The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Listing 10-1. Uses BASIC-52’s real-time clock to count seconds, minutes,
and hours.
10 REM set XTAL to match your crystal’s frequency
20 XTAL=12000000
30 REM set and initialize clock
40 GOSUB 200
50 REM increment clock variables once per minute
60 DO
70 ONTIME 60,500
80 WHILE 1=1
90 END
200 PRINT “Please enter the current time:”
210 INPUT “AM (0) or PM (1)? ”,AP
220 INPUT “Hour (1-12)? ”,H
230 INPUT “Minutes (0-59)? ”,M
240 INPUT “Seconds (0-59)? ”,S
250 REM initialize clock to current seconds
260 TIME=S
270 REM start clock
280 CLOCK 1
290 RETURN
500 REM increment and display time once per minute
510 REM reset seconds
520 TIME=0
530 REM increment minutes
540 M=M+1
550 IF M=60 THEN GOSUB 700
560 REM display current time
570 PRINT “the time is :”
580 PRINT H,"hours"
590 PRINT M,"minutes"
600 IF AP=0 THEN PRINT “ AM” ELSE PRINT “ PM”
610 RETI
700 REM once/hour timekeeping
710 REM reset minutes
720 M=0
730 REM increment hours
740 H=H+1
750 REM at 12:00, toggle am/pm
760 IF H=12 THEN AP=A
770 REM at 1:00, reset hours
780 IF H=13 THEN H=1
790 RETURN
Clocks and Calendars
The Microcontroller Idea Book 173