Implement LED_BUILTIN constant + digitalWrite() overload for RGB LED

[PilnyTomas]

Related area

RGB LED, Blink example

Hardware specification

C3, S3, dev boards with RGB LED

Is your feature request related to a problem?

No issue found

Describe the solution you'd like

Implement LED_BUILTIN constant for dev boards with RGB LED. Where not applicable print a helpful error message - for example, This board does not have builtin LED instead of default error: 'LED_BUILTIN' was not declared in this scope
For boards with RGB LED overload function void digitalWrite(uint8_t pin, uint8_t val) to activate the RGB LED via the appropriate driver.
Sample use of overloaded function:

digitalWrite(LED_BUILTIN, HIGH); // RGB turns full white
digitalWrite(LED_BUILTIN, LOW); // RGB turns off

// Nice to have (1) - ability to control each channel
digitalWrite(RED_LED_BUILTIN, HIGH); // Red channel full brightness
digitalWrite(GREEN_LED_BUILTIN, HIGH); // Green channel full brightness
digitalWrite(BLUE_LED_BUILTIN, HIGH); // Blue channel full brightness
// Similar for LOW -> channel is off

// Nice to have (2) - ability to control brightness
digitalWrite(RED_LED_BUILTIN, 128); // Red channel turns on to 50% brightness

Describe alternatives you've considered

No response

Additional context

No response

I have checked existing list of Feature requests and the Contribution Guide

• I confirm I have checked existing list of Feature requests and Contribution Guide.

[igrr]

Regarding

// Nice to have (2) - ability to control brightness
digitalWrite(RED_LED_BUILTIN, 128); // Red channel turns on to 50% brightness

TBH it looks a bit odd from Arduino API perspective. How about using analogWrite for this purpose, instead? For a normal GPIO connected to an LED, analogWrite(LED_BUILTIN, value) can be used to control LED brightness by means of a PWM output.

[me-no-dev]

This feature is much in line with defining a way for boards to declare what they have on-board. Things like LED, SD/SDMMC, LCD, etc. Also Peripheral Manager :)
Declaring something like (in pins_arduino.h):

#define BOARD_HAS_WS2802
#define LED_BUILTIN 18 // maybe we need to add offset so to not prevent using the pin for normal digitalWrite

Then:

void digitalWrite(uint8_t pin, bool level){
#ifdef BOARD_HAS_WS2802
  if(pin == LED_BUILTIN){
    //use RMT to set all channels on/off
    return;
  }
#endif
  // regular digitalWrite logic
}

[me-no-dev]

that COLOR_LED_BUILTIN idea look odd overall... how would you define the colors? If the user knows they have RGB, then they can use another API to set the color IMHO.

[igrr]

I do agree with @me-no-dev that this should be implemented only to provide compatibility with sketches/libraries which use LED_BUILTIN.

More complex functionality such as setting the color could be handled using a dedicated library (like NeoPixel).

[PilnyTomas]

Ok, we can keep it minimalistic and provide only the basic compatibility.