Beginner having trouble with interrupt..

[mbox]

Hello everyone,
I'm trying to make a simple program that will blink a LED on PortB.2. While waits for an incoming character to PortC.7(with max232). My trouble I think is with the command serin PortC.7, T2400, RX and my PortB.2 doesnt blink. I appreciate any help/guide to understand how I can make this work.

CLEAR 
include "modedefs.bas" 'include serout defines
DEFINE OSC 20
OPTION_REG=%10000101	
				;Bit 7=1 disable pull ups on PORTB 
				;Bit 5=0 selects timer mode 
				;Bit 2=1  }
				;Bit 1=0  } sets Timer0 pre-scaler to 64
				;Bit 0=1  }
				
INTCON=%10100000	;Bit 7=1 Enables all unmasked interrupts
				;Bit 5=1 Enables Timer0 overflow interrupt 
				;Bit 2 flag will be set on interrupt and has to be cleared 
				;in the interrupt routine.  It is set clear to start with.

RX   var byte    ; holds incoming character 
symbol led1=PORTB.0
symbol led2=PORTB.1
i    var byte    ;counter
Cnt  var byte
TRISB   = %11110000	;sets the 3 output pins in the B port
PORTB = %00000000	;sets all pins low in the B port
TRISC = %10000000   ;Set portC.7 for  serial input				
ON INTERRUPT GOTO INTERUPTROUTINE	
			
				
MAINLOOP:		
        High PORTB.2       
        For i = 0 to 200	
	    	Pause 1   	
		Next i		
        Low PORTB.2     
        For i = 0 to 200	
	        Pause 1   
		Next i 
        
   ''''''''''''''''''''''''''''''''''''''''''''''''
    IF Cnt=49 then
            led1=1-led1
            pause 100
            Cnt=0
      endif   

      if Cnt=50 then
            led2=1-led2
            pause 100
            Cnt=0
      endif
GOTO MAINLOOP	;end of loop
				
DISABLE	
INTERUPTROUTINE:	
   serin PortC.7, T2400, RX 'Receive menu  
   Cnt = Rx
ENDINTERRUPT:	;
INTCON.2 = 0		;clears the interrupt flag.
RESUME		;resume the main program
ENABLE		;DISABLE and ENABLE must bracket the int routine
END

mbox

[Archangel]

Hello mbox,
are you sure your PIC has portC interrupts? Which PIC are you using? I guess my question is what is the source of the interrupt? The timer ? I do not see anything to trigger the interrupt.

[mbox]

Hi joe,
I'm using Pic16F877A, to be honest I'm trying to learn how to use Interrupt command and got stuck...

regards,
mbox

[Bruce]

With a prescaler of 64 at 20MHz, Timer0 will overflow in about 3.2mS. With your loop from 0 to 200, you're going to have to send a LOT of serial data before it can make it through your loop and toggle PORTB.2 off.

Every 3.2mS it jumps to your SERIN interrupt routine. It's going to sit there until it receives a byte of data. So it's going to take a good while before making it through your first loop.

And, you really don't know what the count in Timer0 is when you exit your interrupt routine, so you might want to clear Timer0 before exiting just in case.

You can check to make sure your Timer0 interrupt is happening by commenting out the SERIN line, and toggling another LED in your interrupt routine.

Something like this;

DISABLE	
INTERUPTROUTINE:	
   'serin PortB.3, T2400, RX 'Receive menu  
   'Cnt = Rx
   TOGGLE PORTB.3
   TMR0=0           ;clear Timer0 before exit
   INTCON.2 = 0   ;clears the interrupt flag.
   RESUME            ;resume the main program
   ENABLE            ;DISABLE and ENABLE must bracket the int routine

[mbox]

Hi Bruce,
Thanks for the reply, I realized that when I used the code PortB.3, T2400, RX PortB.2 starts to blink. Is this mean that Portc.7 can not be used as my rx?


thanks in advance,
mbox

[Bruce]

Is this mean that Portc.7 can not be used as my rx?

No. I was just pointing out why your original code didn't work as you expected.

It doesn't matter which pin you use for receiving serial data. You just need to understand why your original code isn't working as expected.

Every 3.2mS or so your program jumps to the SERIN interrupt routine. And it just sits there until it receives a byte of data.

This means it will take a very long time before your first loop can finish because it gets interrupted too often for your first loop to complete.