Fix Ublox C030 SARA-R412M onboard cellular device serial (target default instance).

[phlegx]

Summary of changes

Whit the following mbed_app.json config example, the ONBOARD_UBLOX target is used and the serial connection to the Ublox module can work properly.

"target_overrides": {
    "*": {
        "platform.stdio-convert-newlines": true,
        "platform.stdio-baud-rate": 115200,
        "platform.default-serial-baud-rate": 115200,
        "mbed-trace.enable": true,
        "cellular.debug-at": true,
        "cellular.use-apn-lookup": true,
        "nsapi.default-cellular-apn": "\"some.provider.com\"",
        "nsapi.default-cellular-plmn": null,
        "nsapi.default-cellular-sim-pin": null,
        "nsapi.default-cellular-username": null,
        "nsapi.default-cellular-password": null,
        "lwip.ipv4-enabled": true,
        "lwip.ipv6-enabled": true,
        "lwip.tcp-enabled": true,
        "lwip.ethernet-enabled": false,
        "lwip.ppp-enabled": false,
        "ppp.ipv4-enabled": false,
        "ppp.ipv6-enabled": false
    },
    "UBLOX_C030_R412M": {
        "target.features_add": ["STORAGE"],
        "target.components_add": ["SD"],
        "target.network-default-interface-type": "CELLULAR",
        "storage.storage_type": "FILESYSTEM",
        "sd.SPI_MOSI": "SPI_MOSI",
        "sd.SPI_MISO": "SPI_MISO",
        "sd.SPI_CLK": "SPI_CLK",
        "sd.SPI_CS": "SPI_NSS"
    }
}

Bugfix on ONBOARD_UBLOX.cpp#L30 adding line serial.set_flow_control(SerialBase::RTSCTS, MDMRTS, MDMCTS);.

With the correct serial flow control, the serial connection is stable and don't get lost.

CellularDevice *CellularDevice::get_target_default_instance()
{
#if defined(TARGET_UBLOX_C030_R41XM)
    static UARTSerial serial(MDMTXD, MDMRXD, MBED_CONF_PLATFORM_DEFAULT_SERIAL_BAUD_RATE);
    serial.set_flow_control(SerialBase::RTSCTS, MDMRTS, MDMCTS); // <---------- Bugfix
    static ONBOARD_UBLOX_AT device(&serial);
#elif defined(TARGET_UBLOX_C030_N211)
...

The discussion about this bug can be found on issue #15143.

Impact of changes

Migration actions required

Documentation

None

---

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)

---

Test results

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

---

Reviewers

@RobMeades @0xc0170

---

[RobMeades]

Looks good to me.

[phlegx]

Thanks to @RobMeades for his advice and help in this issue!

[ciarmcom]

@phlegx, thank you for your changes.
@0xc0170 @RobMeades @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

Thank you both for fixing this

[phlegx]

@RobMeades you need to re-review.

[RobMeades]

Looks good to me still! Thanks for all the hard work @phlegx.

[andypowers]

Approved.

[phlegx]

Thank you all for the reviews. I'm not authorized to merge this pull request. Are any of you authorized?

[0xc0170]

Thank you all for the reviews. I'm not authorized to merge this pull request. Are any of you authorized?

I'll start CI . Once green, we can merge.

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 9 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_example-test-lts

[phlegx]

I don't know what kind of test this is and if it is related to this bugfix.

[0xc0170]

Test passed but then timeouted, not related. I restarted it

[mbed-ci]

Test run: FAILED

Summary: 1 of 9 test jobs failed
Build number : 2
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_example-test-lts

[0xc0170]

This is internal error, I'll report it to our test team

[mbed-ci]

Test run: FAILED

Summary: 1 of 9 test jobs failed
Build number : 3
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_example-test-lts

[0xc0170]

We need to review internal CI to find a root cause for this. We will get back to this PR once it's resolved

[mbed-ci]

Test run: SUCCESS

Summary: 9 of 9 test jobs passed
Build number : 4
Build artifacts

[phlegx]

Notice: could also be added like in related Ublox files of other targets:

#if DEVICE_SERIAL_FC
    if (MDMRTS != NC && MDMCTS != NC) {
        tr_debug("Modem flow control: RTS %d CTS %d", MDMRTS, MDMCTS);
        serial.set_flow_control(SerialBase::RTSCTS, MDMRTS, MDMCTS);
    }
#endif