/*ADC采樣函數，采樣第7通道信號，采樣分辨率256*/
unsigned char get_ad(void) {

	unsigned char i;
	
	ADMUX = 0x67;			/*基準AVCC、左對齊、通道7*/
	ADCSRA = 0xC2;			/*使能、開啟、4分頻*/
	while(!(ADCSRA & (1 << ADIF)));	/*等待*/
	i = ADCH;
	ADCSRA &= ~(1 << ADIF);		/*清標志*/
	ADCSRA &= ~(1 << ADEN);		/*關閉轉換*/

	return i;
}