AD7686用示波器采集SDO的SCK，为什么SCK和SDO的时钟频率是相同的？

对AD7686进行测试，采用三线无忙模式，用示波器采集SDO的SCK，发现SDO不受SCK控制;换句话说，SCK和SDO的时钟频率是相同的。是因为AD7686模块故障吗
以下是初始化SPI的参数:
SPI_InitStructure。SPI_Direction = SPI_Direction_2Lines_RxOnly;
SPI_InitStructure。SPI_Mode = SPI_Mode_Master;
SPI_InitStructure。SPI_DataSize = SPI_DataSize_8b;
SPI_InitStructure。SPI_CPOL_Low;//SPI_CPOL_High;
SPI_InitStructure。spi_cpa_2 = spi_cpa_2;
SPI_InitStructure。SPI_NSS = SPI_NSS_Soft;
SPI_InitStructure。spi_baudrateprecaler_256;
SPI_InitStructure。SPI_FirstBit = SPI_FirstBit_MSB;
SPI_InitStructure。SPI_CRCPolynomial = 7;
SPI_Init (SPI1 SPI_InitStructure);