How do I read a string from USART, given known delimiter

[zristic]

This program demonstrates how to read a string from USART.
Tested on 18F452, 8MHz, EasyPIC3 board.

Code: Select all

// Date  2005-08-16   ZR
program ReadString

var txt, delim: string[20];

// Reads chars from usart until delimiter sequence is detected.
// Read sequence is stored in output.
// Delimiter sequence is not stored in output.
// This is a blocking call, it is expected that the sequence arrives,
//    otherwise the procedure exits after 20 received chars
procedure Usart_Read_Text(var output, delimiter: string[20]);
var i, j, data, data2: byte;
    lasti: byte;
label exit;
begin
  i := 0;
  j := 0;
  lasti := 0;
  data2 := delimiter[0];
  while true do
    begin
      if Usart_Data_Ready <> 0 then    // something has arrived
        begin
          data := USART_Read;          // read char
          output[i] := data;           // store char in data (delimiter is stored too)
          if data = data2 then         // does it match the sequence?
            begin
              lasti := i - j;          // indicate where output should end
              inc(j);                  // next letter in sequence
            end
          else
            begin
              j := 0;                  // sequences do not match, reset counters
              lasti := 0;
            end;

          data2 := delimiter[j];       // get next delimiter

          if data2 = 0 then            // stop if the end of delimiter is reached
            break;

          inc(i);
          if i > 20 then  // too many chars received
            begin
              lasti := 19;
              break;
            end;
        end;
    end;
  output[lasti] := 0;  // place the end of string before delimiter
end;


// writes text to usart.
// text has to be zero terminated
procedure Usart_Write_Text(var uart_text: string[20]);
var i, data: byte;
begin
  i := 0;
  data := uart_text[0];
  while data <> 0 do
    begin
       USART_Write(data);
       inc(i);
       data := uart_text[i];
    end;
end;

// test program
begin
  USART_init(9600);
  delim := 'OK';
  while true do
    begin
      if Usart_Data_Ready = 1 then
         begin
           Usart_Read_Text(txt, delim); // read text until "OK" is received
           Usart_Write_Text(txt); // write back what has been read
         end;
    end;
end.