I believe I’ve found the pin definition file location I’d need to add an entry to -
_packages_\packages\arduino\hardware\samd\1.8.6\variants<board> where in this case board is <arduino_zero> . This is the contents of the variant.h file which looks to be the full hardware pinmap for the Arduino Zero implementation of the Cortex SAMD21.
So - one step closer, I think. Hoping someone knows more about this than I do 
/*
Copyright © 2014-2015 Arduino LLC. All right reserved.
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*/
#ifndef VARIANT_ARDUINO_ZERO
#define VARIANT_ARDUINO_ZERO
// The definitions here needs a SAMD core >=1.6.10
#define ARDUINO_SAMD_VARIANT_COMPLIANCE 10610
/*----------------------------------------------------------------------------
----------------------------------------------------------------------------/
/** Frequency of the board main oscillator */
#define VARIANT_MAINOSC (32768ul)
/** Master clock frequency */
#define VARIANT_MCK (48000000ul)
/*----------------------------------------------------------------------------
----------------------------------------------------------------------------/
#include “WVariant.h”
#ifdef __cplusplus
#include “SERCOM.h”
#include “Uart.h”
#endif // __cplusplus
#ifdef __cplusplus
extern “C”
{
#endif // __cplusplus
/*----------------------------------------------------------------------------
----------------------------------------------------------------------------/
// Number of pins defined in PinDescription array
#ifdef __cplusplus
extern “C” unsigned int PINCOUNT_fn();
#endif
#define PINS_COUNT (PINCOUNT_fn())
#define NUM_DIGITAL_PINS (20u)
#define NUM_ANALOG_INPUTS (6u)
#define NUM_ANALOG_OUTPUTS (1u)
#define analogInputToDigitalPin§ ((p < 6u) ? § + 14u : -1)
#define digitalPinToPort§ ( &(PORT->Group[g_APinDescription[P].ulPort]) )
#define digitalPinToBitMask§ ( 1 << g_APinDescription[P].ulPin )
//#define analogInPinToBit§ ( )
#define portOutputRegister(port) ( &(port->OUT.reg) )
#define portInputRegister(port) ( &(port->IN.reg) )
#define portModeRegister(port) ( &(port->DIR.reg) )
#define digitalPinHasPWM§ ( g_APinDescription[P].ulPWMChannel != NOT_ON_PWM || g_APinDescription[P].ulTCChannel != NOT_ON_TIMER )
/*
- digitalPinToTimer(…) is AVR-specific and is not defined for SAMD
- architecture. If you need to check if a pin supports PWM you must
- use digitalPinHasPWM(…).
-
-
https://github.com/arduino/Arduino/issues/1833
*/
// #define digitalPinToTimer§
// LEDs
#define PIN_LED_13 (13u)
#define PIN_LED_RXL (25u)
#define PIN_LED_TXL (26u)
#define PIN_LED PIN_LED_13
#define PIN_LED2 PIN_LED_RXL
#define PIN_LED3 PIN_LED_TXL
#define LED_BUILTIN PIN_LED_13
/*
- Analog pins
*/
#define PIN_A0 (14ul)
#define PIN_A1 (15ul)
#define PIN_A2 (16ul)
#define PIN_A3 (17ul)
#define PIN_A4 (18ul)
#define PIN_A5 (19ul)
#define PIN_DAC0 (14ul)
static const uint8_t A0 = PIN_A0;
static const uint8_t A1 = PIN_A1;
static const uint8_t A2 = PIN_A2;
static const uint8_t A3 = PIN_A3;
static const uint8_t A4 = PIN_A4;
static const uint8_t A5 = PIN_A5;
static const uint8_t DAC0 = PIN_DAC0;
#define ADC_RESOLUTION 12
// Other pins
#define PIN_ATN (38ul)
static const uint8_t ATN = PIN_ATN;
/*
- Serial interfaces
*/
// Serial (EDBG)
#define PIN_SERIAL_RX (31ul)
#define PIN_SERIAL_TX (30ul)
#define PAD_SERIAL_TX (UART_TX_PAD_2)
#define PAD_SERIAL_RX (SERCOM_RX_PAD_3)
// Serial1
#define PIN_SERIAL1_RX (0ul)
#define PIN_SERIAL1_TX (1ul)
#define PAD_SERIAL1_TX (UART_TX_PAD_2)
#define PAD_SERIAL1_RX (SERCOM_RX_PAD_3)
/*
- SPI Interfaces
*/
#define SPI_INTERFACES_COUNT 1
#define PIN_SPI_MISO (22u)
#define PIN_SPI_MOSI (23u)
#define PIN_SPI_SCK (24u)
#define PERIPH_SPI sercom4
#define PAD_SPI_TX SPI_PAD_2_SCK_3
#define PAD_SPI_RX SERCOM_RX_PAD_0
static const uint8_t SS = PIN_A2 ; // SERCOM4 last PAD is present on A2 but HW SS isn’t used. Set here only for reference.
static const uint8_t MOSI = PIN_SPI_MOSI ;
static const uint8_t MISO = PIN_SPI_MISO ;
static const uint8_t SCK = PIN_SPI_SCK ;
/*
- Wire Interfaces
*/
#define WIRE_INTERFACES_COUNT 1
#define PIN_WIRE_SDA (20u)
#define PIN_WIRE_SCL (21u)
#define PERIPH_WIRE sercom3
#define WIRE_IT_HANDLER SERCOM3_Handler
static const uint8_t SDA = PIN_WIRE_SDA;
static const uint8_t SCL = PIN_WIRE_SCL;
/*
- USB
*/
#define PIN_USB_HOST_ENABLE (27ul)
#define PIN_USB_DM (28ul)
#define PIN_USB_DP (29ul)
/*
- I2S Interfaces
*/
#define I2S_INTERFACES_COUNT 1
#define I2S_DEVICE 0
#define I2S_CLOCK_GENERATOR 3
#define PIN_I2S_SD (9u)
#define PIN_I2S_SCK (1u)
#define PIN_I2S_FS (0u)
#ifdef __cplusplus
}
#endif
/*----------------------------------------------------------------------------
----------------------------------------------------------------------------/
#ifdef __cplusplus
/* =========================
- ===== SERCOM DEFINITION
- =========================
*/
extern SERCOM sercom0;
extern SERCOM sercom1;
extern SERCOM sercom2;
extern SERCOM sercom3;
extern SERCOM sercom4;
extern SERCOM sercom5;
extern Uart Serial;
extern Uart Serial1;
#endif
#ifdef __cplusplus
extern “C” {
#endif
unsigned int PINCOUNT_fn();
#ifdef __cplusplus
}
#endif
// These serial port names are intended to allow libraries and architecture-neutral
// sketches to automatically default to the correct port name for a particular type
// of use. For example, a GPS module would normally connect to SERIAL_PORT_HARDWARE_OPEN,
// the first hardware serial port whose RX/TX pins are not dedicated to another use.
//
// SERIAL_PORT_MONITOR Port which normally prints to the Arduino Serial Monitor
//
// SERIAL_PORT_USBVIRTUAL Port which is USB virtual serial
//
// SERIAL_PORT_LINUXBRIDGE Port which connects to a Linux system via Bridge library
//
// SERIAL_PORT_HARDWARE Hardware serial port, physical RX & TX pins.
//
// SERIAL_PORT_HARDWARE_OPEN Hardware serial ports which are open for use. Their RX & TX
// pins are NOT connected to anything by default.
#define SERIAL_PORT_USBVIRTUAL SerialUSB
#define SERIAL_PORT_MONITOR Serial
// Serial has no physical pins broken out, so it’s not listed as HARDWARE port
#define SERIAL_PORT_HARDWARE Serial1
#define SERIAL_PORT_HARDWARE_OPEN Serial1
#endif /* VARIANT_ARDUINO_ZERO */