// программа измерения времени с прерываениями 

#include <reg51.h>

unsigned char sec, min;
unsigned char count=0, time;

intt0() interrupt 1 // counter 
{
	P2 = 0;
	TH0 = (-50000) >> 8;
	TL0 = (-50000);			// const recalc too
	count++;				//counter overflov
	time += 10;
	if (20 == count)
		{
			sec++;
			count = 0;
		}
	if (60 == sec)
		{
			min++;
			sec = 0;
		}
	P2=0x7f;
}

main()
{
	TMOD = 1;				// 16 - bit timer Tm0
	ET0 = 1;				// mask TF0
	TR0 = 1;				// turn on count Tm0
	EA = 1;					// turn on interrupt
	while(1)
	{
		P1 = sec;
	}
}