More than 32 bytes cannot be written via I2C

[arcao]

When I try to write more than 32 bytes (2B for address + >30B data) via I2C to AT24C256 EEPROM device, next written byte is NAKed. I guess it's because a new transaction is started and EEPROM device thinks that is a new (PAGE) WRITE command. AT24C256 supports up to 64 bytes to be transferred with that command.

Note: Between PAGE WRITE and any other command has to be a delay (max 5ms by datasheet) to be data physically written, so this is the reason of NAK.

Test code:

#include <Arduino.h>
#include <Wire.h>

#define PIN_SDA  21  // I2C SDA
#define PIN_SCL  22  // I2C SCL

void setup() {
  int i2cdevice = 0x50;  // 24C256 EEPROM address with A0=0, A1=0, A2=0
  int addr;
  int res;

  Serial.begin(115200);
  Serial.println("Initializing I2C");
  Wire.begin(PIN_SDA, PIN_SCL);

  addr = 0;

  Wire.beginTransmission(i2cdevice);
  Wire.write(addr >> 8);
  Wire.write(addr & 0xff);
  
  for (uint8_t i = 1; i <= 64; i++) {
    Wire.write(i);
  }
  res = Wire.endTransmission();
  if (res != 0) {
    Serial.println("Write error!");
  }

}

void loop() {}

[me-no-dev]

@arcao if you have time, you can try to play with the i2c code here and see if not sending the address again on the second iteration of the loop will help you.

[arcao]

I will try it later today. I'm not at home just now.

[kriswiner]

The 32 byte limit is a common hard coded limit used in the Wire library. You can change this in the Wire.h library somewhere but I usually just accept this and buffer data writes in 32 byte chunks to keep the code platform agnostic.

[lonerzzz]

@me-no-dev @kriswiner Does it make sense to add an API to set the Wire library to change the byte limit from a set default? Is there any known implementation where this has been done before?

[kriswiner]

Just change it yourself. 32 bytes is standard for avr arduinos. I have bumped mine up to 64, which is enough for most tasks. You can also just read/write twice!

…

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[copercini]

Fixed with I2C RC1