Battery internal resistance goes way up at low temperature. The exact
temperature depends on the type of battery. I assume you are using some
type of lithium primary battery if you are at -29C. I had to use lithium
thionyl to get reliable performance below -40C.

Bob



On Mon, May 21, 2012, at 07:20 PM, zipwize@comcast.net wrote:
> I have a product based on the PIC18F4580. It works well with RS-232,
> RS-485 and SDI-12. But I have one application at 4000 meters of altitude
> where the EEPROM settings are getting corrupted at unusual times. Once
> the EEPROM is wiped, I loose contact with the unit. There is a 5V LDO
> regulator. This application has a radio with 1/10W output. The case is
> just aluminum. The antenna is a 3db omni that is about 5cm from the PC
> board. The ambient temperature did not drop below -29C this past winter.
> The PIC18F4580 is rated for -40. I can't believe the radio would cause
> this problem. Out of 40 stations, 15 were made non-functional because the
> address, baud rate, communication mode are all stored in EEPROM. Power is
> turned on every hour for 40 seconds by a data logger with RTCC. I have
> hundreds of these units in remote areas using RS-232 with no problems. I
> use the same UART for all communication modes. I use the same command
> parsing and EEPROM Read/Write for all modes. I just enable!
>   the appropriate IC for the mode. The 40 stations all use the SDI-12
>   communication. I ran it in the office for weeks without issue.=20
>=20
> I would like to hear about any theories on why this is happening.=20

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