Arduino ATMEGA 2560 serial communication with another ATMEGA2560

javid123 · February 27, 2024, 5:27pm

hi,
i am facing an issue while doing serial communication with two ATmegas.
the code of 1st AT MEGA IS

int MESP_ID_Time_AvgAT_v=0;

int MESP_ID_Time_AvgRH_v=34;
void senddata();
void setup() 
{
  Serial.begin(115200);
  Serial2.begin(9600);
  Serial.println("welcome...");
}

void loop() 
{
  String str="";
  while(Serial2.available())
  {
    
    str=Serial2.readString();
    Serial.print("DISPLAY DATA: ");Serial.println(str);
    if(str=="as\r\n")
    {
      MESP_ID_Time_AvgAT_v=1;
      Serial.println("SENDING DATA TO 2nd ATMEGA");
    }
    else if(str=="ap\r\n")
    {
      MESP_ID_Time_AvgAT_v=0;
      Serial.println("cut DATA TO 2nd ATMEGA");
    }
  }
  delay(100);
  senddata();
  delay(100);
}
void senddata()
{
  if (MESP_ID_Time_AvgAT_v > 0)
  {

    Serial.println("INSIDE SENDDATA FUN");
    Serial2.print("at.txt=\"");
    Serial2.print(MESP_ID_Time_AvgAT_v);
    Serial2.print("\"");
    Serial2.write(0xff);//Serial2.write(0x03ff);
    Serial2.write(0xff);
    Serial2.write(0xff);
    delay(100);
    Serial2.print("rh.txt=\"");
    Serial2.print(MESP_ID_Time_AvgRH_v);
    Serial2.print("\"");
    Serial2.write(0xff);
    Serial2.write(0xff);
    Serial2.write(0xff);
    delay(100);
  }
}

the code of 2nd AT MEGA IS

String serial2read(void)
{
  
  String str="";
  while(Serial2.available())
  {
    Serial.println("READING DATA FROM SERIAL 2 ");
    str=Serial2.readStringUntil('\n');
    Serial.println("done READING DATA to SERIAL 2 ");
  }
  if(str!=NULL)Serial.println(str);
  
  return str;
}

void Serial2write(void)
{
  while(Serial.available())
  {
    Serial.println("WRITTING DATA FROM SERIAL 2 ");
    Serial2.write(Serial.read());
    Serial.println("done WRITTING DATA to SERIAL 2 ");
  }
}
void setup() 
{
  Serial.begin(115200);
  //Serial.setDebugOutput(true);
  Serial2.begin(9600);
  Serial.println("HELLO");
  Serial2.write("AT\r\n");
}

void loop() 
{
  serial2read();
  delay(100);
  Serial2write();
  delay(100);
}

the out put of 2nd AT MEGA IS

22:38:28.636 -> WRITTING DATA FROM SERIAL 2 
22:38:28.636 -> done WRITTING DATA to SERIAL 2 
22:38:29.951 -> READING DATA FROM SERIAL 2 
22:38:55.719 -> done READING DATA to SERIAL 2

when ever my 1st AT MEGA is sending data to 2nd AT MEGA, my 2nd MEGA is always doing Serial2.readString() not printing until and unless i am disconnecting my 1st ATMEGA.
more over if i remove all

Serial2.write(0x0ff);
Serial2.write(0x0ff);
Serial2.write(0x0ff);

still same issue
i dont understand why?
can any one please help me.

J-M-L · February 27, 2024, 5:39pm

I would suggest to study Serial Input Basics to handle this

you posted in an unsuitable category, I moved your post. Please make sure you post in a suitable category in the future.

javid123 · February 27, 2024, 5:40pm

means am i doing anything wrong?
please share me where to study Serial Input Basics.

J-M-L · February 27, 2024, 5:41pm

click on the link, it will take you there

you use blocking code functions with a timeout for a process that is asynchronous. Second guessing the timing of an asynchronous task is a recipe for failure.

blh64 · February 27, 2024, 5:50pm

From your second Mega:

You are reading until you get a newline, but nowhere in your First Mega code do you every send a newline. Typically, this is done with a Serial2.println() function which tacks on a trailing newline

javid123 · February 27, 2024, 5:56pm

you use blocking code functions with a timeout for a process that is asynchronous. Second guessing the timing of an asynchronous task is a recipe for failure.

i did not understand can you please elaborate about the timeout part.

javid123 · February 27, 2024, 5:59pm

i change it to

str=Serial2.readString();

still same issue

J-M-L · February 27, 2024, 6:02pm

you use readStringUntil() to expect an '\n' but you are not sending any, so the function will read whatever is coming and then terminate after a 1s timeout (by default as documented, see specs)

javid123 · February 27, 2024, 6:11pm

but it is not happening like that.
when i send "as" my ATMEGA starts sending some data and my other ATMEGA is stuck at line and doing nothing until i disconnect the ATMEGA then it is printing what ever it has read.
i changed it to Serial2.readString() still same result

J-M-L · February 27, 2024, 6:27pm

try to rephrase this, it's very unclear

call your Arduino MEGA1 and MEGA2
document which code runs on which
clarify how you hooked them up (GND connected ?)
tell us what you type into the Serial monitor of which arduino and how the Serial monitor is configured
tell us exactly what you see on the other side

javid123 · February 27, 2024, 6:37pm

MEGA 1 TX PIN IS CONNECTED TO RX PIN OF MEGA2.
MEGA 1 RX PIN IS CONNECTED TO TX PIN OF MEGA2.
MEGA 1 GND CONNECTED TO MEGA2 GND.

MEGA 2 sends AT message to MEGA 1 via serial2. MEGA 1 is receiving the message displaying on serial monitor here there is no problem.

when MEGA 2 sends "as\r\n" then MEGA 1 received the message then stated sending all the data in senddata() function but my MEGA 2 is not receiving it and when remove MEGA 2 usb cable from pc then MEGA 2 is printing all data it has read from MEGA 1.

that mean MEGA 2 is always reading [Serial2.readString()] or we can say MEGA 2 is stuck at Serial2.readString() line.
how i know this this is

String str="";
  while(Serial2.available())
  {
    Serial.println("READING DATA FROM SERIAL 2 ");
    str=Serial2.readStringl();
    Serial.println("done READING DATA to SERIAL 2 ");
}

out put is

3:28:46.694 -> done WRITTING DATA to SERIAL 2 
23:28:46.694 -> WRITTING DATA FROM SERIAL 2 
23:28:46.742 -> done WRITTING DATA to SERIAL 2 
23:28:50.756 -> WRITTING DATA FROM SERIAL 2 
23:28:50.756 -> done WRITTING DATA to SERIAL 2 
23:28:50.756 -> WRITTING DATA FROM SERIAL 2 
23:28:50.756 -> done WRITTING DATA to SERIAL 2 
23:28:50.756 -> WRITTING DATA FROM SERIAL 2 
23:28:50.756 -> done WRITTING DATA to SERIAL 2 
23:28:50.756 -> WRITTING DATA FROM SERIAL 2 
23:28:50.756 -> done WRITTING DATA to SERIAL 2 
23:28:52.073 -> READING DATA FROM SERIAL 2

see the last message

javid123 · February 27, 2024, 7:56pm

yes you are right.
it is true that when you use Serial.readString() function there must be a gap of 1000 milliseconds otherwise it will continuously reads the data from serial input buffer.
thanks.

J-M-L · February 27, 2024, 10:00pm

another example of why

javid123 · February 28, 2024, 3:42am

do you have any website link to learn about the timing, asynchronous and synchronous tasks.

J-M-L · February 28, 2024, 6:31am

You could look at

you might benefit from studying state machines. Here is a small introduction to the topic: Yet another Finite State Machine introduction

javid123 · February 28, 2024, 9:44am

do you have any best way to send and receive json format via serial.
the challenge i am facing is, i dont know when mega 1 will send json to mega 2.
i dont know when mega 2 will send json to mega 1.
we must synchronous the timing?
we should not miss the data.
how to deal with this situation?

J-M-L · February 28, 2024, 11:36am

Json or whatever, as I said, I would suggest to study Serial Input Basics to handle this

the code listens asynchronously to what's coming on the Serial port (byte by byte, in a non blocking way) and upon detection of the end marker takes an action

system · August 26, 2024, 11:36am

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Arduino ATMEGA 2560 serial communication with another ATMEGA2560

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