[gen2][electron] modem brownout or unexpected reset causes system to think it still is connected [ch64465]

[technobly]

Problem

Electron was found to suffer from a modem reset while antenna was disconnected in low signal environment on battery only (and also with USB and battery). This leads to C*REG=2 verbose mode + URCS being reset to C*REG=0 non-verbose mode + no URCs, which causes the system firmware to not be notified that the modem has deregistered. NO_ACK publishes can continue to be sent into the breeze, and a user's firmware will not know Particle.connected() or Cellular.ready() are false.

#2215 addresses the root cause of the problem, but just in case for unexpected reasons the following solution offers a recovery method.

Solution

To periodically poll for C*REG? every 30 seconds look for changes to CREG verbosity. If there is a valid response, and we see C*REG: 0,x, URCs are disabled. Notify the system of disconnect immediately, which will reinitialize the modem.

Steps to Test

Run unit tests: TEST=wiring/no_fixture_long_running CELLULAR_03 & CELLULAR_04

References

ch64465
#2215 addresses the root cause of the problem
ch63632

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• N/A Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)