DS18B20 setup time at power on ?

Hi ,

I use lot of DS18B20 for years. I purchased new pieces last month and they gave me a fixed value of 25°C.
I made some tests.
1 - they work well when powered continuously. The problem appears only at power up.
2 - 1sec delay before convert T command doesnt work...
3 - but, doing twice convert T command (with normal delay) solves the problem !

In the past I got some DS18S20 giving only negative values

Your infos are welcome

After 58 posts you really should know how to post a proper question. Or do you really think this info is enough to get useful suggestions?

No code.
No link to specific sensor.
No schematics or anything else to go by.

Are you saying you think they are knock-off versions? Is the full part number the same as previous batches?

Does "work well" mean 100% reliability? I've used an array of 20 genuine DS18B20's 24/7 logged continuously
for years with absolutely no problems.

MarkT:
Are you saying you think they are knock-off versions?

I think that maybe something changed in the design of DS18B20 (or maybe defective?)... new devices seems not able to give a value after only one conversion at power up.

Is the full part number the same as previous batches?

old devices (marked 1812C4) readRom exemple cd(0x33) 28 ED 77 45 92 02 02 6A
new devices (1827C4) readRom exemple cd(0x33) 28 F8 D9 79 97 14 03 3C
(left byte is first to be read)

Does "work well" mean 100% reliability?

yes

I've used an array of 20 genuine DS18B20's 24/7 logged continuously
for years with absolutely no problems.

  1. pb appears at power up and ...
  2. pb concern recently purchased devices and ...
  3. I use more than 20 DS18B20 (from older purchases) for years with absolutely no problems.

wvmarle:
Or do you really think this info is enough to get useful suggestions?

The device is DS18B20 … I dont know how to be more explicit
No code needed : I can reformulate the subject of my post as : “abnormal DS18B20 answer to a specific command (convert T) after standard delay”.

DS18B20 has 3 legs : 1 at VCC ; 1 at GND ; the last is data (with pullup of 4,7k as specified in doc) ;
Sorry I forgoted to mention that.

I found how to use the device (as I indicated) and (if someone meets the same problem) other infos are welcome :slight_smile:

Maybe you have a DS18B20-PAR? Marked DA18B20P?

zoomx:
Maybe you have a DS18B20-PAR? Marked DA18B20P?
https://datasheets.maximintegrated.com/en/ds/DS18B20-PAR.pdf

No "P" ... and P devices have same timings than no-P

I never noticed that since i leave this sensor always on. My sensors are quite old but I will check for the mark just below DS18B20.

Do you use library?

To close the subject...

using larger conversion time solved definitely the issue...

ie 500mS instead of 375mS

AFAIK the conversion time for 12bit precision is 750ms

Indeed, 750 ms.
That’s why you have the async mode. Don’t get stuck waiting for conversion to complete, tell it to start and come back later to take the reading.