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Download the relevent files here: arduinophp.zip

The zip file contains three files: index.php, serial.php and arduino_serial.pde. Two more files are required: the jQuery JavaScript library, and the PHP Serial class.

The code isn’t exactly elegant. It was just something I was playing around with, so there is probably scope for a lot of improvement. I’m not always great at explaining things like this, but I’ll give it a crack:

index.php

This doesn’t actually need to be a PHP file, since it only contains HTML and JavaScript. It’s just force of habit that caused me to save it as .php instead of .html.

This is the file I navigated to on my iPod touch. It consists of a HTML form containing two option boxes; one to select the LED and another to select the brightness. The value attributes on the options contains the actual data that’s sent to the Arduino.

An onClick event is added to the submit button that triggers the AJAXness, which is done using jQuery’s getJSON function. It calls serial.php with the values from the option boxes as GET variables. It also displays the returned values from serial.php on the page.

serial.php

This file does the actual communication with the Arduino. If first checks that the necessary GET variables are set, then does a bit of input sanitising and then constructs a packet to send via the serial port to the Arduino. The packet is three bytes:

• The first byte is an @ symbol, which denotes the start of the packet to the Arduino.
• The second byte is the index (0, 1, 2) of the LED to adjust, encoded to ASCII using PHP’s chr function.
• The third byte is the desired brightness (0 – 255), similarly encoded using chr.

This is then sent, and the response from the Arduino is echoed.

arduino_serial.pde

This is the code loaded onto the Arduino itself. The most important part of it is the nest of ifs that check for and interpret the data packet sent by serial.php:

if (Serial.available() > 2) { // if there are at least 3 bytes in the serial buffer
  int p,i;
  incomingByte = Serial.read(); // read first byte
  if(incomingByte == 64){ // check if it is packet start (@ = 64 in ASCII)
    p = Serial.read(); // read second byte
    if(p >= 0 && p < numPins){ // check it is within a valid range
      i = Serial.read(); // read the third byte
      if(i >=0 && i <=255){ // check it is within a valid range
        set(p,i); // set LED p to brightness i
        printVals(); // respond with status
      }
    }
  }
}

That’s basically it. Hopfully that makes sense…

Whilst I really haven’t got a clue what I might use it for, I thought it would be cool if I could create a simple web-based system to control hardware attached to the Arduino. My server-side language of choice is PHP, so I looked for a way to allow it to send and receive data from a serial port. I didn’t have to look very far before I found the PHP Serial class, which looked just the ticket. Unfortunately whilst it caters for Windows and Linux, it doesn’t work under OS X. Fortunately it was just a matter of adding a few more else if‘s to allow for the slight differences in the way stty works in OS X.

I thought it would all be plain sailing from here, but this was not the case. The Diecimila resets whenever a serial connection is opened, so it can be ready to receive a new program. However this can be a problem; suppose I open a serial connection to the Arduino and tell it to light up an LED at 50% brightness. Then later I decide I want it at 100% brightness, so I open up a serial connection again, at which point the Arduino resets and the LED goes out. Not very desirable, and positively useless if the Arduino has been collecting data that you wanted to retrieve.

After some Googling it became apparent that I wasn’t the only one with this issue. Most suggestions were to either remove a tiny SMT capacitor from the board to permanently disable the auto-reset function, or to suppress the behaviour by disabling DTR (in Windows) or HUPCL (in *nix). That sounded better, so I tried it out. The commands required were:

nhup sleep 999 < /dev/cu.usbserial-A7004WqM &
stty -f /dev/cu.usbserial-A7004WqM -hupcl

Now, I’m no command-line/serial expert, so I only have the slightest understanding about what goes on when those two commands are run. What I do know is that it solves the problem so long as the sleep process is still running. It’s hardly an ideal solution (in my opinion). I suspect this might be a problem unique to OS X, as it seemed to work much better when I tried it in Debian.

Then! I found a forum post lurking at the bottom of a thread on the Arduino Forum. It suggests putting a 110 ohm resistor between the 5V and Reset pins, or a 47 ohm between the 3.3V and Reset pins. I had a 47 ohm resistor handy so I whacking it in there and lo! No more automatic resets. Brilliant. The post does go on to say that using the reset button with the 47 ohm resistor is place may be risky, so I’ll have to try and remember to remove it if I ever need to press it.

So now the PHP to Arduino link is useable. Here’s an example of some PHP code to send a character to the Arduino:

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<?php
    //Include the PHP Serial class
    include "php_serial.class.php";
    //Define the serial port to use (in Windows something like 'COM1')
    define('SERIALPORT','/dev/cu.usbserial-A7004WqM');
    //The character to send
    $c = 0;
 
    $serial = new phpSerial; 
    $serial->deviceSet(SERIALPORT);
    $serial->deviceOpen();	
    $serial->sendMessage(chr($c));
?>

… and an example of the Arduino code to interpret this and do something:

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int incomingByte = 0;
 
void setup()
{
    Serial.begin(9600); //Start serial connection, 9600 baud
}
 
void loop()
{
    if(Serial.available() > 0){
        incomingByte = Serial.read();
        if(incomingByte = 0){
            analogWrite(9, 128); //Pin 9 to 50% if 0 received
        }
        else if(incomingByte = 1){
            analogWrite(9, 255); //Pin 9 to 100% if 1 received
        }
    }
}

This works brilliantly. With only slightly more complex code (thanks to a post on Principia Labs for a great explanation on effective serial commands), and a bit of AJAXy goodness, I have something like this:

Now all I have to do if think of a use for it…