ALIENTEK MiniSTM32 扩展实验25 ATK-SIM900A GSM模块测试实验中下面程序为何实现不了？

ALIENTEK MiniSTM32 扩展实验25 ATK-SIM900A GSM模块测试实验中下面程序为何实现不了
//sim900a短信测试
//用于读短信或者发短信
//返回值:0,正常
//    其他,错误代码
u8 sim900a_sms_test(void)
{
u8 key;
u8 timex=0;
if(sim900a_send_cmd("AT+CMGF=1","OK",200))return 1;//设置文本模式
if(sim900a_send_cmd("AT+CSCS="UCS2"","OK",200))return 2;//设置TE字符集为UCS2
if(sim900a_send_cmd("AT+CSMP=17,0,2,25","OK",200))return 3;//设置短消息文本模式参数
sim900a_sms_ui(40,30);
while(1)
{
key=KEY_Scan();
if(key==1)
{
sim900a_sms_read_test();
sim900a_sms_ui(40,30);
timex=0;
}else if(key==2)
{
sim900a_sms_send_test();
sim900a_sms_ui(40,30);
timex=0;
}else if(key==3)break;
timex++;
if(timex==20)
{
timex=0;
LED0=!LED0;
}
delay_ms(10);
sim_at_response(1);//检查GSM模块发送过来的数据,及时上传给电脑
}
sim900a_send_cmd("AT+CSCS="GSM"","OK",200);//设置默认的GSM 7位缺省字符集
return 0;
}