Файл: Microcontroller Programming. Thi Micro Chip PIC (Julio Sanchez, 2007).pdf

;The prescaler is assigned to Timer0 and set up so

;that the timer runs at 1:2 rate. This means that

;every time the counter reaches 128 (0x80) a total

;of 256 machine cycles have elapsed. The value 0x80

;is detected by testing bit 7 of the counter

;register.

;Note:

;The Timer0 register provides the low-order level

;of the count. Since the counter counts up from zero,

;in order to ensure that the initial low-level delay

;count is correct the value 128 - (xx/2) must be calculated

;where xx is the value in the original countL register.

;First calculate xx/2 by bit shifting

;Routine tests timer overflow by testing bit 7 of

;the TMR0 register.

cycle:

;At this point TMR0 bit 7 is set

;Clear the bit

;Subtract 256 from beat counter by decrementing the

;mid-order byte

decfsz countM,f

zero

;At this point the mid-order byte has overflowed.

;High-order byte must be decremented.

decfsz countH,f goto cycle

; At this point the time cycle has elapsed return

;==============================

;set register variables for

;one-half second delay ;==============================

;Procedure to initialize local variables for a

;delay of one-half second on a 16F84 at 4 Mhz.

;Timer is set up for 500,000 clock beats as

;follows: 500,000 = 0x07 0xa1 0x20

;|

;|_____ * indicates set up values

;===================================================== ; variables in PIC RAM ;===================================================== ; Local variables

cblock 0x0d

countH countM countL old_w old_STATUS endc

;========================================================

; m a i n p r o g r a m

;========================================================

;=============================

;main program ;============================= main:

;Clear the Watchdog Timer and reset prescaler clrf TMR0

clrwdt

;Set up the OPTION register bit map

;|__________________________ RBPU (Pullup enable)

;Port-A is not used in this program ;============================

;set up interrupts ;============================

;Clear external interrupt flag (INTF = bit 1)

;Enable global interrupts (GIE = bit 7)

;Enable RB0 interrupt (inte = bit 4)

;do-nothing loop ;============================

;All work is performed by the interrupt handler mloop:

goto mloop ;==============================

;set register variables for

;one-half second delay ;==============================

;Procedure to initialize local variables for a

;delay of one-half second on a 16F84 at 4 Mhz.

;Timer is set up for 500,000 clock beats as

;follows: 500,000 = 0x07 0xa1 0x20

;500,000 = 0x07 0xa1 0x20

;—— —— ——

movlw 0x07 movwf countH movlw 0xa1 movwf countM movlw 0x20 movwf countL

;The Timer0 register provides the low-order level

;of the count. Since the counter counts up from zero,

;in order to ensure that the initial low-level delay

;count is correct the value 256 - xx must be calculated

;where xx is the value in the original countL register.

sublw d’255’

; Now w has adjusted result. Store in TMR0

movwf TMR0

return

;=======================================================

;Interrupt Service Routine ;=======================================================

;Service routine receives control when the timer

;register TMR0 overflows, that is, when 256 timer beats

;have elapsed

IntServ:

; First test if source is a Timer0 interrupt

; If so clear the timer interrupt flag so that count continues

;=========================

;interrupt action ;=========================

;Subtract 256 from beat counter by decrementing the

;mid-order byte

decfsz countM,f

zero

;At this point the mid-order byte has overflowed.

;High-order byte must be decremented.

decfsz countH,f

goto exitISR