Read Temperature and activate Leds

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Added by emanueleb on July-17-2007, 7:02 am

Let's see a real-world example of how to read, in real-time, the temperature from the on-board sensor and toggle two LEDs, from a single HTML (+ JavaScript) page. In this example, AHAH (Asynchronous HTTP And HTML) is used to send asyncronous HTTP requests

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<table class="status_table">
  <tr><th colspan="2">Temperature</th></tr>
  <tr><td>System</td><td>%00Â°C</td></tr>
  <tr><th colspan="2">LED Status</th></tr>
  <tr><td>LED 1</td><td>%01</td></tr>
  <tr><td>LED 2</td><td>%02</td></tr>
</table>

Used variables are:

* 00: the temperature;
* 01: the first LED;
* 02: the second LED;

This page will be requested periodically to know the system status.
Now we are going to see the main page (index.htm), that contains two buttons to control the LEDs and, thanks to AHAH, it will also contain the CGI page (loaded into the tempdiv div)

<html>
 <head>
   <title>FTPMicro: Temperature Example</title>
   <script type="text/javascript" src="/ahah.js"></script>
   <script type="text/javascript">
     function refresh() {
	if (lo) return true;     // stop loading when another request is in progress
	noloadAHAH('/Status.cgi','tempdiv','GET'); // request status.cgi and then load it into "tempdiv"
     }
     window.setInterval("refresh()",2000);
   </script>
   <link rel="stylesheet" type="text/css" media="all" href="/style.css" />
 </head>
 <body>
   <h1>FTPMicro</h1>
   <div class="bar">
     The world smallest WebServer and FtpClient with DHCP and UDP features
   </div>
   <div id="tempdiv" class="left">
 	Loading...
   </div>
   <div class="left">
     <form>	
       <table class="status_table">
         <tr><th>LED Toggle</th></tr>
         <tr><td><input type="submit" value="LED 1" onclick="javascript: noloadAHAH('/Status.cgi?t=1','tempdiv','GET'); return false;"></td></tr>
         <tr><td><input type="submit" value="LED 2" onclick="javascript: noloadAHAH('/Status.cgi?t=2','tempdiv','GET'); return false;"></td></tr>
       </table>
     </form>
   </div>
 </body>
</html>

This page is fairly short, but maybe you can't understand how AHAH is used!
In the page header, there is a Javascript that, using the setInterval function, refreshes the CGI page loaded in tempdiv, at regular intervals.
Another important sections is the buttons' one; the use AHAH to send command without refreshing the page. In this example, the parameters "t" is used to identify a "toggle" command, and it is followed by the value of the LED to toggle; so /Status.cgi?t=1 is the command for the first LED, and /Status.cgi?t=2 for the second one. After the command has been executed, the requested page is returned (status.cgi) and it will be always loaded into his div.
In the example, a CSS file is used to define the style, but it's not indispensable.

Below, there are the methods ProcessIO, HTTPExecCmd e HTTPGetVar; here the first:

static char Temperature[8];

static void ProcessIO(void)
{
	signed long temp;

	// Start A/D conversion
	ADCON0bits.GO = 1;

	// Wait until A/D conversion is done
	while(ADCON0bits.GO);

	temp = (long)(*(WORD*)(&ADRESL)) * 322 - 50000; // not a precise conversion...
	itoa(temp/1000, Temperature);
}

This method is called periodically, when the stack is not doing anything else. In this example, the function does an A/D conversion, then it converts the tension to a temperature, and finally the temperature is put in a string.

void HTTPExecCmd(BYTE** argv, BYTE argc)
{
	if (argv[1][0] == 't') {
		switch (argv[2][0] - '0') {
			case CMD_LED1 :
				LED1_IO = !LED1_IO;
				break;
			case CMD_LED2 :
				LED2_IO = !LED2_IO;
				break;
		}
	}
}

We have seen that the only used command is "status.cgi?t=x", where x is the LED to toggle. With this command, when HTTPExecCmd is invoked, the first element of argv will be "status.cgi", and it is left unchanged; the second item will e "t" (the if statement is used to assure that); finally, the third one will be the number of the LED in an ASCII character, so we have to convert it in an integer subtracting '0' from it.

WORD HTTPGetVar(BYTE var, WORD ref, BYTE* val)
{
	switch (var) {
		case VAR_LED1 :
			*val = LED1_IO ? '1' : '0';
			break;
		
		case VAR_LED2 :
			*val = LED2_IO ? '1' : '0';
			break;

		case VAR_TEMPERATURE:
        		*val = Temperature[(BYTE)ref];
        		if(Temperature[(BYTE)ref] == '\0')
            			return HTTP_END_OF_VAR;
			else if(Temperature[(BYTE)++ref] == '\0' )
            			return HTTP_END_OF_VAR;
        		return ref;
		default : break;
	}
    return HTTP_END_OF_VAR;
}

The HTTPGetVar method returns (into val) a LED status or the string Temperature; in the first case we only have to assing '0' or '1' to val, while in the second case, we have to assign to val one byte of the string at a time.

In the source code there are some defines like LEDx_IO, that you can find in Compiler.h:

#define	LED1_IO				(LATAbits.LATA0)
	#define	LED1_TRIS			(TRISAbits.TRISA0)
	#define	LED2_IO				(LATAbits.LATA1)
	#define	LED2_TRIS			(TRISAbits.TRISA1)

In this example, Ethernet LEDs are used, but you can use any I/O pin by changing that defines.

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