从sd卡上的某个文件夹下的txt文件进行写入文件和读取文件的操作-On the sd card from a folder of TXT file for written to the file and reading files operation

上位机将sd卡的数据发送到pc串口，pc端进行解析、并实时显示，将数据保存为txt文件。-PC to send data to the sd card serial pc, pc end for analysis, and real-time display, save the data as a txt file.