NFC entry and exit on an Arduino Uno

// Two NFC readers on one I2C bus, through a TCA9548A.
//
// Every PN532 answers at 0x24 on I2C, and the address cannot be changed. Two
// of them on one bus would both answer every byte. A TCA9548A fixes that: it
// sits at 0x70, and behind it are eight more buses it joins to the main one
// when told to. Here the entry reader is on channel 0 and the exit reader on
// channel 1, and the sketch selects one before it talks to it.
//
//   TCA9548A  SDA, SCL -> SDA, SCL (A4, A5)   VIN -> 5V, GND -> GND
//   entry     SDA, SCL -> SD0, SC0            IRQ -> pin 2
//   exit      SDA, SCL -> SD1, SC1            IRQ -> pin 3
//   both readers: VCC -> 5V, GND -> GND, their switches set to I2C
//
// Press Run, then tap a card on the entry reader: it is signed in. Tap the
// same card on the exit reader and it is signed out. The serial monitor keeps
// the count. The mux's channel lights show which reader the Uno is talking to.
//
// THINGS TO TRY. Move the exit reader's SDA and SCL to SD0 and SC0 beside the
// entry reader's: channel 1 is empty, the sketch says so and stops, and the
// two readers left on channel 0 answer every byte together. Or change
// EXIT_CHANNEL to 2 and move its wires to SD2 and SC2.

#include <Wire.h>
#include <Adafruit_PN532.h>

const uint8_t MUX = 0x70;
const uint8_t ENTRY_CHANNEL = 0;
const uint8_t EXIT_CHANNEL = 1;

// I2C readers: the IRQ pin, and no reset pin (the board pulls RST up).
Adafruit_PN532 entry(2, -1);
Adafruit_PN532 exitReader(3, -1);

// The cards that are in, by UID, and the card on each reader now.
const uint8_t MAX_IN = 4;
uint8_t inside[MAX_IN][7];
uint8_t insideLength[MAX_IN];
uint8_t onEntry[7], onEntryLength = 0;
uint8_t onExit[7], onExitLength = 0;

// One byte to the TCA9548A: bit n joins channel n to the bus.
void selectChannel(uint8_t channel)
{
    Wire.beginTransmission(MUX);
    Wire.write(1 << channel);
    Wire.endTransmission();
}

bool startReader(Adafruit_PN532 &reader, uint8_t channel, const char *name)
{
    selectChannel(channel);
    reader.begin();
    uint32_t version = reader.getFirmwareVersion();
    Serial.print(name);
    if (!version) {
        Serial.print(": no PN532 on channel ");
        Serial.println(channel);
        return false;
    }
    Serial.print(": PN5");
    Serial.print((version >> 24) & 0xFF, HEX);
    Serial.print(" firmware ");
    Serial.print((version >> 16) & 0xFF);
    Serial.print('.');
    Serial.print((version >> 8) & 0xFF);
    Serial.print(" on channel ");
    Serial.println(channel);
    reader.SAMConfig();
    // Look for a card twice, about 30 ms, then answer "none" rather than wait
    // for ever: the other reader needs its turn.
    reader.setPassiveActivationRetries(1);
    return true;
}

void printUid(const uint8_t *uid, uint8_t length)
{
    for (uint8_t i = 0; i < length; i++) {
        if (uid[i] < 0x10) Serial.print('0');
        Serial.print(uid[i], HEX);
        if (i + 1 < length) Serial.print(' ');
    }
}

int findInside(const uint8_t *uid, uint8_t length)
{
    for (uint8_t i = 0; i < MAX_IN; i++) {
        if (insideLength[i] == length && memcmp(inside[i], uid, length) == 0) return i;
    }
    return -1;
}

uint8_t countInside()
{
    uint8_t n = 0;
    for (uint8_t i = 0; i < MAX_IN; i++) {
        if (insideLength[i]) n++;
    }
    return n;
}

// Poll one reader. True, with the UID, when a card has just been laid on it;
// a card that stays on the reader is reported once.
bool newCard(Adafruit_PN532 &reader, uint8_t channel, uint8_t *last, uint8_t &lastLength, uint8_t *uid, uint8_t &length)
{
    selectChannel(channel);
    if (!reader.readPassiveTargetID(PN532_MIFARE_ISO14443A, uid, &length, 100)) {
        lastLength = 0;
        return false;
    }
    if (length == lastLength && memcmp(uid, last, length) == 0) return false;
    memcpy(last, uid, length);
    lastLength = length;
    return true;
}

void setup()
{
    Serial.begin(9600);
    Wire.begin();
    Serial.println("Entry and exit readers");
    bool ok = startReader(entry, ENTRY_CHANNEL, "Entry");
    ok = startReader(exitReader, EXIT_CHANNEL, "Exit") && ok;
    if (!ok) {
        while (1) {
        }
    }
    Serial.println("Tap a card on a reader.");
}

void loop()
{
    uint8_t uid[7];
    uint8_t length;

    if (newCard(entry, ENTRY_CHANNEL, onEntry, onEntryLength, uid, length)) {
        printUid(uid, length);
        if (findInside(uid, length) >= 0) {
            Serial.print(" is already in");
        } else {
            int slot = findInside(uid, 0);
            if (slot < 0) {
                Serial.print(" cannot come in, the list is full");
            } else {
                memcpy(inside[slot], uid, length);
                insideLength[slot] = length;
                Serial.print(" signed in");
            }
        }
        Serial.print(", ");
        Serial.print(countInside());
        Serial.println(" inside");
    }

    if (newCard(exitReader, EXIT_CHANNEL, onExit, onExitLength, uid, length)) {
        printUid(uid, length);
        int at = findInside(uid, length);
        if (at < 0) {
            Serial.print(" was not signed in");
        } else {
            insideLength[at] = 0;
            Serial.print(" signed out");
        }
        Serial.print(", ");
        Serial.print(countInside());
        Serial.println(" inside");
    }
}