Serial.read question ? solved; but why? ? (n00b)

Forum 2005-2010 (read only) Software Syntax & Programs
1

Hi

I set up a little 2x2 matrix of LEDs that I control by "Row Column Scanning".
Works great so far (in fact I'm very proud since I'm completely new to electronics and C programming)

As a next step I want to control the LED by sending a serial 4 byte string ("1111" to turn them on, "0001" turn off all but the last one etc)

First I read the string with this code:

void getData() {
  if (Serial.available() > 0) {
    while (pointer < 4) {
      buffer[pointer] = Serial.read();
      pointer++;
    }
    pointer = 0;
    pxlWrite(buffer[0], buffer[1], buffer[2], buffer[3]);
    Serial.print(buffer[0]);
    Serial.print(buffer[1]);
    Serial.print(buffer[2]);
    Serial.println(buffer[3]);
  }
}

wich returned strange results:

1ÿÿÿ
111ÿ

Then I accidentally added a delay (line 5):

void getData() {
  if (Serial.available() > 0) {
    while (pointer < 4) {
      buffer[pointer] = Serial.read();
      delay(1); //why is this necessary?
      pointer++;
    }
    pointer = 0;
    pxlWrite(buffer[0], buffer[1], buffer[2], buffer[3]);
    Serial.print(buffer[0]);
    Serial.print(buffer[1]);
    Serial.print(buffer[2]);
    Serial.println(buffer[3]);
  }
}

what made it working.
But why?

Thanks mana

2

This is your problem:

if (Serial.available() > 0) {
    while (pointer < 4) {
      buffer[pointer] = Serial.read();
      pointer++;
    }

What if "Serial.available" returns "1"?
You read that single character from the buffer, and the next time you call "Serial.read" the buffer is empty, so you get "-1". (the odd ÿ characters)

Now, at 9600 bps, 1 character takes just over 1ms to transmit.
Your delay allows you time to receive the next character before calling "Serial.read".

I wouldn't rely on this (it won't work if you decide to reduce the line speed), so you need to fix the logic of reading available characters.
So, no, not solved, sorry. :wink:

3

Without the delay, its becoming corrupted probably because of a timing issue. Your code doesn't include what speed serial you're using. It would probably work without the delay if you slowed it down.

4

Hi

Thanks for your responses.

Currently I'm using 9600 Baud.

I don't know what Serial.available() returns. If I do a Serial.println(Serial.available()) only a blank line is shown (should'nt it be 4 when I enter 4 chars, e.g. "1010"?)

so you need to fix the logic of reading available characters

I see, but how?

5

I don't know what Serial.available() returns.

If Serial.available returns a "1", there's no point reading out four characters.

for (pointer = 0; pointer < 4; ++pointer) {
  while (Serial.available () == 0) 
     {;}
  buffer [pointer] = Serial.read ();
}
6

:sunglasses:
Now it works!
Thanks a lot

7

I don't know what Serial.available() returns.

It returns the number of bytes available to read.

8

It returns the number of bytes available to read.

That's what the ref says. I can read, too.
But as noted above running

Serial.println(Serial.available())

returns nothing in my sketch.

9

Try this:

  Serial.println(Serial.available(), DEC) ;