How to send an array of unknown size via I2C (from slave to master)?

Question

When I try something like this, everything works.

Slave:

 # define DEFAULT_DATA_SIZE 256
 
 byte DATA[DEFAULT_DATA_SIZE];
 
 void requestEvent()
 {
 Wire.write( (byte*) DATA, sizeof(DATA));
 }
 
 void receiveEvent(int numBytes)
 {
 for (int i = 0; i < numBytes; i++) {
 DATA[i] = Wire.read();
 }
 }

Master:

 # define DEFAULT_DATA_SIZE 5
 void writeData(byte data[], int numBytes) {
 Wire.beginTransmission(I2C_SLAVE_ADDRESS);
 for (int i = 0; i < numBytes; i++)
 {
 Wire.write(data[i]);
 }
 Wire.endTransmission();
 }
 void readData() {
 byte data[DEFAULT_DATA_SIZE];
 Wire.requestFrom( I2C_SLAVE_ADDRESS, DEFAULT_DATA_SIZE );
 if ( Wire.available() == sizeof( data))
 {
 Wire.readBytes( (byte *) data, DEFAULT_DATA_SIZE);
 }
 Serial.print("Byte Array: ");
 for (int i = 0; i < DEFAULT_DATA_SIZE; i++ ) {
 Serial.print(data[i]);
 Serial.print(" ");
 }
 Serial.println();
 }
 byte data[5] = {0x1, 0x2, 0x3, 0x4, 0x5 };
 
 void setup()
 {
 Wire.begin();
 Serial.begin(9600); 
 
 writeData(data, sizeof(data));
 delay(500);
 readData();
 }
 
 void loop() {
 
 }

But this only works when the master knows how long the array needs to be. If I set the value of DEFAULT_DATA_SIZE not to 5, but more, then I get a set of random numbers.

I tried to make the slave pass the length of the array first, and then the array itself. But for some reason it doesn't work and I also get random numbers.

Slave:

 #include <Wire.h>
 # define I2C_SLAVE_ADDRESS 12
 # define DEFAULT_DATA_SIZE 256
 
 byte DATA[DEFAULT_DATA_SIZE];
 int RECEIVED_NUMBYTES = 0;
 bool isNumBytesSended = false;
 
 void setup()
 {
 Wire.begin(I2C_SLAVE_ADDRESS);
 delay(1000);
 Wire.onRequest(requestEvent);
 Wire.onReceive(receiveEvent);
 }
 
 void loop()
 {
 }
 
 void requestEvent()
 {
 if (!isNumBytesSended) {
 isNumBytesSended = true;
 Wire.write(RECEIVED_NUMBYTES); 
 }
 else {
 isNumBytesSended = false;
 Wire.write( (byte*) DATA, sizeof(DATA)); 
 }
 }
 
 void receiveEvent(int numBytes)
 {
 RECEIVED_NUMBYTES = numBytes;
 for (int i = 0; i < numBytes; i++) {
 DATA[i] = Wire.read();
 }
 }

Master:

 #include <Wire.h>
 # define I2C_SLAVE_ADDRESS 12
 # define DEFAULT_DATA_SIZE 10
 
 
 void writeData(byte data[], int numBytes) {
 Wire.beginTransmission(I2C_SLAVE_ADDRESS);
 for (int i = 0; i < numBytes; i++)
 {
 Wire.write(data[i]);
 }
 Wire.endTransmission();
 }
 
 void readData() {
 Wire.requestFrom( I2C_SLAVE_ADDRESS, 1 );
 int numBytes = Wire.read();
 Serial.println("numBytes: " + String(numBytes));
 
 byte data[numBytes];
 if ( Wire.available() == sizeof( data))
 {
 Wire.readBytes( (byte *) data, numBytes);
 }
 Serial.print("Byte Array: ");
 for (int i = 0; i < numBytes; i++ ) {
 Serial.print(data[i]);
 Serial.print(" ");
 }
 Serial.println();
 } 
 
 byte data[5] = {0x1, 0x2, 0x3, 0x4, 0x5 };
 
 void setup()
 {
 Wire.begin();
 Serial.begin(9600); 
 
 writeData(data, sizeof(data));
 delay(100);
 readData();
 }
 
 void loop() {
 
 }

Master outputs:

01:20:13.844 -> numBytes: 5
01:20:13.878 -> Byte Array: 11 8 212 8 22
01:21:51.222 -> numBytes: 0
01:21:51.255 -> Byte Array:
01:21:54.323 -> numBytes: 5
01:21:54.323 -> Byte Array: 11 8 212 8 22
01:22:01.682 -> numBytes: 0
01:22:01.717 -> Byte Array:
01:22:04.280 -> numBytes: 5
01:22:04.314 -> Byte Array: 11 8 212 8 22
01:22:06.879 -> numBytes: 0
01:22:06.912 -> Byte Array:
01:22:10.386 -> numBytes: 5
01:22:10.420 -> Byte Array: 11 8 212 8 22

Please tell me the best way to solve this problem. Maybe transfer all values one byte at a time, and put a special delimiter between the beginning of new data?

• jsotola: I get a set of random numbers ... the numbers are not random because they repeat ... the numbers are invalid though
• Gil: Try making the first byte the length. That will keep you from sending a zero size but the first byte would be 0. Create a function that will send the array byte by byte. Your receiving would also have to know the format.

Answer 1

yes, the idea of sending one byte at a time and specifying the length of the array worked. I'm surprised that I didn't do that sooner :)

SLAVE:

#include <Wire.h>
# define I2C_SLAVE_ADDRESS 12
# define DEFAULT_DATA_SIZE 256
byte DATA[DEFAULT_DATA_SIZE];
int pointer = 0;
void setup()
{
 Wire.begin(I2C_SLAVE_ADDRESS);
 delay(1000);
 Wire.onRequest(requestEvent);
 Wire.onReceive(receiveEvent);
}
void loop()
{
}
void requestEvent()
{
 if (pointer > DATA[0]) {
 Wire.write(0);
 }
 else {
 Wire.write(DATA[pointer++]);
 }
}
void receiveEvent(int numBytes)
{
 DATA[0] = numBytes;
 for (int i = 1; i <= numBytes; i++) {
 DATA[i] = Wire.read();
 }
 pointer = 0;
}

MASTER:

#include <Wire.h>
# define I2C_SLAVE_ADDRESS 12
void writeData(byte data[], int numBytes) {
 Wire.beginTransmission(I2C_SLAVE_ADDRESS);
 for (int i = 0; i < numBytes; i++)
 {
 Wire.write(data[i]);
 }
 Wire.endTransmission();
}
void readData() {
 Wire.requestFrom( I2C_SLAVE_ADDRESS, 1 );
 int numBytes = Wire.read();
 Serial.println("numBytes: " + String(numBytes));
 byte data[numBytes];
 
 for (int i = 0; i < numBytes; i++) { 
 Wire.requestFrom( I2C_SLAVE_ADDRESS, 1 );
 data[i] = Wire.read();
 } 
 Serial.print("Byte Array: ");
 for (int i = 0; i < numBytes; i++ ) {
 Serial.print(data[i]);
 Serial.print(" ");
 }
 Serial.println();
}
byte data[8] = {0x1, 0x2, 0x3, 0x4, 0x5, 0x6, 0x7, 0x8 };
void setup()
{
 Wire.begin();
 Serial.begin(9600);
 writeData(data, sizeof(data));
 delay(100);
 readData();
}
void loop() {
}