SPI Slave Example Data transfer issue

Question

Hi,

I am using spis example from "SDK v17.0.2" with slight modification in the code as given below

#include "sdk_config.h"
#include "nrf_drv_spis.h"
#include "nrf_gpio.h"
#include "boards.h"
#include "app_error.h"
#include <string.h>

#include "nrf_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.h"

#define SPIS_INSTANCE 1 /**< SPIS instance index. */
static const nrf_drv_spis_t spis = NRF_DRV_SPIS_INSTANCE(SPIS_INSTANCE);/**< SPIS instance. */

#define TEST_STRING "STM32TS"
static uint8_t       m_tx_buf[] = TEST_STRING;           /**< TX buffer. */
static uint8_t       m_rx_buf[sizeof(TEST_STRING) + 1];    /**< RX buffer. */
static const uint8_t m_length = sizeof(m_tx_buf);        /**< Transfer length. */

static volatile bool spis_xfer_done; /**< Flag used to indicate that SPIS instance completed the transfer. */

static uint8_t Tx = 0;
static uint8_t Rx = 0;
uint8_t cnt= 0;

/**
 * @brief SPIS user event handler.
 *
 * @param event
 */
void spis_event_handler(nrf_drv_spis_event_t event)
{
    if (event.evt_type == NRF_DRV_SPIS_XFER_DONE)
    {
        spis_xfer_done = true;
		Tx = m_tx_buf[cnt];
		APP_ERROR_CHECK(nrf_drv_spis_buffers_set(&spis, &Tx, 1, &Rx, 1));
		cnt++;
	}
}

int main(void)
{
    // Enable the constant latency sub power mode to minimize the time it takes
    // for the SPIS peripheral to become active after the CSN line is asserted
    // (when the CPU is in sleep mode).
    NRF_POWER->TASKS_CONSTLAT = 1;

    //bsp_board_init(BSP_INIT_LEDS);

    APP_ERROR_CHECK(NRF_LOG_INIT(NULL));
    NRF_LOG_DEFAULT_BACKENDS_INIT();

    NRF_LOG_INFO("SPIS example");

    nrf_drv_spis_config_t spis_config = NRF_DRV_SPIS_DEFAULT_CONFIG;
    spis_config.csn_pin               = APP_SPIS_CS_PIN;
    spis_config.miso_pin              = APP_SPIS_MISO_PIN;
    spis_config.mosi_pin              = APP_SPIS_MOSI_PIN;
    spis_config.sck_pin               = APP_SPIS_SCK_PIN;

    APP_ERROR_CHECK(nrf_drv_spis_init(&spis, &spis_config, spis_event_handler));

    Tx = m_tx_buf[cnt];
    APP_ERROR_CHECK(nrf_drv_spis_buffers_set(&spis, &Tx, 1, &Rx, 1));
    cnt++;

    while (1)
    {
        memset(m_rx_buf, 0, m_length);
        spis_xfer_done = false;        

        while (!spis_xfer_done)
        {
            __WFE();
        }

        NRF_LOG_FLUSH();

        bsp_board_led_invert(BSP_BOARD_LED_0);
    }
}

On the Master end CS pin is pulled low and send 1 byte(0x45) only at a time. On the BLE I defined "nrf_drv_spis_buffers_set" inside "spis_event_handler" in order to read the next byte in the buffer and add to the buffer automatically.

When the chip selection is low, I received the first byte as correct followed by "0xFF" data as shown below

I have to toggle the chip selection on the master end in order to read the second byte again followed by "0xFF"(MISO high state) data which you can see on the above picture and so on..

Why can't I read the bytes continuously (0x53,0x54,0x4D,...) without toggling chip selection?

As per the documentation "New buffers must be set by calling nrf_drv_spis_buffers_set after every finished transaction. Otherwise, the transaction is ignored, and the default character is clocked out."

Any suggestion on this would be much appreciated.

Regards

Siva