Remove assertion of MDMRST in case of SARA-R4

[wajahat-abbas]

Description

In case of SARA-R4, MDMRST needs to be asserted for 10 seconds before modem actually powers down. This means that modem is initially responsive to AT commands but powers down after 10 seconds unless MDMRST is de-asserted (a call to onboard_modem_init() ). This can create confusion for application as CellularDevice::is_ready() will return TRUE initially and ten seconds later modem will power off without any indication to the application.

This can also cause test cases to fail that rely on system_reset() for MCU reboot. In such case, Cellular state machine will get an OK response from CellularDevice::is_ready() so it will not call the power_on() methods and resume its states however after 10 seconds, modem will power down possibly leaving state machine in an undesired state or leading to failure of test case.

We have evaluated others options to fix this problem and have decided not to assert MDMRST in our start-up code. Other options require us to wait 10 seconds at each reset which is not reasonable.

Pull request type

[ ] Fix
[ ] Refactor
[X] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

Release Notes

[ciarmcom]

@wajahat-ublox, thank you for your changes.
@screamerbg @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

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[mbed-ci]

Test run: SUCCESS

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Build number : 1
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