Why S2LP does not work at low temperatures?

[AKouz]

I use S2LP at 433 MHz with proprietary narrowband protocol. Crystal 25 MHz 10 ppm XRCGB25M000F1H02R0 from Murata; capacitors 18 pF.

Devices operate OK if ambient temperature is positive. When I put them into freezer they stop responding to the base station. Devices are temperature dataloggers. While being in freezer, they continue to log temperatures, and we can see that microcontroller works OK down to -17 C. But S2LP stops responding at temperatures around 0 C.

I suspect that S2LP oscillator does not start if it is cold. Did anyone experienced similar problems?

[AKouz]

The following changes made in a test unit:

• Crystal capacitors changed to 12 pF
• 2 ms delay after software reset command SRES
• Increased waiting interval after command LOCKRX
• As mentioned before, SRES and full re-intialisation every minute or after any error detected, such as incorrect S2LP state or time-out during state transition

Test unit with temperature sensor glued on top of S2LP now works in freezer as expected. Measured S2LP temperature -16.5C.

Probably the extra waiting interval after LOCKRX is not relevant, it was already much greater than required by the datasheet.

[AKouz]

Out of 6 devices tested in a freezer at -18C, 4 passed test, but 2 failed. The failed devices operated down to -14C, but below that communication was lost.

The failed devices did not restore correct operation at room temperature. S2LP oscillator did not work, despite of SRES and re-initialisation every few seconds. Only hardware reset by SHDN pin restored normal operation.

[AKouz]

At last I've got them working at -20C without hardware reset. Apparently most important was to use SRES command only if S2LP error detected or re-init required, and then make a full re-init. Previously I used SRES also if a wrong state detected. In addition I slowed down SPI speed fro 4 Mbps to 2 Mbps and added delays after state changes.

Thus, S2LP at low temperatures can make errors, and it is important to reset S2LP properly with 2 ms delay after reset. Hardware reset is preferred.