[rtl872x] Fix unintentional pin changes for Serial1 RTS/CTS

[eberseth]

Problem

Starting Serial1 causes the RTS and CTS lines to be assigned as hardware handshake signals without regard to whether they are previously set for GPIO.

Solution

Check if flow control is requested on the given interface and if the pins are assigned already. Configure for handshaking only if requested; otherwise, leave pins alone.

Before:

         ___________________
RTS   __/                     ...
          ________  _________
CTS   ___/        ||          ...
          <------->
             50ms

After:

         ___________           ______
RTS   __/           \_________/       ...
          ___________           _____
CTS   ___/           \_________/      ...
          <---------><--------->
              150ms      100ms

Steps to Reproduce

#define SERIAL          (Serial1)
#define SERIAL_RTS      (RTS)
#define SERIAL_CTS      (CTS)

// #define SERIAL          (Serial2)
// #define SERIAL_RTS      (RTS1)
// #define SERIAL_CTS      (CTS1)

// #define SERIAL          (Serial3)
// #define SERIAL_RTS      (RTS2)
// #define SERIAL_CTS      (CTS2)

void setup() {
  Serial.begin(115200);
  waitFor(Serial.isConnected, 10000);  // Wait until we're ready to capture waveforms

  pinMode(SERIAL_RTS, OUTPUT);
  digitalWrite(SERIAL_RTS, HIGH);
  pinMode(SERIAL_CTS, OUTPUT);
  digitalWrite(SERIAL_CTS, HIGH);
  delay(50);
  SERIAL.begin(115200);
  //SERIAL.begin(115200, SERIAL_FLOW_CONTROL_RTS_CTS);
  delay(100);
  digitalWrite(SERIAL_RTS, LOW);
  digitalWrite(SERIAL_CTS, LOW);
  delay(100);
  digitalWrite(SERIAL_RTS, HIGH);
  digitalWrite(SERIAL_CTS, HIGH);
}

References

sc-118986

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)