unsigned short MeasurePeriodTime(void)
{
	// configure timer 1
	TMOD &= 0x0F;
	TMOD |= 0x10;
	TAMOD &= 0xEF;
#pragma asm
$REGUSE MeasurePeriodTime(R0,R1,R2,R6,R7)
	CLR		EA				
.....
.....
	MOV  	A,TL1
	XCH	A,R7
	MOV  	R6,A
TimerOverflow:
	CLR 	TR1
	CLR	TF1
	SETB	EA
#pragma endasm
}