https://blog.gongquiz.com/2023/03/17/%ec%a3%bc%ed%8c%8c%ec%88%98%ea%b0%80-4mhz%ec%9d%98-%ea%b0%95%ec%a4%91%ec%97%90-%ec%a7%84%ed%96%89%ed%95%98%eb%8a%94-%ec%a2%85%ed%8c%8c%ec%9d%98-%ed%8c%8c%ec%9e%a5%ec%9d%80-%eb%8b%a8-%ea%b0%95/