5 years, 3 months ago.

First data transfer from Master is invalid (FRDM-K64F boards)

Hi,

I am new to mbed and i have a question regarding master-slave SPI communication. I have two FRDM-K64F boards connected to each other using SPI pins. My connection looks as below:

#include "mbed.h"
 
SPI spi(PTD2, PTD3, PTD1); // mosi, miso, sclk, ssel
DigitalOut chipSelect(PTD0);
 
Serial pc(USBTX, USBRX); // tx, rx
 
int main() 
{ 
    int valueToSendToSlave = 1;     
    chipSelect = 1; // Deselect device
    spi.format(8,3);    
    spi.frequency(1000000); 
 
    pc.printf("=================== SPI Master ===================================\r\n");
    pc.printf("Press any key to start...\r\n");
    pc.getc(); 
 
    while (1) 
		{
	 chipSelect = 0; 
        int dataFromSlave =  spi.write(valueToSendToSlave);
        chipSelect = 1; // Deselect device
 
        pc.printf("Sent value:%d	Received value:%d\r\n", valueToSendToSlave, dataFromSlave);        
        valueToSendToSlave++;
        
        wait(10); 
    }
}

#include "mbed.h"
#include <stdio.h> 
 
SPISlave device(PTD2, PTD3, PTD1, PTD0); // mosi, miso, sclk, ssel
 
Serial pc(USBTX, USBRX); // tx, rx
 
int main() 
{ 
    pc.printf("=================== SPI Slave ===================================\r\n");
    device.format(8,3);      
    device.frequency(1000000); // 1MHz
 
    int reply = 0x00;
    device.reply(reply);              // Prime SPI with first reply    
		
    while (1) 
		{
        if (device.receive()) 
	{
            int valueFromMaster = device.read();
	    pc.printf("Received value:%d 				Sent value:%d\r\n", valueFromMaster, reply);
            device.reply(++reply);              // Prime SPI with next reply            
        }
    }
}

Output is as below

COM15 is master and COM18 is slave. I have two questions regarding this ouput

• First data which is sent from master is 1 but on the slave the received value is 128. Why is the first data from the master received incorrectly on the slave.
• First reply from slave should be 0 which is correct but the second reply from the slave should be 1. The slave reply is set to 1 in it's second reply but the master is receiving 0. This goes on in the subsequent loops. Is there a reason why slave sends same reply twice and then starts sending the correct reply

I am using keil uvision v5.27 to compile and run the program. The master and slave spi examples were imported from mbed.