CartCom/Arduino/ReceiveCart/ReceiveCart.ino

#include <Wire.h>
#include <inttypes.h>

static const int eeprom_addr = 0b1010000;

// Send a two bytes address via I²C which selects an address within the EEPROM.
void write_addr(uint16_t addr) {
  Wire.write((uint8_t)(addr >> 8));
  Wire.write((uint8_t)addr);
}

void write_page(uint8_t page[128], uint16_t page_addr) {
  uint8_t bytes_written = 0;
  for (int i = 0; i < 5; i++) {
    int ret;
    int write_bytes = min(128 - bytes_written, 30);
    do {
      Wire.beginTransmission(eeprom_addr);
      write_addr(page_addr + bytes_written);
      Wire.write(page + bytes_written, write_bytes);
      ret = Wire.endTransmission();
      delay(5);
    } while (ret != 0);
    bytes_written += write_bytes;
  }
}

bool check_sumcomplement(uint8_t data[128], uint16_t checksum) {
        uint16_t sum = 0;
        for (int i = 0; i<128; i += 2) {
          sum += (data[i+1] << 8) | data[i];
        }
        sum += checksum;
        return !sum;
}

volatile uint8_t pageData[128] = {};
volatile uint16_t pageChecksum = 0;
volatile uint16_t pageAddr = 0;
volatile bool pageOk = false;
volatile bool reicivingCart = false;
volatile uint32_t last_page_update = 0;

void setup() {
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);
  Wire.begin();
  Wire.setClock(400000);
}

void loop() {
  if (reicivingCart && millis() - last_page_update > 1000) {
    Serial.print("TO");
    reicivingCart = false;
    pageOk = false;
    pageAddr = 0;
    digitalWrite(LED_BUILTIN, HIGH);
    delay(500);
    digitalWrite(LED_BUILTIN, LOW);
    delay(500);
    digitalWrite(LED_BUILTIN, HIGH);
    delay(500);
    digitalWrite(LED_BUILTIN, LOW);
  }

  if (pageOk) {
    pageOk = false;
    bool pageValid = check_sumcomplement((uint8_t *)pageData, pageChecksum);
    Serial.print(pageValid ? "ACK" : "NAK");
    if (pageValid) {
      write_page(pageData, pageAddr);
      pageAddr += 128;
    }

 /*  for (int i = 0; i<128; i++) {
      if (i%16 == 0)
        Serial.println();
      Serial.print(pageData[i], HEX);
      Serial.print(" ");
    }
    Serial.println();
    Serial.println(pageChecksum, HEX);*/
  
    Serial.print("OK");
  }
}

void serialEvent() {
  static size_t read = 0;
  digitalWrite(LED_BUILTIN, HIGH);
  if (Serial.available() == 5) {
    char control[4] = {};
    Serial.readBytesUntil('\n', control, 4);
    Serial.read(); /* Drop the '\n' */
    if (!strncmp(control, "CART", 4)) {
      reicivingCart = true;
      Serial.print("OK");
    } else if (!strncmp(control, "DONE", 4)) {
      reicivingCart = false;
    } else {
      Serial.print("KO");
    }
    pageOk = false;
    pageAddr = 0;
    read = 0;
  }

  while (reicivingCart && Serial.available() && read < 128) {
    read += Serial.readBytes((char *)pageData+read, min(128-read, 64));
  }
  if (reicivingCart && read >= 128) {
    pageOk = true;
    Serial.readBytes((char*)&pageChecksum, 2);
    read = 0;
  }
  digitalWrite(LED_BUILTIN, LOW);
  last_page_update = millis();
}