temp_msb = data[1]
-        return (temp_msb << 8 | temp_lsb) / 16
-
-# connect 1-wire temp sensors to GPIO12 for this test
-def test():
-    import time
-    import machine
-    dat = machine.Pin(12)
-    ow = OneWire(dat)
-
-    ds = DS18B20(ow)
-    roms = ow.scan()
-    print('found devices:', roms)
-
-    for i in range(4):
-        print('temperatures:', end=' ')
-        ds.start_measure()
-        time.sleep_ms(750)
-        for rom in roms:
-            print(ds.get_temp(rom), end=' ')
-        print()
+    def read_temp(self, rom):
+        buf = self.read_scratch(rom)
+        return (buf[1] << 8 | buf[0]) / 16