When integrating NFC functionality into ESP32 projects, pin mapping becomes a major pain point across different board variants. Here's how I solved it with a single, universal firmware approach. For my embedded project, I needed NFC card functionality for WiFi onboarding and user interactions.

Adafruit's arduino library example sets up the NFC reader in following manner:

#include <Wire.h>
// Using the SPI breakout, define the pins for SPI communication.
#define PN532_SCK  (2)
#define PN532_MOSI (3)
#define PN532_SS   (4)
#define PN532_MISO (5)
Adafruit_PN532 nfc(PN532_SCK, PN532_MISO, PN532_MOSI, PN532_SS);

These pins are placeholders and they would vary for each ESP32 variant and I had several: ESP-WROOM-32, ESP32-C3 Super Mini, S3, XIAO ESP32S3

I tested the reader across all these variants, different modes (SPI, I2C) wiring and reading different NFC cards.

First challenge was to figure out the mapping of pins. espressif's arduino support is the best resource. For each board the file pins_arduino.h gives us the right pins. For the ESP32-C3 Super Mini board, the above example NFC reader code would become:

#include <Wire.h>
#define PN532_SCK  (4)
#define PN532_MOSI (6)
#define PN532_SS   (7)
#define PN532_MISO (5)
Adafruit_PN532 nfc(PN532_SCK, PN532_MISO, PN532_MOSI, PN532_SS);

But as I iterated through these ESP32 variants it became quite a pain adjusting those pins in the firmware and flashing it. Eventually I realized what would simplify this: I could have a single firmware that will work with everything:

#include <Wire.h>
Adafruit_PN532 nfc(SCK, MISO, MOSI, SS);

These pins, SCK, MISO are just like another constant: LED_BUILTIN, each Arduino board that doesn't use default pin 13, defines it in their pins_arduino.h header. Similarly all the variants have their own respective pins_arduino.h file with right pins. As I select the board inside Arduino IDE, these variables are always available. Further this pattern isn't limited to NFC readers - any SPI peripheral can benefit from this approach.