SPI Slave Question

Question

When NRF52832 as SPI Slave, how can I know how many bytes the slave should read,the API nrf_drv_spis_buffers_set(&spis, m_tx_buf, length, m_rx_buf, length)),does not have the real receive byte

awneil · Answer

This is usually defined by the Slave. So, in the usual use-case of the microcontroller being the master, you would look it up in the Slave's documentation. 
 As you are creating the Slave here, it is you that defines what it does. 
 If you want it to be variable, then you will have to devise some sort of protocol to exchange the information.