Arduino Serial protocol design patterns

[I posted this to the Arduino developer’s mailing list, but figured others might find it useful too]

When I first started with Arduino, I thought Serial.available() was a very loose wrapping of the RXC bit in the USCRA register, i.e. if I didn’t get data out of there fast, it’d be gone. That led to convoluted code like:

 if( Serial.available() ) {
   val1 =;
   while( !Serial.available() );
   val2 =;
   // and so on

Yuck. So you end up designing protocols that are too terse. Or maybe you think you need to buffer so you don’t lose it:

 while( Serial.available() ) {
   commandbuffer[i++] =;

Then parsing becomes a two step process: read serial, parse buffer. Confusing to newbies perhaps, but at least it allows for a better protocol down the line. (And descend into madness as you gaze into the maw of strtok())

Because Serial contains these big comfy buffers, we often don’t need a second buffer to let us easily implement good protocols. My current favorite is to do something like this:

 // protocol is "CCaaaa", two bytes of command, four bytes of args
 if( Serial.available() >= 6 ) {  // command length is 6 bytes
   cmd0 =;
   cmd1 =;
   arg0 =;
   arg1 =;
  // ...etc...

I don’t think I’ve seen any Serial examples that check for a specific number of bytes available. It’s really handy.

Implementing a human-friendly protocol like “command arg0 arg1 arg2”, where command and args are space-separated strings like “servo 12 0xff”, is currently hard with Serial. I do this right now with a 2nd buffer and lots of C hackery:

 char* cmdbuf; char c; int i;
 while( Serial.available() && c!= '\n' ) {  // buffer up a line
   c =;
   cmdbuf[i++] = c;

 int i = 0;
 while( cmdbuf[++i] != ' ' ) ; // find first space
 cmdbuf[i] = 0;          // null terminate command
 char* cmd = cmdbuf;     //
 int cmdlen = i;         // length of cmd

 int args[5], a;         // five args max, 'a' is arg counter
 char* s; char* argbuf = cmdbuf+cmdlen+1;
 while( (s = strtok(argbuf, " ")) != NULL && a < 5 ) {
   argbuf = NULL;
   args[a++] = (byte)strtol(s,NULL,0); // parse hex or decimal arg
 int argcnt = a;         // number of args read

This sort of functionality would be great in a library I think. Maybe not in Serial, but a core class.

Any other protocols people like to use and the Arduino code they use to do it?

34 Replies to “Arduino Serial protocol design patterns”

  1. I found another more reliable method. Basically just do nothing until the next character is available to be read. This gives a nice clean minimal input string:

    String inData ="";
    char ch;
    if (Serial.available() > 0)
    ch =;
    while(ch != ';')//my end of message very reliable character. Then i don't have to worry about
    //newlines and carriage returns
    inData += ch;
    while (Serial.available() <=0)
    ch =;

  2. hi guys,

    I’ve read kasper and todbot’s posts about the leds. I am trying to store 4 values in the arduino as well. I am using the code below, in which i expect to see the value for xStep printed. Instead of that I always get 32 returned by the serialmonitor, does anyone know why?

    cheers, tim

    #define stepPin 26
    #define dirPin 28
    #define enablePin 24

    byte cmd;
    byte xStep;
    byte yStep;
    byte zStep;
    int i = 0;

    void setup()
    // We set the enable pin to be an output
    pinMode(enablePin, OUTPUT);
    pinMode(stepPin, OUTPUT);
    pinMode(dirPin, OUTPUT);

    digitalWrite(enablePin, LOW);
    digitalWrite(dirPin, LOW);
    digitalWrite(stepPin, LOW);

    Serial.print("Program Initiated\n");

    void loop() {
    if( Serial.available() == 4 ) {
    cmd =;
    xStep =;
    yStep =;
    zStep =;

    for(i=0; i<xStep; i++){


    void stepx_p(){
    digitalWrite(stepPin, HIGH);
    digitalWrite(stepPin, LOW);

  3. “How to pass float data ?”
    The arduino converts the value according to what it thinks it is, so declaring it a float arduino-side should work just fine..

    NICE post! far the most usable I have seen the past few days on the subject, tnx.. Btw making me a wifi robot :)

  4. Hi, very good post. I try to make an RC plane where serial communication is very important.
    I wrote this

     long start_time = millis();
       // Permet de rendre la main au reste du programme
         if((millis() - start_time) > 10 )  { // wait at most 10 ms
          // Lecture du byte arrivé  
          byte theByte =;
          //Nouvelle trame et que nous sommes pas dans la construction d'une trame ou la trame met trop longtemps a arrivé
          if((theByte == START_TRAME && newTrame == false) || (millis() - DEB_START_TRAME>OUT_OF_TIME_TRAME))
            newTrame = true;  
            nbBuf = -1;
            DEB_START_TRAME = millis();
          else {  
            // Cas ou nous sommes en construction d'une trame
              // Cas ou le buffer et plein mais nous il ne s'agit pas de la fin de la trame
              // alors on sait que la trame est corrompu pas besoin de la traiter
              if(theByte != END_TRAME)
                 newTrame = true; 
                 nbBuf = -1;
                 for(int i = 0;i<BUFFER_SIZE;i++)
                 Serial.println(" Erreur de trame - ");
              else {
                 // Verification du checksum car la trame est valide
                 // On pointe sur le pointeur cmdbuf
                  if(cmdbuf[11] == checkSum())
                    // Checksum ok
                          Serial.println("Checksum ok");
                        // Ici la trame est ok donc on peux executer une commande en call back  
                        Serial.print("Commande : ");
                        long val  = ((long )cmdbuf[9]) << 8;
                        val |= cmdbuf[10];
                        Serial.print("Val : ");
                  else {
                    // Checksum error $A0000000080I%
                      Serial.print("Erreur dans le check sum : ");
                      Serial.print("Compute checksum : ");
            else {
             //Decodage d'une trame en cours 
              cmdbuf[++nbBuf] = theByte;
              Serial.print(" - ");

    In human readable way, the packet is : $A0000000080I%
    where $ start trame
    A command
    0000000080 Data
    I Checksum
    % end trame

    How to pass float data ?

  5. Hi, very intresting post. I making Rc plane controle by Arduino and Xbee i try to make a light protocol. How could i do to send over serial floar data like lator lng gps data. Could you help me. Protocol seems like this
    Start frame 1 byte
    Command 1 byte
    Data 10 bytes
    Checksum 1 byte
    End frame 1 byte

    But 10 bytes to pass latitudelike -2.153627728 is a good idea?


  6. Hi ryuujin,
    There is the possibility for the lockup you fear when doing something like:

    while( Serial.available() < 5 ) {  // wait for 5 bytes
      // do nothing

    If the host never sends 5 characters, it'll just hang. One wait to get around this is to have a timeout:

    long start_time = millis();
    while( Serial.available() < 5 ) {       // wait for 5 bytes
      if( (millis() - start_time) < 3000 )  // wait at most 3 seconds
  7. but
    while( Serial.available() && c!= ‘\n’ ) {
    requires that Arduino while is waiting for the endline, cannot do anything other.

    I think it is a big problem to solve.


  8. Yes, it’s easy to run into many traps when using malloc(). Since the memory space of the chip in Arduino is so small and the tasks being performed are usually fairly simple, I much prefer to use statically-allocated buffers. You never run the risk of memory leaks then and you can know at compile-time exactly how much RAM you have left. (Arduino doesn’t yet expose this however, but the “avr-size” command can give you static RAM usage)

  9. Hi todbot & Rob –

    *Loved* the code for serial communication- many many thanks…

    In using it I noticed it was getting so far and it was freezing.. going through the code (Rob re-wrote) I noticed that the malloc was been run and using up all the memory.. so put


    somewhere when you are clearing variables and you shouldn’t have the same problem :D

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