;Timekeeping Example, using Timer 0 with simple polling
;http://www.pjrc.com/tech/8051/board4/timers.html

;This examples demonstrates how to use Timer 0 to keep track
;of time.  Timer 0 is used in mode 0, which produces 225
;overflows per second (22.1184 MHz crystal).


.equ    cout, 0x0030            ;Send Acc to serial port
.equ    cin, 0x0032             ;Get Acc from serial port
.equ    phex, 0x0034            ;Print Hex value of Acc
.equ    pstr, 0x0038            ;Print string pointed to by DPTR,
.equ    esc, 0x003E             ;Check for ESC key
.equ    newline, 0x0048         ;print CR/LF (13 and 10)


	.org	0x2000
begin:
	mov	r7, #0		;zero hours, minutes, seconds
	mov	r6, #0
	mov	r5, #0
	clr	tr0		;make sure timer 0 is stopped
	clr	tf0		;clear the overflow flag
	anl	tmod, #0xF0	;set to mode 0 (without touching timer 1)
	mov	th0, #0		;clear the timer 0 value
	mov	tl0, #0
	mov	r2, #225	;start overflow countdown (1 second)
	mov	dptr, #msg_begin
	lcall	pstr
	setb	tr0		;start the timing
	lcall	print_time
timekeeping_loop:
	lcall	esc
	jc	abort
	jnb	tf0, timekeeping_loop ;did timer 0 overflow?
	clr	tf0		;reset overflow flag
	djnz	r2, timekeeping_loop  ;is this 225 overflows (exactly 1 second)
	lcall	inc_time
	lcall	print_time
	mov	r2, #225	;reset countdown for another second
	sjmp	timekeeping_loop
abort:
	mov	dptr, #msg_end
	lcall	pstr
	lcall	cin
	ljmp	0


inc_time:
	inc	r5		;increment seconds
	cjne	r5, #60, inc_time_end
	mov	r5, #0
	inc	r6		;increment minutes
	cjne	r6, #60, inc_time_end
	mov	r6, #0
	inc	r7		;increment hours
	cjne	r7, #24, inc_time_end
	mov	r7, #0
inc_time_end:
	ret


print_time:
	mov	a, r7		;hours
	lcall	print_2digit_int
	mov	a, #':'
	lcall	cout
	mov	a, r6		;minutes
	lcall	print_2digit_int
	mov	a, #':'
	lcall	cout
	mov	a, r5		;seconds
	lcall	print_2digit_int
	lcall	newline
	ret


print_2digit_int:
	mov	b, #10		;divide by 10...
	div	ab
	add	a, #'0'		;quotient is tens digit
	lcall	cout
	mov	a, b		;remainder is ones digit
	add	a, #'0'
	lcall	cout
	ret


msg_begin:
	.db	13,10,13,10,13,10
	.db	"Timekeeping Example (Polling)",13,10
	.db	"Press ESC to stop...",13,10,13,10,0
msg_end:
	.db	13,10,13,10,"Stopped.  Press a key to reboot",0