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Classes for driving the DS18x20 sensor with the onewire protocol for Pycom MicroPython

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Micropython driver for onewire and the DS18x20 sensor

This two classes support the DS18x20 sensor and the onewire protocol used by this sensor on Pycom devices. Similar classes exists for the Micropython.org version of MicroPython, which make use of the built-in onewire driver.

Class Onewire

ow = Onewire(Pin_object)

Pin_object must be an output caspable pin. It will be configured by driver.

Methods

reset([required=False])

Resets the bus. If the parameter required is True, AssertError is raised if no device on the bus responds

value = readbit()

value = readbyte()

buffer = readbytes(count)

readinto(buffer)

Read data form the bus, either a single bit, or a byte, or a sequence of bytes. readbytes expects the numer of bytes to be read. These will be returned. Readinto expects a buffer to be supplied, and len(buffer) bytes will be read.

writebit(bit [, powerpin=None])

writebyte(byte [, powerpin=Nobe])

writebytes(buffer)

Write data to the bus. powerpin, if supplied, must be a Pin object. This pin will be set if the bit is tranferred resp. the last bit of a byte is transferred. writebytes send all bytes in the buffer.

select_rom(rom)

Send the message to select a specific device based on the rom number. This number will be obtained by scan(). The selected device will respond to further read and write calls.

devices = scan()

Return the list of rom numbers of all devices on the onwire bus.

_search_rom()

Internal function. Search devices on the bus and obtain the rom number.


Class DS18X20

sensor = DS18X20(onewire)

onewire must be an instance of the Onewire class.

Methods

devices = scan()

Return the list of DS18x2x devices on the bus. Only devices with rom type 0x10, 0x22 and 0x22 are selected.

mode = powermode([pin_object=None])

Test the powermode on the bus. If the bus has strong power, 1 is returned. In case of paraisitic power, 0 is returned. pin_object must be a pin object, which is output mode capable. It will be use to switch between soft and strong pullup for parasitic power. To enable string pull-up, the pin will be set to 1 (high). The power has then to be supplied by an external circuit. In case of just a few active devices on the devices, a small signal schottky diode (e.g. 1N5817) between the pin_object and the data bus is sufficient.

convert_temp([rom=None])

Start temperture conversion. If rom is supplied, only this device will start conversion. Otherwise, all devices on the bus will be started.

data = read_scratch(rom)

Read the scratchpad memory of the addressed device. 9 bytes of data will be returned

write_scratch(rom, data)

Write to the scratchpad of the addressed devices. data shall be three bytes. The first two bytes are the high and low alarm temperature. The third by is the configuration. See the DS18B20 data sheet for details.

read_temp(rom)

Get the temperature reading of the addressed device as degree Celsuis. In case of an CRC error, None is returned.

Warning: After power up and before a conversion cycle has been performed, the DS18x20 sensors will return the value 85°C. Since this is also a valid return value, the calling app must decide, whether it is a reasonable value in the given context.

res = resolution(rom [, bits])

Get or set the resolution of a sensor. If the paramter bits is specified, the resolution is set to that value. Valid values are 9 though 12. If bits is not specified, the actual resolution is returned. The value set is not permanent. After a power cycle it is reset to 12. The conversion time depends on the resoltion. The figures are:

bits Conversion time (ms)
9 94
10 188
11 375
12 750

deg_f = fahrenheit(celsius)

deg_k = kelvin(celsius)

Three fuctions converting the celsius value returned by read_temp() into other scales.


Class DS18X20Single

sensor = DS18X20Single(onewire)

Convenience Class derived from DS18X20. It makes using the clase with a single sensor a little bit easier by not requiring a rom parameter for convert_temp() and read_temp(), and calling scan() as part of the init procedure. If more than one device is connected to the bus, an AssertionError is raised.

Specific Methods

convert_temp()

Start temperature conversion with the single device on the bus.

read_temp([rom=None])

Get the temperature reading of the addressed device as degree Celsuis from the single device on the bus. In case of an CRC error, None is returned.

Warning: After power up and before a conversion cycle has been performed, the DS18x20 sensors will return the value 85°C. Since this is also a valid return value, the calling app must decide, whether it is a reasonable value in the given context.

Besides these specific methods, all other methods of the master classe work as documented.

Example

Copy ds18x20.py and onewire.py to the board, and the file ds18x20_example.py. Connect the data line of the sensor to P10, GND to GND and Vcc of the sensor to the 3.3V output of your device. You need also to attach a pull-up resistor of 4.7kOhm between the data line and Vcc of the sensor. The internal pull-up of the ES32 may not be strong enough.

Then you may start that example with:

import ds18x20_example

The code of the example reads as:

import time
from machine import Pin
from ds18x20_new import DS18X20
from onewire_new import OneWire

#DS18B20 data line connected to pin P10
ow = OneWire(Pin('P10'))
temp = DS18X20(ow)
roms = temp.scan()

temp.convert_temp()
while True:
    time.sleep(1)
    for rom in roms:
        print(temp.read_temp(rom), end=" ")
    print()
    temp.convert_temp()

The same example using parasitic power and setting the resolution of the first device:

import time
from machine import Pin
from ds18x20 import DS18X20
from onewire import OneWire

#DS18B20 data line connected to pin P10
ow = OneWire(Pin('P10'))
temp = DS18X20(ow)
pm = temp.powermode(Pin('P11'))
print("Powermode = ", pm)
roms = temp.scan()

temp.resolution(roms[0], 9)
print("Resolution", temp.resolution(roms[0]))

temp.convert_temp()
while True:
    time.sleep(1)
    for rom in roms:
        print(temp.read_temp(rom), end=" ")
    print()
    temp.convert_temp()

Example using the class DS18X20Single

import time
from machine import Pin
from ds18x20_single import DS18X20Single
from onewire import OneWire

# DS18B20 data line connected to pin P10
ow = OneWire(Pin('P10'))
temp = DS18X20Single(ow)
print("Powermode = ", temp.powermode(Pin('P11')))

temp.convert_temp()
while True:
    time.sleep(1)
    print(temp.read_temp())
    temp.convert_temp()

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Classes for driving the DS18x20 sensor with the onewire protocol for Pycom MicroPython

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