Issue setting DS18B20 resolution (Onewire_DS18X20)

[FredrikOfors]

Hi,

I use the AZ-Delivery 1M cable DS18B20 digital stainless steel temperature sensor with the Onewire_DS18X20 library on an ESP32-WROOM-32.

Converting and reading temperature works fine, but I'm trying to change the resolution for faster conversation times without luck.
VCC of the DS18B20 is connected to Vin, so 5V.

My code is a slightly modified example from the library, and the result printed is 12 bit resolution, not the anticipated 9 bit.

I don't know where to start looking., so any help with ideas for troubleshooting would be welcome!

Edit: checked the ROM number, 28-ff-64-1f-41-b6-9b-e7, which does not follow the expected pattern 28-xx-xx-xx-xx-00-00-crc. Perhaps it's a hardware issue with the (fake) sensor?

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

#DS18B20 data line connected to pin P10
ow = OneWire(Pin((27), Pin.OPEN_DRAIN))
temp = DS18X20(ow)
#print("Powermode = ", temp.powermode(Pin(P11)))
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()

[davefes]

Not sure if this helps but ...

import onewire, ds18x20
from machine import Pin
import time


ow1 = onewire.OneWire(Pin(18))   # create a OneWire bus on GPIO18
ow2 = onewire.OneWire(Pin(19))   # create a OneWire bus on GPIO19
ds1 = ds18x20.DS18X20(ow1)       # room temp
ds2 = ds18x20.DS18X20(ow2)       # hot water tank temp


def one_wire_init():

  # scan for DS18X20B device on bus 1
    roms1 = ds1.scan()

  # to display 1-wire IDs
    print (roms1)


def room_temp():

  # read 1-wire room temperature
    try:
        ds1.convert_temp()
    except onewire.OneWireError as error:
       try:
            with open('errors.txt', 'a') as outfile:
                outfile.write('room temp ' + str(error) + '\n')
       except OSError:
            pass

    time.sleep_ms(1000)  # min of 750ms for 1wire conversion

    try:
        room_temp = ds1.read_temp(bytearray(b'(\xe7q\xee\x04\x00\x00\xd0'))
    except Exception as error:
        try:
            with open('errors.txt', 'a') as outfile:
                outfile.write('room temp ' + str(error) + '\n')
        except OSError:
            pass

    print ('Room temp = ', str(room_temp))

    return room_temp

I wonder if the 1 second delay helps?

[FredrikOfors]

There is a 1 second delay between the conversion and read in the code I'm using.

I was hoping to reduce that second to 100ms by changing the resolution from 12 to 9 bit.

[Josverl]

Check and change the wiring to match Roberts recommended connection, 3.3v and an additional pull-up

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.

[FredrikOfors]

I didn't think of it but as I'm using Vin it's the USB 5V. Will change that to 3.3V!

I do have the external 4.7k pullup.

[jomasnash]

Hi,

I use the AZ-Delivery 1M cable DS18B20 digital stainless steel temperature sensor with the Onewire_DS18X20 library on an ESP32-WROOM-32.

Converting and reading temperature works fine, but I'm trying to change the resolution for faster conversation times without luck. VCC of the DS18B20 is connected to Vin, so 5V.

My code is a slightly modified example from the library, and the result printed is 12 bit resolution, not the anticipated 9 bit.

I don't know where to start looking., so any help with ideas for troubleshooting would be welcome!

Edit: checked the ROM number, 28-ff-64-1f-41-b6-9b-e7, which does not follow the expected pattern 28-xx-xx-xx-xx-00-00-crc. Perhaps it's a hardware issue with the (fake) sensor?

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

#DS18B20 data line connected to pin P10
ow = OneWire(Pin((27), Pin.OPEN_DRAIN))
temp = DS18X20(ow)
#print("Powermode = ", temp.powermode(Pin(P11)))
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()

If the 'Resolution' printed is 12-bits while you selected 9-bits, you most likely have a 'fake' DS18B20.
I also have some and I prefer them above the 'original' ones. The fake ones are as accurate as the original ones but convert much faster at 12 bit resolution.

[FredrikOfors]

Thanks for the input. I haven't tested the conversion time on my sensors but will see if it is short enough!

[robert-hh]

I hooked up a known genuine DS18x20 with that code and it works as expected.

• the resolution is reported as 9 bits.
• with a sleep(0.1), the temperature changes reasonable between two readings when applying a coolant.

25.5 
22.5 
18.0 
12.0 
6.5 
2.5 
-0.5 
-3.0 
-5.0 
-6.0 
-7.5 
-8.0 
-9.0 
-9.5 
-10.0 
-10.5 
-11.0 

A nice reading about fake DS18B20 is here: https://github.com/cpetrich/counterfeit_DS18B20

[Josverl]

Thanks Robert,
Ill need to run some tests on some long probes that I have. I have a strong suspicion that they are not genuine parts.
do you know if there is a micropython version of discover-classify_fake_DS18B20.ino ?

[scruss]

As a former reseller of electronic components: most devices sold as DS18B20s are not made by Analog Devices Inc./Maxim Integrated. That doesn't make them less useful, and the whole "fake" tester code things only makes customer service people sad when they tell the customer that they're not getting a refund.

• If you paid less than about €8, it's probably a clone. DS18B20+/MAX31820 sensors are not cheap.
• Unless the reseller says it's genuine (like Adafruit and companies like RS, Digikey, etc) it will be a clone.
• If it's in a steel jacket, it's probably a clone.
• Readings from an IC temperature sensor made more frequently than once a second (especially one in a steel jacket with some thermal mass) will mostly be lost in the fog of τ (time constant). Changing resolution for faster reading might only result in more noise, especially with clone sensors. The real thing's only ±½ °C accuracy.

Clone sensors are usually good enough. There are no perfect temperature sensors, and you get what you pay for.

[aliarifat]

The OneWire_DS18X20 MicroPython library often does not support resolution change unless it’s explicitly implemented. The standard ds18x20 driver used in MicroPython only supports temperature conversion and reading. Even if your sensor supports variable resolution, the driver may not write to the configuration register.
You mentioned this ROM:
28-ff-64-1f-41-b6-9b-e7
Which starts with 28, meaning it's likely a DS18B20. However:

ROM codes vary. The bytes 00-00 in the middle are not mandatory. Your sensor has a legitimate-looking CRC (e7).

Clones/fakes may not support configuration changes or may ignore them.
Check if the conversion time is faster after attempting to change the resolution. This is the best indicator if the resolution changed even if the library doesn't report it. Also check if all of the connections that you made are OK. For a flawless connection, you can make a compact circuit like this if your application supports it. https://www.pcbway.com/project/shareproject/DIY_Thermometer_with_TTGO_T_Display_and_DS18B20_2bf4fa79.html