无法spi读

环境CC2640  blestack5, IAR

char wb[2];
wb[0] = 0x0B;
wb[1] = addr;
char rb[2];
masterTransaction.count = 2;
masterTransaction.txBuf = wb;
masterTransaction.arg = NULL;
masterTransaction.rxBuf = rb;

bool r = SPI_transfer(s_spiHandle, &masterTransaction);

示波器看到发了2个字节数据出去，然后就没有clk了，收不到应答。

问题：SPI_transfer是怎么知道应该收多少数据？