前言

字符型液晶显示模块是一种专门用于显示字母、数字、符号等点阵式 LCD，目前常用 161，162，202 和 402 行等的模块。
上面指的是以字符为单位，如161，也就是1行16列，最多能显示16个字符。其中，每个字符都是有几行乘几列的像素点组成，如168的像素点构成一个字符。本次介绍的液晶是12864。

正文

如上图所示，就是我要讲的液晶显示。CS1,CS2用于片选操作，也就是选择左半屏和右半屏，其中每一屏是64位，也就是说每一屏都是64x64dots，这里的液晶显示是4行16列，也就是说，一个字符，高是16dots，宽是8dots(一共128x64dots)。

引脚定义：

VSS 为接地
VDD 接 5V 正电源
RS 为寄存器选择，高电平时选择数据寄存器、低电平时选择指令寄存器
R/W 为读写信号线，高电平时进行读操作，低电平时进行写操作。当 RS 和 R/W
共同为低电平时可以写入指令或者显示地址，当 RS 为低电平 R/W 为高电平时可以读忙信
号，当 RS 为高电平 R/W 为低电平时可以写入数据
E 端为使能端，当 E 端由高电平跳变成低电平时，液晶模块执行命令。
D0～D7 为 8 位双向数据线

下面说说指令集：

1602 液晶模块的读写操作、屏幕和光标的操作都是通过指令编程来实现的。（说明：1 为
高电平、0 为低电平）

指令 1：清显示，指令码 01H,光标复位到地址 00H 位置。
指令 2：光标复位，光标返回到地址 00H。
指令 3：光标和显示模式设置 I/D：光标移动方向，高电平右移，低电平左移 S:屏幕上所有
文字是否左移或者右移。高电平表示有效，低电平则无效。
指令 4：显示开关控制。 D：控制整体显示的开与关，高电平表示开显示，低电平表示关显
示 C：控制光标的开与关，高电平表示有光标，低电平表示无光标 B：控制光标是否闪烁，
高电平闪烁，低电平不闪烁。
指令 5：光标或显示移位 S/C：高电平时移动显示的文字，低电平时移动光标。
指令 6：功能设置命令 DL：高电平时为 4 位总线，低电平时为 8 位总线 N：低电平时为单
行显示，高电平时双行显示 F: 低电平时显示 5x7 的点阵字符，高电平时显示 5x10 的点阵
字符。
指令 7：字符发生器 RAM 地址设置。
指令 8：DDRAM 地址设置。
指令 9：读忙信号和光标地址 BF：为忙标志位，高电平表示忙，此时模块不能接收命令或
者数据，如果为低电平表示不忙。
指令 10：写数据。
指令 11：读数据。

关于如何写数据

1.让引脚RW置高电平
2.让引脚RS置高电平
3.如果是片选一，则让CS1置高电平，否则让CS2置高电平
4.让引脚RW置低电平（RS高,RW低表示可以写入数据）
5.将数据写入引脚中(STM32每个GPIO口16个引脚)
6.让引脚E置高电平
7.延时1us
8.让引脚E置低电平
9.延时1us
10.让引脚RW置高电平
11.如果是片选一，则让CS1置低电平，否则让CS2置低电平

代码理解

 LCD_PRINT(0,0,lcd_show,"Input Card pwd:");

调用LCD_PRINT函数,对应4个参数

#define  LCD_PRINT(row, colum, buf, fmt, args...) do{\memset(buf, 0, sizeof(buf));\sprintf(buf, fmt,##args);\lcd_clr_row(row);\lcd_write(row, colum, buf, strlen(buf));}while(0)

发现,该函数是一个宏定义的函数,args表示参数缺省，首先先将将数组buf清空
int sprintf(char *str, const char *format, …) 发送格式化输出到 str 所指向的字符串
也就是让buf的值 = fmt的值 = Input Card pwd:
接着调用函数lcd_clr_row(row)，看这名字，应该是lcd显示清空,row为行号
接着往lcd里面开始写入数据，我们先来看函数lcd_clr_row(row)，如下

lcd_clr_row

 void lcd_clr_row(unsigned char row)
{unsigned char i;//CHAR_ROW_PIXEL = 16,LCD_PAGE_ROW_NUM = 8lcd_set_dsp_position(row*(CHAR_ROW_PIXEL/LCD_PAGE_ROW_NUM), 0);                       //选中左屏//LCD_COLUMN_PIXEL_SUM = 128for(i = 0; i < LCD_COLUMN_PIXEL_SUM/2; i++){    //一行两页，一次清除一页,其中一次只能清除8像素行，需要循环64次，清除lcd_write_data(0, CONCTROLLER_CS1);       //也就是清除row的内容.}lcd_set_dsp_position(row*(CHAR_ROW_PIXEL/LCD_PAGE_ROW_NUM) + 1, 0);                 for(i = 0; i < LCD_COLUMN_PIXEL_SUM/2; i++){lcd_write_data(0, CONCTROLLER_CS1);}lcd_set_dsp_position(row*(CHAR_ROW_PIXEL/LCD_PAGE_ROW_NUM), 64);                      //选中右屏for(i = 0; i < LCD_COLUMN_PIXEL_SUM/2; i++){lcd_write_data(0, CONCTROLLER_CS2);}lcd_set_dsp_position(row*(CHAR_ROW_PIXEL/LCD_PAGE_ROW_NUM) + 1, 64);for(i = 0; i < LCD_COLUMN_PIXEL_SUM/2; i++){lcd_write_data(0, CONCTROLLER_CS2);}
}

首先，根据上面传递的参数，row的值为0，表示选中左屏，那么一开始lcd_set_dsp_position(0,0),进入for循环，重复64次写入数据1，其中，CONCTROLLER_CS1 为1，CONCTROLLER_CS2为2。第二次的是lcd_set_dsp_position(1, 0),重复64次写入数据1。接着就是lcd_set_dsp_position(0, 64)和lcd_set_dsp_position(1, 64)；这里我们有必要看看函数lcd_set_dsp_position，如下

lcd_set_dsp_position

/* set display positionpage: page num(0~7), column:column pixel num(0~127)*/
void lcd_set_dsp_position(unsigned char page, unsigned char column)
{unsigned char controller_cs;if( column >= LCD_COLUMN_PIXEL_SUM/2 )controller_cs = CONCTROLLER_CS2;else    controller_cs = CONCTROLLER_CS1;lcd_write_cmd(0xC0, controller_cs); //rowlcd_write_cmd(0x40 + (column & 0x3F), controller_cs);                 //collcd_write_cmd(0xB8 + (page & 0x07), controller_cs);                    //page
}

根据前面，我们知道page的范围是0-7,column像素的范围是0-127 (16行8个像素)，首先往0xC0这个地址写入片选数据1，接着往0x40写入数据1，往0xB8 写入数据1，这里，我们研究左屏。0xC0表示设备起始行的地址，0x40表示设备的列地址，0xB8表示页地址。现在我们不难理解，这段函数的功能，那就是选中对应的显示位置。
lcd_write_data(0,1)，数据为0表示清除该数据。
这里，这个函数的功能是清除第row行，那么如果我们要清除全部行(4行)怎么写呢？

lcd_clr

 void lcd_clr(void)
{unsigned char i, x=0;for(x=0; x<8; x++){lcd_set_dsp_position(x, 0);                       //选中左屏//clean all lcd//8 times = 8 page  left 64bit right 64bit // one page 8 pixel row 64 columnfor(i=0; i<64; i++){//clean one column 8 pixel rowlcd_write_data(0, CONCTROLLER_CS1); }lcd_set_dsp_position(x, 64);                      //选中右屏for(i=0; i<64; i++){lcd_write_data(0, CONCTROLLER_CS2);}}
}

这里，一次lcd_wirte_data，只能清除1列，8像素行（1页），所以x循环8次8页，循环128次128列。

lcd_write

/*row num(0~3), column num(0~15), lcd can display 4*16 chars*/
int lcd_write(unsigned char row, unsigned char column, char* data, unsigned char len)
{unsigned char controller_cs;   unsigned char page_addr_cmd;    unsigned char column_addr_cmd;  unsigned char i,j,page_index,column_index;  unsigned char code_idx1,code_idx2;  if((row > LCD_ROW_NUM -1 ) || (column > LCD_COLUMN_NUM - 1) || (data == NULL)){return -1;   }if((len + column) > LCD_COLUMN_NUM){len = LCD_COLUMN_NUM - column;    //超出范围，那么超出的部分不显示}//128*64//for (page_index=0;page_index<2;page_index++)for(page_index = 0; page_index < (CHAR_ROW_PIXEL/LCD_PAGE_ROW_NUM); page_index++){//for (column_index = 0,i=0;column_index<(3);column_index++,i++)for(column_index = column, i = 0; column_index < (column + len); column_index++, i++){//if (column_index>=16/2=8)if(column_index >= (LCD_COLUMN_NUM / 2)){controller_cs = CONCTROLLER_CS2;    //2}else{controller_cs = CONCTROLLER_CS1;  //1}lcd_write_cmd(0xC0, controller_cs);     //行地址page_addr_cmd = 0xB8 | row*(CHAR_ROW_PIXEL/LCD_PAGE_ROW_NUM) + page_index;   //页地址lcd_write_cmd(page_addr_cmd, controller_cs);   //写入页地址column_addr_cmd = 0x40 | ((column_index * CHAR_COLUMN_PIXEL) & 0x3f);   //列地址lcd_write_cmd(column_addr_cmd, controller_cs);//send data to lcdif(data[i] & 0x80){//chineseprintf("not support display chinese\r\n");}else{//CHAR_COLUMN_PIXEL = 8for(j = 0; j < CHAR_COLUMN_PIXEL; j++){code_idx1 = data[i]-ASCILL_TO_CODE_INDEX;  //32code_idx2 = j+LCD_PAGE_ROW_NUM*page_index;//j + 8 x page_indexlcd_write_data(char_code_8x16[code_idx1][code_idx2], controller_cs);}}}}return 0;
}

这段代码的意思就是，每一次写数据，假设data[] = “input”，前面说过，一个字符是宽8x高16个像素点，高16个像素点，就占两页，也就是外循环需要两次，内循环是字符串的长度列，其中每一列占8个像素点，就是最内层的j循环8次，j循环结束后，也就是1页 8x8个像素点。lcd_write_data是数据写入。

除了以上数据写入之外，我们还需要对液晶的初始化，如下

lcd_init

void lcd_init (void)
{lcd_gpio_init();   //引脚的初始化//打开背光GPIO_ResetBits(GPIOD, LCD_BL);    //BL = 0//resetGPIO_ResetBits(GPIOD, LCD_RST); //RST = 0，复位delay_ms(100);GPIO_SetBits(GPIOD, LCD_RST); //RST = 1delay_ms(100);GPIO_ResetBits(GPIOE, LCD_E);   //E = 0 使能delay_us(1);lcd_write_cmd(0xC0, CONCTROLLER_CS1);                            //设置显示起始行(左)lcd_write_cmd(0x3F, CONCTROLLER_CS1);                            //显示打开lcd_write_cmd(0xC0, CONCTROLLER_CS2);                             //设置显示起始行(右)lcd_write_cmd(0x3F, CONCTROLLER_CS2);                           //显示打开lcd_clr();                                                        //清屏
}

STM32液晶显示完整代码

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include "stm32f10x.h"
#include "stm32f10x_it.h"
#include "systick.h"
#include "LCD12864.h"
#define CONCTROLLER_CS1 1
#define CONCTROLLER_CS2 2
int main( void )
{lcd_init();    //lcd_initshow_logo();  //显示logo}
void lcd_init (void)
{lcd_gpio_init();//打开背光GPIO_ResetBits(GPIOD, LCD_BL);//resetGPIO_ResetBits(GPIOD, LCD_RST);delay_ms(100);GPIO_SetBits(GPIOD, LCD_RST);delay_ms(100);GPIO_ResetBits(GPIOE, LCD_E);delay_us(1);lcd_write_cmd(0xC0, CONCTROLLER_CS1);                            //set first row adr leftlcd_write_cmd(0x3F, CONCTROLLER_CS1);                            //show openlcd_write_cmd(0xC0, CONCTROLLER_CS2);                          //set first row adr rightlcd_write_cmd(0x3F, CONCTROLLER_CS2);                          //show openlcd_clr();                                           //clear lcd
}
void show_logo(void)
{lcd_disp_full_img(newLandEduLogo); //myname是一维字符数组,大小是16 * 64delay_ms(1000);   //延时1000mslcd_clr();    //清除lcd内容
}
void lcd_disp_full_img(unsigned char *img)
{//img一维数组，需要自己使用字摸工具,使用。unsigned char i=0,j=0;for(i=0;i<8;i++){lcd_set_dsp_position(i, 0);                  //一次性一页，一共八页for(j=0;j<128;j++){if( j == 64 ){lcd_set_dsp_position(i, j);                   //选中右屏}if(j < 64)lcd_write_data(*img, CONCTROLLER_CS1);    //一次一页,因为数据引脚8位elselcd_write_data(*img, CONCTROLLER_CS2);img++;   //img++，指向下一个位置}}
}
void lcd_set_dsp_position(unsigned char page, unsigned char column)
{unsigned char controller_cs;if( column >= LCD_COLUMN_PIXEL_SUM/2 )controller_cs = CONCTROLLER_CS2;    //2elsecontroller_cs = CONCTROLLER_CS1;    //1lcd_write_cmd(0xC0, controller_cs);  //0x40 + 0x80 = 0xc0 row//µÚÒ»ÐÐµÄµØÖ·ÊÇ80H = 0x80   lcd_write_cmd(0x40 + (column & 0x3F), controller_cs);                 //collcd_write_cmd(0xB8 + (page & 0x07), controller_cs);                    //page
}
void lcd_write_cmd(unsigned char cmd, unsigned char controller_cs)
{uint16_t data;if(controller_cs != CONCTROLLER_CS1 && controller_cs != CONCTROLLER_CS2) {printf("controller_cs(%d)error...\r\n", controller_cs);return;}//while(is_lcd_busy(controller_cs));GPIO_SetBits(GPIOD, LCD_RW);    // RW= 1set_lcd_data_gpio_to_output();//init lcd_rw outputGPIO_ResetBits(GPIOD, LCD_RS);   //RS = 1if(controller_cs == CONCTROLLER_CS1)GPIO_SetBits(GPIOD, LCD_CS1);elseGPIO_SetBits(GPIOD, LCD_CS2);GPIO_ResetBits(GPIOD, LCD_RW); //RW = 0data = GPIO_ReadInputData(GPIOE);data &= 0x00ff;data |= (cmd << 8);GPIO_Write(GPIOE, data);GPIO_SetBits(GPIOE, LCD_E);delay_us(1);GPIO_ResetBits(GPIOE, LCD_E);delay_us(1);GPIO_SetBits(GPIOD, LCD_RW);if(controller_cs == CONCTROLLER_CS1)GPIO_ResetBits(GPIOD, LCD_CS1);else if(controller_cs == CONCTROLLER_CS2)GPIO_ResetBits(GPIOD, LCD_CS2);
}
/* send lcd data to show */
void lcd_write_data(unsigned char wdata, unsigned char controller_cs)
{uint16_t data;if(controller_cs != CONCTROLLER_CS1 && controller_cs != CONCTROLLER_CS2) {printf("controller_cs(%d)error...\r\n", controller_cs);return;}//while(is_lcd_busy(controller_cs));GPIO_SetBits(GPIOD, LCD_RW);set_lcd_data_gpio_to_output();  //Éè±¸Êý¾ÝÒý½ÅGPIO_SetBits(GPIOD, LCD_RS);if(controller_cs == CONCTROLLER_CS1)GPIO_SetBits(GPIOD, LCD_CS1);elseGPIO_SetBits(GPIOD, LCD_CS2);GPIO_ResetBits(GPIOD, LCD_RW);data = GPIO_ReadInputData(GPIOE);  //读取数据data &= 0x00ff;  //取低8位data |= (wdata << 8);  //wadata = 0x40GPIO_Write(GPIOE, data);    //写入数据GPIO_SetBits(GPIOE, LCD_E);delay_us(1);GPIO_ResetBits(GPIOE, LCD_E);delay_us(1);GPIO_SetBits(GPIOD, LCD_RW);if(controller_cs == CONCTROLLER_CS1)GPIO_ResetBits(GPIOD, LCD_CS1);elseGPIO_ResetBits(GPIOD, LCD_CS2);
}
unsigned char newLandEduLogo[]=
{//New land edu logo,128*64
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x80,0xC0,0xE0,0xF0,0xF8,0xFC,0xFE,
0xFE,0xFE,0xFC,0xF8,0xF0,0xE0,0xC0,0x80,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x60,0xE0,0xE0,0x78,0x78,0xE0,0xE0,0x60,0x60,0xE0,0xE0,0x60,0x60,0x60,
0x20,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xE0,0xE0,0xE0,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0xE0,0xE0,0xE0,0xE0,0xE0,0xE0,0xC0,0xC0,
0xC0,0xC0,0xE0,0xE0,0xC0,0xC0,0xC0,0x00,0x00,0x00,0x00,0x00,0x60,0x60,0xF0,0xF0,
0xF0,0xE0,0xC0,0xE0,0x20,0xE0,0xF0,0xF0,0xC0,0xC0,0xC0,0xC0,0x00,0x00,0x00,0x00,
0x60,0x60,0x60,0xE0,0xE0,0xE0,0xE0,0x70,0x70,0x70,0x60,0xE0,0xE0,0x60,0x60,0x60,
0x00,0xC0,0xE0,0xE0,0xF0,0xF8,0xFC,0xFE,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,
0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFF,0xFE,0xF8,0xF8,0xE0,0xE0,0x80,0x80,
0x00,0x00,0xFA,0xFA,0xFB,0xFF,0xFE,0xFF,0xFB,0x9A,0x9A,0xFF,0xFF,0x02,0xFE,0xFE,
0xFE,0x02,0x00,0x00,0x00,0x03,0x03,0x03,0x03,0x83,0xE3,0xFB,0xFF,0xFF,0xFF,0x83,
0x83,0x03,0x03,0x03,0x03,0x00,0x00,0x00,0xFF,0xFF,0xFF,0xBF,0xFF,0xE0,0xE6,0xE6,
0xE6,0x06,0xFF,0xFF,0x06,0xE6,0xE6,0x06,0x00,0x00,0x00,0x00,0xBA,0x9A,0x9F,0xDF,
0xFF,0xFF,0xBB,0xBB,0x03,0x1F,0x7F,0xF8,0xF0,0xFF,0x0F,0x00,0x00,0x00,0x00,0x00,
0x00,0xFB,0xFB,0xFB,0x6B,0x6B,0x6B,0x6B,0x6B,0x6B,0x6B,0x6B,0xFB,0xFB,0x02,0x00,
0x00,0x00,0x01,0x03,0x87,0x8F,0xCF,0xEF,0x33,0xBB,0xCC,0xEE,0x31,0xBB,0xC7,0xEF,
0x1F,0xBF,0x7F,0xFF,0xFF,0xFF,0xFF,0xFF,0x3F,0xBF,0x9F,0x0F,0x07,0x03,0x01,0x00,
0x00,0x00,0x0F,0x07,0x03,0x0F,0x0F,0x0F,0x01,0x03,0x07,0x07,0x03,0x00,0x0F,0x0F,
0x0F,0x00,0x00,0x00,0x00,0x0C,0x0E,0x06,0x07,0x03,0x03,0x01,0x00,0x00,0x03,0x03,
0x07,0x06,0x06,0x0E,0x0C,0x00,0x00,0x00,0x0F,0x0F,0x0F,0x01,0x01,0x01,0x07,0x0F,
0x0F,0x0C,0x0F,0x0F,0x0C,0x0F,0x0F,0x00,0x00,0x00,0x00,0x00,0x01,0x0D,0x0D,0x0F,
0x0F,0x07,0x00,0x0C,0x0E,0x06,0x03,0x03,0x03,0x07,0x06,0x0E,0x00,0x00,0x00,0x00,
0x00,0x0F,0x0F,0x0F,0x03,0x03,0x03,0x03,0x03,0x03,0x0B,0x0F,0x0F,0x0F,0x00,0x00,
0x00,0x0C,0x0E,0x37,0x3B,0xDB,0xEC,0xEE,0x33,0xFB,0xCC,0xEE,0x33,0xBB,0xCC,0xEE,
0x33,0xBB,0xCC,0xEF,0x33,0xBB,0xCC,0xEE,0x33,0xBB,0xCC,0xEE,0x26,0x3A,0x18,0x00,
0x00,0x00,0xFE,0xFE,0x3E,0x78,0xF0,0xC0,0xFE,0xFE,0xFE,0xE0,0xF8,0xB8,0x98,0xD8,
0xF8,0x78,0xB8,0x28,0xF8,0xF0,0x80,0xE0,0xF8,0xF8,0xE0,0xC0,0xF8,0x38,0xFE,0xFE,
0xFE,0xE0,0xF8,0x38,0x38,0x18,0x18,0x38,0xF8,0xE0,0xE0,0xF0,0xF8,0x38,0x18,0x38,
0xF8,0xE0,0xE0,0xF0,0xF8,0x38,0x18,0x18,0x38,0xFF,0xFF,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0xFE,0xFE,0x66,0x66,0x66,0x66,0x66,0x06,0x00,0xE0,0xF8,0x38,0x18,
0x18,0x38,0x00,0xFE,0xFE,0x00,0xF8,0xF8,0x00,0x00,0x00,0xF8,0xF8,0xF8,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x02,0x03,0x0B,0x0C,0x2E,0x33,0xBB,0xCC,0xEE,
0x33,0xBB,0xCC,0xEE,0x33,0x3B,0x0C,0x0E,0x03,0x03,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x0F,0x0F,0x00,0x00,0x01,0x03,0x0F,0x0F,0x0F,0x01,0x07,0x07,0x0F,0x0C,
0x0E,0x06,0x07,0x00,0x00,0x03,0x0F,0x07,0x01,0x03,0x0F,0x0F,0x03,0x00,0x0F,0x0F,
0x0F,0x01,0x07,0x07,0x0E,0x0E,0x0E,0x06,0x0F,0x0F,0x0F,0x0F,0x0F,0x00,0x00,0x00,
0x0F,0x0F,0x03,0x03,0x0F,0x0E,0x0E,0x0E,0x0F,0x07,0x03,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x0F,0x0F,0x0E,0x0E,0x0E,0x0E,0x0E,0x0E,0x00,0x03,0x07,0x0E,0x0E,
0x0E,0x0E,0x07,0x03,0x00,0x00,0x03,0x0F,0x0E,0x0E,0x0E,0x0F,0x07,0x03,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x02,
0x03,0x03,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
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0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00
};

总结

通过以上，我们掌握了基本的LCD使用方法。但这里，我们把LCD封装成函数，那么我们只要关心如何调用就好，如下：

show_logo() 显示logo，使用字摸工具128*64
LCD_PRINT(0,0,lcd_show,“Connect to sever”); 第0行0列,字符数组,显示的内容
lcd_clr_row(row) 清除第row行的内容(0-3)
lcd_clr(); //清除lcd所有内容

疑惑

每个GOIP口都有16个引脚，那么上述中的

data = GPIO_ReadInputData(GPIOE);
data &= 0x00ff;
data |= (cmd << 8);
其中cmd假设是0x40,无符号字符型，首先读取GPIOE的数据，存储到data,而data是两个字节的数据，接着跟0x00ff按位与，那么必然舍弃高8位，假设读取到的数据是0101 0101 1110 1110,那么按位与之后就是0000 0000 1110 1110，接着
data = data | (cmd<<8)，疑惑就是cmd是8位的数据,往左移8位，按C语言语法来看，左移8位，低位自动补0，那么需要数据cmd的作用是什么？

这里，我猜测可能是编译器问题，当cmd的精度不够，它会自动转换成精度更高的数据类型，这里是uint16_t

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