ds18x20.py

"""
DS18x20 temperature sensor driver for MicroPython.

This driver uses the OneWire driver to control DS18S20 and DS18B20
temperature sensors.  It supports multiple devices on the same 1-wire bus.

The following example assumes the ground of your DS18x20 is connected to
Y11, vcc is connected to Y9 and the data pin is connected to Y10.

>>> from pyb import Pin
>>> gnd = Pin('Y11', Pin.OUT_PP)
>>> gnd.low()
>>> vcc = Pin('Y9', Pin.OUT_PP)
>>> vcc.high()

>>> from ds18x20 import DS18X20
>>> d = DS18X20(Pin('Y10'))

Call read_temps to read all sensors:

>>> result = d.read_temps()
>>> print(result)
[20.875, 20.8125]

Call read_temp to read the temperature of a specific sensor:

>>> result = d.read_temp(d.roms[0])
>>> print(result)
20.25

If only one DS18x20 is attached to the bus, then you don't need to 
pass a ROM to read_temp:

>>> result = d.read_temp()
>>> print(result)
20.25

"""

from onewire import OneWire

class DS18X20(object):
    def __init__(self, pin):
        self.ow = OneWire(pin)
        # Scan the 1-wire devices, but only keep those which have the
        # correct # first byte in their rom for a DS18x20 device.
        self.roms = [rom for rom in self.ow.scan() if rom[0] == 0x10 or rom[0] == 0x28]

    def read_temp(self, rom=None):
        """
        Read and return the temperature of one DS18x20 device.
        Pass the 8-byte bytes object with the ROM of the specific device you want to read.
        If only one DS18x20 device is attached to the bus you may omit the rom parameter.
        """
        rom = rom or self.roms[0]
        ow = self.ow
        ow.reset()
        ow.select_rom(rom)
        ow.write_byte(0x44)  # Convert Temp
        while True:
            if ow.read_bit():
                break
        ow.reset()
        ow.select_rom(rom)
        ow.write_byte(0xbe)  # Read scratch
        data = ow.read_bytes(9)
        return self.convert_temp(rom[0], data)

    def read_temps(self):
        """
        Read and return the temperatures of all attached DS18x20 devices.
        """
        temps = []
        for rom in self.roms:
            temps.append(self.read_temp(rom))
        return temps

    def convert_temp(self, rom0, data):
        """
        Convert the raw temperature data into degrees celsius and return as a float.
        """
        temp_lsb = data[0]
        temp_msb = data[1]
        if rom0 == 0x10:
            if temp_msb != 0:
                # convert negative number
                temp_read = temp_lsb >> 1 | 0x80  # truncate bit 0 by shifting, fill high bit with 1.
                temp_read = -((~temp_read + 1) & 0xff) # now convert from two's complement
            else:
                temp_read = temp_lsb >> 1  # truncate bit 0 by shifting
            count_remain = data[6]
            count_per_c = data[7]
            temp = temp_read - 0.25 + (count_per_c - count_remain) / count_per_c
            return temp
        elif rom0 == 0x28:
            temp = (temp_msb << 8 | temp_lsb) / 16
            if (temp_msb & 0xf8) == 0xf8: # for negative temperature
                temp -= 0x1000
            return temp
        else:
            assert False