Hello.

Please note that I'm new to both 8051 and English language but I'll try my best ^^;

According to the RTC tutorial found here

http://www.8052.com/tutrtc.phtml

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HOURS EQU 07Ch ;Our HOURS variable
MINUTES EQU 07Dh ;Our MINUTES variable
SECONDS EQU 07Eh ;Our SECONDS variable
TICKS EQU 07Fh ;Our 20th of a second countdown timer
RESET_VALUE EQU 19456 ; ***** I modified this line

ORG 0000h ;Start assembly at 0000h
LJMP MAIN ;Jump to the main routine

ORG 001Bh ;This is where Timer 1 Interrupt Routine starts
PUSH ACC ;We'll use the accumulator, so we need to protect it
PUSH PSW ;Protect PSW flags
CLR TR1 ;Turn off timer 1 as we reset the value
MOV TH1,#HIGH RESET_VALUE ;Set the high byte of the reset value
MOV TL1,#LOW RESET_VALUE ;Set the low byte of the reset value
SETB TR1 ;Restart timer 1 now that it has been initialized
DJNZ TICKS,EXIT_RTC ;Decrement TICKS, if not yet zero we exit immediately
MOV TICKS,#20 ;Reset the ticks variable
INC SECONDS ;Increment the second varaiable
MOV A,SECONDS ;Move the seconds variable into the accumulator
CJNE A,#60,EXIT_RTC ;If we haven't counted 60 seconds, we're done.
MOV SECONDS,#0 ;Reset the seconds varaible
INC MINUTES ;Increment the number of minutes
MOV A,MINUTES ;Move the minutes variable into the accumulator
CJNE A,#60,EXIT_RTC ;If we haven't counted 60 minutes, we're done
MOV MINUTES,#0 ;Reset the minutes variable
INC HOURS ;Increment the hour variable
EXIT_RTC:
POP PSW ;Restore the PSW register
POP ACC ;Restore the accumulator
RETI ;Exit the interrupt routine

MAIN:
MOV TH1,#HIGH RESET_VALUE ;Initialize timer high-byte
MOV TL1,#LOW RESET_VALUE ;Initialize timer low-byte
MOV TMOD,#10h ;Set timer 1 to 16-bit mode
SETB TR1 ;Start timer 1 running
MOV HOURS,#00 ;Initialize to 0 hours
MOV MINUTES,#00 ;Initialize to 0 minutes
MOV SECONDS,#00 ;Initialize to 0 seconds
MOV TICKS,#20 ;Initialize countdown tick counter to 20
SETB EA ;Initialize interrupts
SETB ET1 ;Initialize Timer 1 interrupt

END ; The compiler said I should put 'END' here
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Ofcourse this code is just an implementation of RTC. We can include main program. I want to understand how the code work so I try to run and debug the program as is.

Using a simulator called Isis 6 professional, I found few problems which I do not understand why.

-Question 1.-
After the FIRST execution of the command

SETB ET1 ;Initialize Timer 1 interrupt

The program immediately jump into timer1 interrupt routine.

It should only jump into that routine when the timer1 overflow, is it not? @_@ ?

-Question 2-
After it jump into the interrupt routine, I continue debugging.

After this line

DJNZ TICKS,EXIT_RTC ;Decrement TICKS, if not yet zero we exit immediately

The program exit interrupt routine, which is normal but...

It returns to MAIN and continue executing EVERY lines in MAIN which include MOV TICKS,#20. Resulting in TICKS becomes 20 again. We are going to decrement it every time the timer1 overflow aren't we? But why this code set it back to 20 after each decrement?

I think I must be misunderstadning something. Can anyone clear this up for me please?

Thank you and please spare me for my horrible programming skill >_< ;