KL25Z as SPI slave not able to send data correctly

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KL25Z as SPI slave not able to send data correctly

Contributor I

Hi everybody! Smiley Happy

I'm working on a project that involves the FreedomBoard (KL25Z) and an Arduino UNO microcontroller. The two boards should communicate through the SPI bus: Arduino acts as the master, the KL25Z as the slave. The problem is that although the FreedomB. is able to receive correctly the data from Arduino, it is not able to write into the transmit buffer. I report below what I've done so far.

The involved pins are:

Arduino pin13121110

The Arduino code is very simple: through the official SPI library I set the clock frequency to 4 MHz, the SPI mode to 0 (CPOL=0, CPHA=0), MSBF (Most Significant Bit First). Then in the main loop, using the Serial Monitor, I can ask Arduino to send a byte to the other board and to print the byte received from the KL25Z.

From the Freedom side, the code that I wrote is the following (I use CodeWarrior, without processor expert):

void setupSPIslave() {

  SIM_SCGC5 |= SIM_SCGC5_PORTC_MASK; // clock to port C

  SIM_SCGC5 |= SIM_SCGC5_PORTD_MASK; // clock to port D

  // setup of multiplexer for selecting CLK, MOSI, MISO and SS

   // PTC5 as CLK (pin 13 on Arduino UNO)


   // PTC6 as MOSI (pin 11 on Arduino UNO)


  // PTC7 as MISO (pin 12 on Arduino UNO)


   // PTD0 as CS (pin 10 on Arduino UNO)


  SIM_SCGC4 |= SIM_SCGC4_SPI0_MASK; // clock to SPI0 module

  // SPI slave; SPRF interrupts; Most-Significant-Bit-First; SPI_MODE0 (CPOL=0, CPHA=0); SS=ChipSelect


  SPI0_C1 |= SPI_C1_SPE_MASK; // enable SPI module

  NVIC_ISER |= NVIC_ISER_SETENA(1<<10); // enables interrupts handling for the SPI0 module


unsigned volatile char val = 0;

// interrupt handler for SPI0 module interrupts

// an interrupt should be fired any time the module receives data

void SPI0_IRQHandler() {

  // wait until SPRF is set (should be executed once)

  while((SPI0_S & SPI_S_SPRF_MASK)==0);

  unsigned char data = SPI0_D; // read the data register


  if(data==0xFF) // 0xFF means switch on a LED


  else if(data==0x00) // 0x00 means switch off the LED


  val++; // counter that tells me how many interrupts have been fired

  // wait until the data register is ready to be written

  while((SPI0_S & SPI_S_SPTEF_MASK)==0);

  RED_TOG; // toggle the red on-board LED (it tells me that the interrupt has been served)

  SPI0_D = SPI_D_Bits(0x02); // write a value into the data register


The setup simply configures the module as a slave and enables the interrupts (they'll be fired whenever new data arrives from Arduino).

The interrupt handler reads the received byte; if the value is 0 (255) it switches off (on) a LED.

Then there is the 'hot' section: the interrupt routine should write into the data register (SPI0_D) a value (0x02, it's just an example). Unfortunately, when I run the Arduino sketch, the received values, instead of being 0x02, is always 0x00 or 0xFF (that is, the bytes that I send from Arduino!). Thanks to the red LED I know that the interrupts are fired and served; the additional LED that I drive through the interrupt blinks properly (I therefore know that the SPI module is working well).

Any idea? :smileyconfused:

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2 Replies

NXP TechSupport
NXP TechSupport

Hi Franco Fusco,

    I have the SPI slave and master sample code based on FRDM-KL25 for your reference.

  I already test my code, and the code works ok on my side, you can refer to my configuration and check your code, whether it is correct.

    Please refer to the attachment at first.

  Any question after you check the code, please let me know!

Have a great day,

Note: If this post answers your question, please click the Correct Answer button. Thank you!

Contributor I

Thanks a lot Jingjing for your answer! :smileyhappy:

I had a look at your code: it seems that the setups are quite the same, so I deduced that the problem was in the hardware. In fact, it seems that the mess involved the voltage levels. I already knew that Arduino pins work at 5V, but I thought it was not a problem for the KL25Z. And I was probably wrong!!! :smileyconfused:  I just added a couple of resistors to reduce the voltage to 3.3V and now it works :smileywink:

Thanks again!

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