include the mbed library with this snippet

#include "mbed.h"
#include <DigitalOut.h> 
//DigitalOut myled(LED1);
DigitalOut myled(PD_13);

SPI ser_port(PB_5,PB_4,PB_3);
DigitalOut cs(PB_9);//私有对象成员cs
DigitalOut ldac(PE_0);//私有对象成员ldac
DigitalOut reset(PE_8);//私有对象成员reset

//SPI ser_port(PB_5,PB_4,PB_3);
//char switch_word;
int switch_wordx;
int switch_wordy;
//Serial usart1(PB_6,PB_7,9600);//RS232通讯端口为PD8,PD9
//Serial usart3(PD_8,PD_9,9600);//RS232通讯端口为PD8,PD9
//Serial usart3(PB_10,PB_11,9600);//RS232通讯端口为PD8,PD9 
//Serial usart3(PC_10,PC_11,9600);//RS232通讯端口为PD8,PD9
//Serial usart1(PA_9,PA_10,9600);//RS232通讯端口为PD8,PD9
//Serial usart1(PA_9,PB_7,9600);//RS232通讯端口为PD8,PD9
//Serial usart2(PA_2,PA_3,9600);//RS232通讯端口为PD8,PD9
/////Serial usart6(PC_6,PC_7,9600);//RS232通讯端口为PD8,PD9
int main() {
/*    while(1) {
        myled = 1;
        wait(0.5);
        myled = 0;
        wait(0.5);    
        //usart1.printf("Hello World\n");
        //usart2.printf("Hello World\n");          
        usart6.printf("Hello World\n");   
    }
*/
    ser_port.format(16);
    ser_port.frequency(1000000);
    while(1)
        {
            ldac =1;    
            cs =1;
            
            cs =0;//拉低cs电平，SCLK正常工作、使能移位寄存器（时钟输出=cs+sclk）
            ldac =1;    
                
            switch_wordx=0xA0A0;
            switch_wordy=0xA1A1;
            ser_port.write(switch_wordx);    
            wait_us(1);
            ser_port.write(switch_wordy);
            wait_us(1);
            cs =1;
            
            ldac=0;
            wait_us(1);
            ldac=1;
                        
            wait_us(20);
        }            
}