8051: Example 6

Using Timer T2

This example describes the use of Timer T2 configured to operate in Auto-Reload mode. In this very case, LEDs are connected to port P3 while the push button used for forced timer reset (T2EX) is connected to the P1.1 pin.
Program execution is similar to the previous examples. When timer ends counting, an interrupt is enabled and subroutine TIM2_ISR is executed, thus rotating a logic zero (0) in accumulator and moving the contents of accumulator to the P3 pin. At last, flags which caused an interrupt are cleared and program returns to the loop LOOP1 where it remains until a new interrupt request arrives...
If push button T2EX is pressed, timer is temporarily reset. This push button resets timer, while push button RESET resets the microcontroller.

;************************************************************************
;* PROGRAM NAME : Timer2.ASM
;* DESCRIPTION: Program rotates log. "0" on port P3. Timer2 determines
;* the speed of rotation and operates in auto-reload mode
;************************************************************************

;BASIC DIRECTIVES

$MOD53
$TITLE(TIMER2.ASM)
$PAGEWIDTH(132)
$DEBUG
$OBJECT
$NOPAGING

;DEFINITION OF VARIABLES

        T2MOD    DATA     0C9H
;STACK
        DSEG     AT       03FH
STACK_START:     DS       040H

;RESET VECTORS
        CSEG     AT       0
        JMP      XRESET                   ; Reset vector

        ORG      02BH                     ; Timer T2 Reset vector
        JMP      TIM2_ISR

        ORG      100H

XRESET: MOV      SP,#STACK_START          ; Define Stack pointer
        MOV      A,#0FFH
        MOV      P3,#0FFH
        MOV      RCAP2L,#0FH              ; Prepare 16-bit auto-reload mode
        MOV      RCAP2L,#01H
        CLR      CAP2                     ; Enable 16-bit auto-reload mod 
        SETB     EXEN2                    ; Pin P1.1 reset is enabled
        SETB     TR2                      ; Enable Timer T2 
        MOV      IE,#0A0H                 ; Interrupt is enabled
        CLR      C

LOOP1:  SJMP     LOOP1                    ; Remain here

TIM2_ISR:        RRC      A               ; Rotate contents of Accumulator to the right through
                                          ; Carry flag
                 MOV      P3,A            ; Move the contents of Accumulator A to PORT3
                 CLR      TF2             ; Clear timer T2 flag TF2 
                 CLR      EXF2            ; Clear timer T2 flag EXF2 
                 RETI                     ; Return from interrupt

                 END                      ; End of program