CAUTION: Bricked XBee3 Cellular LTE-M/NB-IoT

I think it has been mentioned on other posts briefly, but if you are using a XBee3 Cellular LTE-M/NB-IoT modem make sure it is updated to the newest cellular firmware immediately ( A.02.19 firmware).  A known issue on the early modem revisions was discovered which leads to corrupted memory in the ublox modem portion of the unit and they become “bricked” and unusable and they can not be recovered from this state. The newest firmware makes modems “less susceptible” to this issue, and new units “shipped after May 15, 2019 should contain u-blox firmware app version A.02.01 and do not need to be updated.”

https://www.digi.com/support/knowledge-base/u-blox-memory-housekeeping-issue-on-xb3-cat-m-cell

Modems which power cycle often are more susceptible to this issue than modems which remain on all or most of the time.  Additionally, Digi recommended that modems which cycle on and off a lot be shut down using their “clean shutdown” procedure which issues the “SD” AT command and returns OK before power can be removed from the modem. I think this is a different procedure and desired outcome than placing the modem into airplane mode and then using PinSleep to put it to sleep, but I might be wrong about that.

https://www.digi.com/resources/documentation/Digidocs/90002258/#reference/r_clean_shutdown.htm

My modem is a XB3-C-A2-UT-001 Revision D.  After I installed the Xbee power adapter I didn’t have any further issues with the modem hanging and freezing the Mayfly, which I think was related to the known power/brownout issue. There have been a few missed data points however these are surely due to the poor signal as it is deployed in a very remote location pretty far from any towers.  It has worked very reliably for over a year with a Verizon sim and phone plan through RevXSystems that I purchased at DigiKey.

My logger stopped uploading data on January 28th and I assumed it was related to the ThingSpeak server migration which was mentioned in another post. I discovered that the problem was actually the corrupted modem memory, but it could have been a combination of the two. Interestingly, my SD card was also corrupted when I went to check on the saved file.  Digi let me know that my modem was not salvageable and that it has to be replaced, however there is no stock available at any vendor. I will be trying the new LTEBee modems, but I hope this information helps anybody interested. It is very important to check in on the Digi website for any new issues or firmware updates and make sure you integrate that into your site maintenance schedule.