spi发送不了，亲们，帮我看看

xinjitmzy-34588 提问时间：2012-12-19 14:54 /

     SPI_InitTypeDef SPI_InitStructure;
    GPIO_InitTypeDef GPIO_InitStructure;

    /* SPI2 Periph clocks enable */
    RCC_APB1PeriphClockCmd (RCC_APB1Periph_SPI2, ENABLE);
    /* GPIOB clock enable */
    RCC_APB2PeriphClockCmd (RCC_APB2Periph_GPIOB| RCC_APB2Periph_AFIO, ENABLE);

    /* SCK, MISO and MOSI PB13=CLK,PB14=MISO,PB15=MOSI*/
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_13 | GPIO_Pin_14 | GPIO_Pin_15;
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF_PP;
    GPIO_Init(GPIOB, &GPIO_InitStructure);

    /* PB12 作片选*/
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_12;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP;
    GPIO_Init(GPIOB, &GPIO_InitStructure);
    GPIO_SetBits(GPIOB, GPIO_Pin_12); //预置为高

    /* SPI1 configuration */
    SPI_Cmd(SPI2, DISABLE);                                             //必须先禁能,才能改变MODE
    SPI_InitStructure.SPI_Direction = SPI_Direction_2Lines_FullDuplex; //两线全双工
    SPI_InitStructure.SPI_Mode = SPI_Mode_Master;       //主
    SPI_InitStructure.SPI_DataSize = SPI_DataSize_8b;      //8位
    SPI_InitStructure.SPI_CPOL = SPI_CPOL_Low;         //CPOL=0 时钟悬空低
    SPI_InitStructure.SPI_CPHA = SPI_CPHA_1Edge;       //CPHA=0 数据捕获第1个
    SPI_InitStructure.SPI_NSS = SPI_NSS_Soft;        //软件NSS
    SPI_InitStructure.SPI_BaudRatePrescaler = SPI_BaudRatePrescaler_4; //4分频=9M < 25M (LOW READ)
    SPI_InitStructure.SPI_FirstBit = SPI_FirstBit_MSB;      //高位在前
    SPI_InitStructure.SPI_CRCPolynomial = 7;

    SPI_Init(SPI2, &SPI_InitStructure);
    SPI_Cmd(SPI2, ENABLE);

这是简单的配置
然后发送字节这样
uint8_t SPI_SendByte(uint8_t data)
{
while((SPI2->SR & SPI_I2S_FLAG_TXE)==RESET); /*等待发送寄存器空*/
SPI2->DR = data;                              /*发送一个字节*/
while((SPI2->SR & SPI_I2S_FLAG_RXNE)==RESET); /* 等待接收寄存器有效*/
return(SPI2->DR);
}、

然后测试发送的时候，用示波器总是看不到波波、