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nRF52832 SPI dummy clock cycles after TX (SDK v14.2.0)

Hi,

I am using the nRF52832 on a custom PCB and I am interfacing it with a flash memory chip through SPI.

I am testing the SPI communication using the spi example (nRF52 SDK 14.2.0\examples\peripheral\spi).

Using an oscilloscope I can see that each transmitted byte (from m_tx_buf) is followed by 24 clock cycles during which the MOSI line is low.

I would like to know how to transmit continuously, without these 24 additional clock cycles.

I have tried to feed nrf_drv_spi_transfer with a NULL pointer for the rx buffer, so that no rx is expected, but nothing changed.

Here is main.c:

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#include "nrf_drv_spi.h"
#include "app_util_platform.h"
#include "nrf_gpio.h"
#include "nrf_delay.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 SPI_INSTANCE 0 /**< SPI instance index. */
static const nrf_drv_spi_t spi = NRF_DRV_SPI_INSTANCE(SPI_INSTANCE); /**< SPI instance. */
static volatile bool spi_xfer_done; /**< Flag used to indicate that SPI instance completed the transfer. */
static int m_tx_buf[] = {170, 170}; /**< TX buffer. */
static uint8_t m_rx_buf[1]; /**< RX buffer. */
static const uint8_t m_length = sizeof(m_tx_buf); /**< Transfer length. */
static const uint8_t m_rx_length = sizeof(m_rx_buf);
/**
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

And here are the oscilloscope traces (the yellow one is MOSI and the green one is SCK):

As mentioned above, the transmitted byte 10101010 is followed by 24 clock cycles where MOSI is low.

Thank you in advance for your help.