Parsing data from serial port

Hi,

I communicate with a DsPIC

i have a constant format for a message:

HEADER1 HEADER2 OPCODE DATA CHECKSUM (no spaces)

in my MATLAB GUI i want to read the incoming message, chk if i get the HEADER1 I EXPCT, then chk to see i get HEADER2 i expect , if so: get OPCODE and DATA and do with that something

how can i split my message?

thought about switch;case

wanted to know if there is something more "elegant"

Thanks

[Walter Roberson]

Is that a text format or a numeric format? are the headers variable length? How can you tell when HEADER1 stops and HEADER2 begins?

[Daniel]

headers ARE FIXED

header1: 55
header2: AA

if i get a sequence of 55AA i know now comes the opcode then the data

if i get 55555555555 i wait for AA because HEADER1 was recieved

if i get 55AF i wait to get 55 again because sequence 55AA was not recieved

Answer 1

[Walter Roberson]

You need a Finite State Machine. The Mathworks tools for that are http://www.mathworks.com/discovery/finite-state-machine.html but you probably do not need anywhere near the full power of that.

current_state = 0;

while true
   this_input = fread(s, 1, 'uint8');
   if feof(s); break; end
   if state == 0 && this_input == 88   %0x55
      current_state = 1;
   elseif current_state == 1 && this_input == 170   %0xAA
      current_state = 2;
      now read the opcode and data and so on.
      When you are through with it
      current_state = 0;
   else
      current_state = 0;
   end
 end

[Daniel]

ok, so here is what i tried: i have a button ('READ') that is intended to check if any message exist in the buffer. analyze the message i.e. check if the message is valid (HEADERs) GET THE OPCODE, and according to opcode - write to 3 different boxes. example:

    55  AA  02 1B CHKSUM
    H1  H2  OP DATA CHKSUM
OPCODE 02 : write to box 'temperature' 27(0X1B)
OPCDODE 03: write to box 'voltage' the DATA

BUT NOTHING APPEARS IN THE BOX... here is my actual code for the button 'READ' press:

%%--- Executes on button press in rxButton.
function rxButton_Callback(hObject, eventdata, handles)
current_state = 0;
while (handles.SerPIC.BytesAvailable >= 5)
  this_input = fread(handles.SerPIC,1, 'uint8');
%     if feof(SerPIC);
%         break;
%     end

    switch current_state
        case 0 % state 0  wait for HEADER_1
            if (this_input == 85)
                current_state = 1;
            else 
                current_state = 0;
            end

        case 1 % state 1  wait for HEADER_2
            if (this_input == 170) %was 170
                current_state = 2;
                recieved_checksum = 255;
            elseif (this_input == 85)
                current_state = 1;
            else
                current_state = 0;
            end 

        case 2 % state 2  wait for OPCODE
             OPCODE = this_input;
             current_state = 3;
             recieved_checksum = mod(recieved_checksum + this_input,255);
        case 3 % state 3  wait for DATA
             DATA = this_input;

             current_state = 4;
             recieved_checksum = mod(recieved_checksum + this_input,256);
        case 4 % state 4  wait for CHECKSUM
             current_state = 0;
             recieved_checksum = recieved_checksum - this_input;
             if (recieved_checksum == 0)
                % data is valid and can be read
                if (OPCODE == 2)
                    set(handles.temp_read,'string',(DATA))
                elseif (OPCODE == 3)
                    set(handles.voltage_read,'string',DATA)
                elseif (OPCODE == 4)
                    set(handles.current_read,'string',DATA)
                end
             end
    end
end
guidata(hObject,handles)
end

[Walter Roberson]

case 2, you use 255 as your mod, case 3 your use 256 as your mod.

case 2, you are starting with receive_checksum of 255 of type double. this_input is type uint8 . double + uint8 is evaluated by converting the uint8 to double, doing the addition, and converting the result to uint8, including "saturating" at 255 . uint8(255 + 27) is 255, take that mod 255 to get uint8(0) . Or uint8(255) if the mod is changed to 256. This is probably not what you wanted.

Similar saturation problem with case 3.

Case 4... remember that subtraction of uint8 underflows to 0, so you are going to have more 0's than you want. If you want to test equality, test equality.

Now around all of this is that you probably should not be using while() on there being at least 5 bytes available. Not all the bytes might have arrived yet, and your code does not wait for 5, it quits if there are not 5 or more available. Just read the bytes as needed. You can watch out for timeouts if you are worried about data going missing.

[Daniel]

thanks!! corrected the mod256 got rid of the uint8 changed the while to while >=1 now its working. thanks a lot!!! still not sure about the while though... concerned it might stuck my problem if there is a situation where the buffer always has something stored...

[Daniel]

Hi (again), Ok so Sending data is done by initiating a button press. My issue now is with recieving data. At the moment i have a state machine (see above)that runs with:

While(bytes.avaliable>=1)
But what i really want is an interrupt.

I can recieve at any moment data. I need the interrupt to jump to the state machine and remember at which state i am now. I have no lead on how to do so, especially since i havent figured out how to "remember" parameters between callbackd

[Walter Roberson]

"persistent". Or a shared variable if you need to be able to reset the state.

[Daniel]

I'll try here: so i set in the "connect" button the following:

 handles.SerPIC.BytesAvailableFcn = {'@intcon1',handles};
    set(handles.SerPIC,'BytesAvailableFcnMode','byte');  
    set(handles.SerPIC,'BytesAvailableFcnCount', 1);

and the call back function is:

if true
function  intcon1(hObject, eventdata, handles)
  persistent current_state
  persistent L;
  persistent i;
  persistent DATA;
  persistent recieved_checksum;
  % current_state = 0;
  % while (handles.SerPIC.BytesAvailable >= 1)
      this_input = fread(handles.SerPIC,1);
      switch current_state
          case 0 % state 0  wait for HEADER_1
              L=0;
              i=0;
              DATA=0;
              recieved_checksum=0;
              if (this_input == 85)
                  current_state = 1;  
              else 
                  current_state = 0;
              end
          case 1 % state 1  wait for HEADER_2
              if (this_input == 51) %was 170
                  current_state = 2;
                  recieved_checksum = 136;
              elseif (this_input == 85)
                  current_state = 1;
              else
                  current_state = 0;
              end 
          case 2 % state 2  wait for message length
               MESSAGE_LENGTH = this_input;
               L=MESSAGE_LENGTH;
               i=1;
               current_state = 3;
               recieved_checksum = mod(recieved_checksum + this_input,255);
          case 3
  %         if (s.BytesAvailable == MESSAGE_LENGTH)
              DATA(i) = this_input;
              recieved_checksum = mod(recieved_checksum + this_input,255);
              i=i+1;
              L = L - 1;
              if L > 0
                  current_state = 3;
              else
                  current_state = 4;
              end
          case 4
                recieved_checksum = recieved_checksum - this_input;
                if (recieved_checksum ==0)
                    set(handles.temp_read,'string',[num2str(DATA(1))])
                    set(handles.voltage_read,'string',[num2str(DATA(2))])
                    set(handles.current_read,'string',[num2str(DATA(3))])
                      current_state = 0;
                else
                    current_state = 0;
                end
      end
          guidata(hObject,handles)
end

but nothing seems to happen. and i get an error:

if true
Error using feval
Invalid function name '@intcon1'.
  Error in instrcb (line 36)
          feval(val{1}, obj, eventStruct, val{2:end});
  Warning: The BytesAvailableFcn is being disabled. To enable the callback property
  either connect to the hardware with FOPEN or set the BytesAvailableFcn property.  
end