Hi Mr.Karas,

Thanks for your reply. You are right in stating that "can see lots and lots of usage of the A register in this mainline code" and I also agree with the fact that the accumulator value at any instance is going to be trashed by my ISR. BUT I am using the PUSH ACC and POP ACC to take a 'backup' of the value and place it in the Stack pointer for that very specific reason, but thanks for pointing me that out. Something that you said that had brought my attention and that it actually solved my main problem is the first part of your reply where you state that "the interrupt programming is likely to break almost all the time". I agree 100% and hence I have modified the program to enable the external and global interrupt just before waiting for a flag to occur. I also admit that I am somewhat not using interrupts as they should. I mean, the main advantage of an interrupt is that you can save cycles from checking a software flag in the first place. But here i have used the ISR for the sole reason to be 100% sure that the ADC0804 (analog to digital converter) digital value is ready to be read. In the ADC0804 datasheet the manufacturer states that the digital value must not be read immediatley after the conversion is ready, hence to avoid reading an erroneous digital value from the adc i used the ISR.

Yes, I am considering to redisign the whole program to be more elegant and inline with more traditional use of interrupts.

Regarding the reply about comments, yes I apologise for writing the whole program--including comments=--in capital letters. I will sort that out when I'll write a new code as stated before.

Anyway, the program now works almost fine. I have analyzed the received samples via a software which is similar to Realterm (freeware). It displays received data in HEX and confirmed the data received is as follows:

N0$N1ÿN2ÿN3ÿN4ÿN5ÿN6ÿN7ÿN8ÿN9N0$N1ÿN2ÿ....etc etc

Honestly I am still thinking of a way to avoid these unwanted 'ÿ' (as in N9N0--note there is no ÿ). Since I am using only one ADC the data received makes sense but I must avoid sending continously a ÿ. Perhaps I have to include pulldown resistors for the bits going from the ADC to port0 of the 8051....I'll try and report....But If the problem is related to the code written please, do point it out.

Below is the updated code:

;PROGRAM MODIFIED TO USE EXTERNAL INTERRUPT INT0
;NOTE: MAKE SURE TO MODIFY BREADBOARD WIRING!!<-OK

ORG 0000H

LJMP MAIN ;DO JUMP TO AVOID GETTING A 'RET ASSEMBLY ERROR' ON PROGRAM STARTUP

ORG 0003H ;STANDARD EXTERNAL INTERRUPT INT0 ENTRY POINT
EXTINT0:
	MOV A,P0
	SETB P3.7 ;HIGH TO LOW TRANSITION TO CLR INTERRUPT LINE AND ENABLE OUTPUT LATCHES
	CLR P3.7 ;NOTE: P3.7 = RD PIN ON 8051
	MOV R5,#0FFH			      
RETI
	
ORG 0030H ;STANDARD MAIN PROGRAM ENTRY POINT (WHEN USING INTERRUPTS)
MAIN:
MOV SP,#256-32
  
MOV SCON,#01010000B
MOV TMOD,#00100101B ;T1=MODE 2(8-BIT AUTO RELOAD MODE),TIMER
MOV PCON,#10000000B ;TURN SMOD HIGH (DOUBLE BAUD)
		    
MOV TH1,#0FBH	    ;TH1 = 256 - ((Crystal / 192) / Baud) WHEN SMOD HIGH
                    ;TH1 = 256 - ((18.432E6 / 192) / 19200 ) = 251 
 		    ;THEREFORE AT TH1=256 BAUD RATE = 19200

SETB IT0            ;SET IT0 IN TCON SFR TO MAKE INT0 -VE EDGE TRIGGERED <BE SURE>
        	    ;FIRST 8 ADCS (N0 TO N7)

MOV A,#01111111B    ;ENABLE N7 ADC
MOV P2,#11111111B
MOV R0,#1
SETB TR1            ;INITIALIZE TIMER1

N0N7:
	PUSH ACC
        MOV P1,A				      
    	JNB P1.0,HERE				      
    	JNB P1.1,HERE2				      	
    	JNB P1.2,N5				      
    	JNB P1.3,N4				      
    	JNB P1.4,N3				
    	JNB P1.5,N2				
    	JNB P1.6,N1				
    	JNB P1.7,N0
	CONT: SETB P3.6     ;HIGH TO LOW PULSE TO ADC0804 WR PIN
	CLR P3.6 
	NOP
	NOP    
	SETB P3.6           ;LOW TO HIGH PULSE TO ADC0804 WR PIN (START CONVERSION)
	MOV IE,#01H         ;ENABLE EXTERNAL INTERRUPT (INT0)
	SETB EA             ;ENABLE GLOBAL INTERRUPT BIT IN IE REGISTER (OR SFR)				      	
    	HERE5: CJNE R5,#0FFH,HERE5
	CLR EA
	MOV R5,#00H
	LCALL TX_BYTE
	POP ACC				      
	RR A					      	
	INC R0					      
	CJNE R0,#9,N0N7			      
	LCALL N8N9
LJMP N0N7		        	 		

HERE:
LJMP N7

HERE2:
LJMP N6      

N0:
        MOV A,#'N'
	CALL TX_BYTE
	MOV A,#'0'
	CALL TX_BYTE
LJMP CONT

N1:
        MOV A,#'N'
	CALL TX_BYTE
	MOV A,#'1'
	CALL TX_BYTE
LJMP CONT

N2:
        MOV A,#'N'
	CALL TX_BYTE
	MOV A,#'2'
	CALL TX_BYTE
LJMP CONT

N3:
        MOV A,#'N'
	CALL TX_BYTE
	MOV A,#'3'
	CALL TX_BYTE
LJMP CONT

N4:
        MOV A,#'N'
	CALL TX_BYTE
	MOV A,#'4'
	CALL TX_BYTE
LJMP CONT

N5:
        MOV A,#'N'
	CALL TX_BYTE
	MOV A,#'5'
	CALL TX_BYTE
LJMP CONT

N6:
        MOV A,#'N'
	CALL TX_BYTE
	MOV A,#'6'
	CALL TX_BYTE
LJMP CONT

N7:
        MOV A,#'N'
	CALL TX_BYTE
	MOV A,#'7'
	CALL TX_BYTE
LJMP CONT

N8:
        MOV A,#'N'
	CALL TX_BYTE
	MOV A,#'8'
	CALL TX_BYTE
RET

N9:
        MOV A,#'N'
	CALL TX_BYTE
	MOV A,#'9'
	CALL TX_BYTE
RET

N8N9:   ;LAST 2 ADCS (N8 AND N9 	
    	MOV P1,#11111111B                             
    	MOV P2,#01111111B                             
    	LCALL N8                                       
    	MOV A,P0                                
    	CALL TX_BYTE                            
    	MOV P2,#10111111B                       
    	LCALL N9                                       
    	MOV A,P0                                      
    	MOV A,#01111111B
	MOV P2,#11111111B                                
    	MOV R0,#1                                   
RET

TX_BYTE: 
    PUSH IE
    CLR TI
    MOV SBUF,A
    JNB TI,$
    CLR TI
    POP IE
RET

END

Thanks in advance for any help!

Kai Busuttil
Industrial Electronics Student