ensure that PTP is low after reset

[houkhouk]

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PR Description section

Description and dependencies

After power-on or a reset, it can happen that the PTP line stays high due to some parasitic capacitance on the line combined to the high impedance default configuration of GPIOs. It has been observed that this behavior can trigger interrupts that sends the µcontroller to the hardfault handler.
Forcing GPIOs to a low level during start-up is recommended.

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• [ x] Bug fix (non-breaking change which fixes an issue)

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[nicolas-rabault]

Nice catch.