Disable the cell AUX UART by default on EP_AGORA

[trowbridgec]

Summary of changes

Disables the auxiliary UART of the cell module on the EP_AGORA target by default. This was causing issues in the mbed-os-example-pelion example because the auxiliary UART is muxed with the default standard output UART. Therefore, if the auxiliary UART on the cell module is enabled, it has the undesired effect of receiving and attempting to parse the debug output as an AT command. When the parsing fails, it echoes back an error in the form:

<input string> + CME ERROR: unknown

In the mbed-os-example-pelion example, this would result in undesired characters being received via standard input and subsequently undesired application behavior.

So, the best path forward is to disable the auxiliary UART by default and let a user enable it if/when needed.

From the Telit ME910C1 AT command reference guide, for the AT#PORTCFG command (page 76) a Variant value of 0 enables the main cell module UART and 2 USB interfaces and does not enable the auxiliary UART (USIF1):

#PORTCFG: Variant=0: AT= USIF0 USB0 USB1
#PORTCFG: Variant=3: AT= USIF0 USIF1 USB0
#PORTCFG: Variant=8: AT= USB0 USB1
#PORTCFG: Variant=13: AT= USIF0 USB0; CMUX

Therefore, changing from a Variant value of 3 to a value of 0 should effectively disable the auxiliary UART.

NOTE: As stated in the AT command reference guide, this setting change does not take effect until the cell module is rebooted, but should only ever have to be configured once.

Impact of changes

The auxiliary UART of the cell module is disabled by default.

Migration actions required

None.

Documentation

None.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@linlingao

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[trowbridgec]

@ARMmbed/team-embeddedplanet @maclobdell

[0Grit]

@ARMmbed/team-embeddedplanet

[trowbridgec]

I pushed new changes which makes the port configuration (AT#PORTCFG) configurable via a user's mbed_app.json file (default is still 0: auxiliary UART disabled). The new setting is exposed as ep-agora.port-configuration-variant.

[linlingao]

This line is missing a "%d".
Should be _at->at_cmd_discard("#PORTCFG", "=", "%d", EP_AGORA_PORT_CONFIGURATION_VARIANT);

[trowbridgec]

Ah, you're right! Change added.

[ciarmcom]

@trowbridgec, thank you for your changes.
@linlingao @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[0xc0170]

We are investigating, CI tests are not executing, might restart CI

[0xc0170]

All good, no execution of tests as this is target is not in the CI , thus skipped that stage.