// 單片機演奏消防車的報警聲音
// 作者：chenming
// 出處：偉納電子論壇www.wllar.com


#include <reg51.h>
#include <intrins.h>
sbit	SPK = P3^7;
unsigned char frq; 
unsigned int flag;			
void delayms(unsigned char ms);
void main()
{	
	TMOD = 0x01;
	frq = 0x00;
	TH0 = 0x00;
	TL0 = 0xff;	
	TR0 = 1;
	IE = 0x82;
	flag=0;
	while(1)
	{
		switch(flag)
		{	case 0:frq++;break;			 //flag標志置0時,frq遞增
			case 1:frq--;break;			 //flag標志置1時,frq遞減
		}
		if (!(frq^0xff)) flag=1;		 //當frq增加到FFH時,flag置1,準備frq遞減	
		if (!(frq^0x00)) flag=0;		 //當frq遞減到00H時,flag置0,準備frq遞增
		delayms(15);	
	}
}		
void timer0() interrupt 1 using 1
{
	TH0 = 0xfe;
	TL0 = frq;
	SPK = ~SPK;	
}

void delayms(unsigned char ms)	
// 延時子程序
{						
	unsigned char i;
	while(ms--)
	{
		for(i = 0; i < 120; i++);
	}
}