Why do you need delay

Using Arduino Programming Questions
1

Hi I am doing a learning project using arduino ATMEGA1280 and a LED and a small 1.5V motor. Essentially I wanted to see how I can send serial commands from VB.net and read that series of commands in Arduino and execute the commands. Here is what I did:

VB:
When you press a button on the form and a checkmark is ticked (or not ticked) following gets executed:

Try
With serialPort1
.PortName = "COM19"
.BaudRate = 9600
.Parity = IO.Ports.Parity.None
.DataBits = 8
.StopBits = IO.Ports.StopBits.One
.DtrEnable = True
.RtsEnable = True
.ReceivedBytesThreshold = 1
.Encoding = System.Text.Encoding.ASCII
.NewLine = Chr(13) + Chr(10)
End With
serialPort1.Open()
serialPort1.DiscardInBuffer()
Dim command As Byte()
Dim curspeed As Integer
command = {1, curspeed, 41}
serialPort1.Write(command, 0, command.Length - 1)

''''''''''''''''' {1, curspeed, 41} = {status of the LED=0 or 1, PWM speed for motor(0 to 255), end of command character}

Arduino:
void loop()
if (Serial.available()>0) {

light=int(Serial.read());
delay(100);

curspeed=int(Serial.read());

temp=Serial.read();

}

if (light==1) digitalWrite(52, HIGH); // there is a LED on pin 52
if (light==0) digitalWrite(52, LOW);
analogWrite(9, curspeed); //PWM pin9 is used to drive the motor
}
}

My question is:
If i dont use the delay(100) command after the "light=Serial.read();" line, this program doesnt work and "curspeed" gets a value of 255. This only works with a delay of any value put between the two Serial.read() commands. Why is this the case?

Also, if someone can point me to the maze of how these serial communications work, i would appreciate it very much. I am quiet confused as to the formats VB and arduino talk to each other in. I beleive VB is sending integers/characters and arduino is reading bytes and then converting them to integers or characters. I need to understand the serial communications protocols properly. Help is highly appreciated.

Cheers
Omi

2

I found this on serial communications fundamentals

3

You check if there is 1 byte available to read, and then you read. 3

if (Serial.available()>0)  {  //<<<< try changing from ">0" to  ">2"
      
        light=int(Serial.read());
        delay(100);
 
        curspeed=int(Serial.read());
  
        temp=Serial.read();
4

ahhh .... i see what maybe happening ... so the read commands should wait till all the three commands in the sequence are in the serial buffer and then read them sequentially. Is that correct? Also, i guessed as much but just to confirm: once you read a byte from the buffer using Serial.read(), that byte is pushed off the list?

5

Wait for 3, yes.
Or check that 1 is available prior to each read.
Once read, are gone, yes.

6

so the read commands should wait till all the three commands in the sequence are in the serial buffer and then read them sequentially.

That's one possibility.

Or check that 1 is available prior to each read.

That is another possibility.

However, both approaches rely on serial communication being 100% successful. Serial data delivery does not follow the UPS delivery guarantee (they guarantee to deliver your package). Instead, it follows the USPS delivery guarantee (the guarantee to try to deliver your package).

Serial data CAN get lost or corrupted. You need to consider this possibility, and plan for it happening.

If you send three values, for an RGB LED, for example, r, g, and b, and g gets lost, the LED will be lit using r1, b1, r2, then b2, g2, r3, then g3, r3, b4. Not a good thing.

If you send [, r, g, b, and ], then a lost value will be more easily recognized.

7

PaulS:
If you send [, r, g, b, and ], then a lost value will be more easily recognized.

You can also throw in a checksum just to be sure too. :wink:

8

Thanks a lot everyone ... here is what I am trying to make and based on the feedback I got the bot moving yesterday ... here is first few trials...