Nuvoton: Fix UART flow control CTS/RTS

[ccli8]

Description

This PR tries to enhance the case in which RTS pin output would drive to low when RTS is disabled. This allows the peer to be able to send data even though its corresponding CTS is still enabled.

Related targets

• NUMAKER_PFM_NANO130
• NUMAKER_PFM_NUC472
• NUMAKER_PFM_M453
• NUMAKER_PFM_M487/NUMAKER_IOT_M487
• NUMAKER_PFM_M2351

Pull request type

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

[ciarmcom]

@ccli8, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[cmonr]

Interesting. LGTM

[cmonr]

Waaait. Asking a question.

[cmonr]

RTS pin output would drive to low when RTS is disabled.

Is there something amis with the way Nuvoton's hw flow control works that it needs a software bypass? I'm not sure why this fix is needed.

[ccli8]

@cmonr There're two reasons for this change:

1. When RTS is first configured and configured to disabled, original control flow doesn't do RTS-related pin configuration. So RTS output pin status is dependent on previous pin configuration.
2. When RTS is configured to disabled from enabled, Nuvoton's H/W would drive RTS output pin to low by default. But this default behavior may be changed elsewhere. So guarantee it by manually driving RTS output pin to low (except NANO130 which doesn't support this).

[cmonr]

CI started

[mbed-ci]

Test run: SUCCESS

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