Wrong data from DS18B20 temperature sensor after long power off

Question

I have my Arduino Uno connected to the PC via USB when prototyping. I am developing a thermometer with memory, using DS18B20 sensors.

When PC is shut down, there is no power on the USB and the Arduino is off. Sometimes, when my PC is turned off for several hours and then, when I turn it on, the Arduino is started, but it behaves "strange" - the program is showing buggy values on the display, looking like if there is some bug in the sketch. However, when I upload the sketch again, it works fine.

The buggy values are the averages for the run, computed simply by summing a float and dividing by int. What I see is e.g. instead of seeing something around 23.5 I see 85.1. The current values read from the sensor look OK (show 23.5 as expected), but I cannot tell for sure if perhaps some value read early might be wrong.

It is not easy to reproduce, I see it only sometimes, always when powering Arduino on after several hours without power.

My first idea is either the sketch got corrupted during the long power down, or variables are initialized wrong, and the sample sum / sample count is not zero.

However I though the sketches are uploaded into the Flash RAM, which should be non-volatile according to the docs. Is there something obvious I am missing, some explanation why is the sketch working differently after long power down?

Answer 1

There should be no difference between turning on your computer and plugging in a USB cable. The embedded devices typically plugged into a USB ports such as mice, keyboards, wireless adapters, hard drives, etc don't have their FLASH memory corrupted when the computer is turned on so it is unlikely that your Arduino would.

To confirm there isn't something dodgy going on with the USB power supplied to the board, use a multimeter to monitor the 5V pin of the Arduino. If the voltage isn't going too high or too low then everything should be fine. Even if it goes too low, the microcontroller will have brown-out-detect enabled to shut it down in this event.

You mentioned a display. Perhaps, and this is pure speculation, some how the display is being initialised wrongly due to the Arduino being reset by USB connection. If the display has a general reset command, or a reset pin, enable it at the start of your sketch to ensure the display is in its default state.

Answer 2

I researched this once when I was getting consistent, exactly 85 degC readings from my sensors. The Maxim DS18b20 data sheet (rev 042208, page 4, in a footnote to Table 1) says:

"The power-on reset value of the temperature register is +85°C.

Double check that the Arduino code is follows the sensor communication protocol correctly.

Answer 3

The number you are seeing instead - 85.1, is suspiciously close to a Farenheit reading instead of Celcius - my guess is that there maybe a setup step for the Dallas DB18B20 that you are issuing too early for it, before it has initialised, and so it is ignoring it.

My guess would be steps being a unit setting, and a calibration setting.

can you attempt to wait a little time (perhaps 100 millis) before initialising the temperature control library.