Baljit Riar

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10 years ago.

How to enable duplex SPI communication with two F401RE boards?

Hi

Could you please help me to setup a duplex communication i.e. How can I read data sent by slave SPI board? So far, I have been able to send data from master to slave and the code is as follows:

Thanks a lot

//======================== SPI Master====================================
#include "mbed.h"
 
Serial pc(USBTX, USBRX); 
SPI deviceMaster (PB_5, PB_4, PB_3);// mosi, miso, sclk
DigitalOut cs(PA_9);

int main() {
    
    pc.printf("Communicating with PC...\n");
    cs = 1; // Chip must be deselected
 
    deviceMaster.format(8,2);
    deviceMaster.frequency(1000000);
    int values[5] = {0, 5, -15, 10, 55};   
    int i = 0;
    int val = 0;
    
    while(1) {
        if (i < 4){
            i++;
            }
            else
            {
                i = 0;
                }
        val = values[i];        
        cs = 0; // Select the device by seting chip select low  
        deviceMaster.write(val);
        cs = 1;
                
        //int value = deviceMaster.read();
            
            pc.printf("Master Transmission...\n");
            pc.printf("%d  \r\n", val); 
            wait_us(1000000); 
    }
}

//======================== SPI Slave ====================================

// Reply to a SPI master as slave
 
 #include "mbed.h"
 #include <SPISlave.h>

Serial pc(USBTX, USBRX); 
SPISlave device(PB_5, PB_4, PB_3, PA_4); // mosi, miso, sclk, ssel

 int main() {
     pc.printf("Communicating with PC...\n");
     int v = 0;
     device.format(8,2);
    device.frequency(1000000);     
     device.reply(0x00);              // Prime SPI with first reply
     while(1) {
         if(device.receive() == 1) {
             v = device.read();   // Read byte from master
             //device.reply(v);         // Make this the next reply 
            pc.printf("Slave Transmission...\n");
            pc.printf("%d  \r\n", v);        
         }
     }
 }

1 Answer

Wim Huiskamp
9 years, 5 months ago.

The SPI Master is always in control of the communication with the Slave. A Slave can never send data on its own initiative. It will send its data while it receives a byte from the Master. Try this code

//Master 
....

char slavedata;

cs = 0; // Select the device by setting chip select low  
slavedata = deviceMaster.write(val);  // Master is sending and receiving at the same time
cs = 1;

pc.printf("Master Received..%d\r\n", slavedata);        
...

// Reply to a SPI master as slave
...
     device.reply(0x00);              // Prime SPI with first reply, this data will be send to the Master

     if (device.receive() == 1)  {  // check to see if anything was received from the Master
        v = device.read();        // Read byte from master
        //device.reply(v);         // Make this the next reply 
        pc.printf("Slave Received..%d\r\n", v);        
     }
...

Accepted Answer