Allow USB CDC Serial to be disabled, redirect to UART

[h2zero]

This will set the default Serial output to UART if USB CDC is disabled by setting USB_CDC_DEFAULT_SERIAL to 0.

If the board has Serial1 pins defined they will be redefined to Serial.

[Copilot]

Pull request overview

This PR makes USB_CDC_DEFAULT_SERIAL overridable so that when USB CDC is disabled (USB_CDC_DEFAULT_SERIAL=0), Serial is redirected to the UART hardware port (using the board’s Serial1 pins where applicable).

Changes:

• Make USB_CDC_DEFAULT_SERIAL in several nRF52840 variants conditional (#ifndef ... #define ...) to allow build-time overrides.
• Replace #ifdef/#ifndef USB_CDC_DEFAULT_SERIAL with value-based #if USB_CDC_DEFAULT_SERIAL checks in core utilities.
• Introduce UART pin remapping logic intended to convert Serial1 pins into Serial pins when USB CDC is not the default.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
variants/nRF52840_Dongle/variant.h	Make USB_CDC_DEFAULT_SERIAL override-friendly while defaulting to 1.
variants/itsybitsy_nrf52840_express/variant.h	Same: allow override, default to USB CDC serial.
variants/feather_nrf52840_sense/variant.h	Same: allow override, default to USB CDC serial.
variants/feather_nrf52840_express/variant.h	Same: allow override, default to USB CDC serial.
variants/clue_nrf52840/variant.h	Same: allow override, default to USB CDC serial.
variants/circuitplayground_nrf52840/variant.h	Same: allow override, default to USB CDC serial.
variants/Seeed_XIAO_nRF52840_Sense/variant.h	Same: allow override, default to USB CDC serial.
cores/nRF5/utils/debug_utils.cpp	Use value-based checks so USB_CDC_DEFAULT_SERIAL=0 behaves correctly.
cores/nRF5/libc/printf/putchar.cpp	Gate Serial.flush() based on the macro’s value (USB CDC vs UART).
cores/nRF5/Uart.h	Switch to value-based macro logic and add pin remapping for UART-as-Serial.
cores/nRF5/Uart.cpp	Instantiate Serial UART object when USB_CDC_DEFAULT_SERIAL is 0.
cores/nRF5/Arduino.h	Ensure variant.h is included early and provide a fallback default for the macro.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 1 comment.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 3 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 16 out of 16 changed files in this pull request and generated 2 comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 17 out of 17 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

cores/nRF5/TinyUSB/Adafruit_TinyUSB_ArduinoCore/Adafruit_USBD_CDC.h:102

• SerialTinyUSB is conditionally macro-mapped to Serial based on USB_CDC_DEFAULT_SERIAL. Since USB_CDC_DEFAULT_SERIAL may not be defined yet in some translation units (e.g., files that include Adafruit_USBD_CDC.h before Arduino.h/variant.h), this can produce inconsistent symbol names (Serial vs SerialTinyUSB) across TUs and lead to link errors. Please ensure USB_CDC_DEFAULT_SERIAL is defined before including this header everywhere it’s used (e.g., include Arduino.h earlier in those TUs or centralize the macro definition in a common config header included before Adafruit_USBD_CDC.h).

// Built-in support "Serial" is assigned to TinyUSB CDC
// CH32 defines Serial as alias in WSerial.h
#if defined(USE_TINYUSB) && !defined(ARDUINO_ARCH_CH32) && USB_CDC_DEFAULT_SERIAL
#define SerialTinyUSB Serial
#endif

extern Adafruit_USBD_CDC SerialTinyUSB;

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.