What are memset() and nrf_driv_spis_buffers_set() doing in the spi_slave example?

Question

I have two NRF52840 dev kits that are running what are essentially the spi master and spi slave examples. They both work and I've altered them slightly. I'm just kind of confused by this block of code in the spi slave example: 
 while (1)
 {
 if (spis_xfer_done)
 {
 NRF_LOG_FLUSH();
 bsp_board_led_invert(BSP_BOARD_LED_1);
 memset(spi_rx_buf, 0, spi_length);
 spis_xfer_done = false;
 APP_ERROR_CHECK(nrf_drv_spis_buffers_set(&spis, spi_tx_buf, spi_length, spi_rx_buf, spi_length));
 }
 } 
 
 Why do I need to reset the rx_buf after each transaction? Why can't I have the master keep shifting in data without the slave running memset() and nrf_driv_spis_buffers_set()? 
 Also just to confirm my understanding of how this works, when a spi transaction comes in, CS goes low, data gets shifted into some buffer in memory. Then CS goes high and an interrupt is triggered (in the slave). When this interrupt is triggered, a handler will read out the data and do whatever I tell it to do with it. Is this using the easyDMA? 
 Sorry if these are dumb questions, I appreciate all/any help someone can give me.

Karl Ylvisaker · Accepted Answer

Hi Ryan, Looking at the SPI code you have provided I dont immediately see where it goes wrong. You are doing all the right things, but I suspect the order in which you do them is the cause of your problems. In particular, I think moving nrf_drv_spis_buffers_set to the event handler might be were it goes wrong, since the callback function can be called before returning from the function. Is there a particular reason why you would like to do this in your event handler? Here is the relevant exempt from the SPIS API Reference documentation: 
 When either the memory buffer configuration or the SPI transaction has been completed, the event callback function will be called with the appropriate event nrfx_spis_evt_type_t . The callback function can be called before returning from this function, because it is called from the SPI slave interrupt context. 
 Furthermore, I would advice you to use the __WFE() wait-for-event function, rather than wasting the full cycles in the while(1) loop. Using __WFE will give you considerably lower power consumption. 
 ryerye120 said: As always, thank you. I've really appreciated your help. 
 Thank you for saying that! It is no problem at all, I am happy to help! :) Best regards, Karl

What are memset() and nrf_driv_spis_buffers_set() doing in the spi_slave example?

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