Can't use two SPI devices at the same time

Question

I can use the MFRC522 using the following code:

#include <SPI.h>
#include <MFRC522.h>

MFRC522 mfrc522(10, 9);

void setup() {
    SPI.begin();
    mfrc522.PCD_Init(); 
}

void loop() {
    if (  mfrc522.PICC_IsNewCardPresent() && mfrc522.PICC_ReadCardSerial() ) {
         // Do stuff
    }
}

And it works great. I can also use the dot-matrix (8x8) using the following code:

#include "LedControl.h"

LedControl lc = LedControl(12,11,8,1);

void setup() {
    lc.shutdown(0,false);
    lc.setIntensity(0,3);
    lc.clearDisplay(0);

    lc.setLed(0,2,5,true);
    lc.setLed(0,5,5,true);

    lc.setLed(0,2,2,true);
    lc.setLed(0,3,1,true);
    lc.setLed(0,4,1,true);
    lc.setLed(0,5,2,true);
}

void loop() {
}

And it works just fine as well. However, when I try to use both of them using the following code:

#include <SPI.h>
#include <MFRC522.h>
#include "LedControl.h"

LedControl lc = LedControl(12,11,8,1);
MFRC522 mfrc522(10, 9);

void setup() {
    SPI.begin();
    mfrc522.PCD_Init();

    lc.shutdown(0,false);
    lc.setIntensity(0,3);
    lc.clearDisplay(0);

    lc.setLed(0,2,5,true);
    lc.setLed(0,5,5,true);

    lc.setLed(0,2,2,true);
    lc.setLed(0,3,1,true);
    lc.setLed(0,4,1,true);
    lc.setLed(0,5,2,true); 
}

void loop() {
    if (  mfrc522.PICC_IsNewCardPresent() && mfrc522.PICC_ReadCardSerial() ) {
         // Do stuff
    }
}

In that case only one of them works (the MFRC522). I know that since they are connected in SPI mode they need to have different SS pins, so I used pin 10 for MFRC522 and pin 8 for dot-matrix. So, what's wrong? Why dot-matrix doesn't work at the same code with MFRC522??

Answer 1

Without the datasheets at hand, I'd first suspect that the two SPI devices have clock rates that are incompatible. You need to find the clock rates for each one and either clock them off two different timers, or switch timing on a single timer to provide the correct clock rate for the currently selected device. Incompatible clock rates has been the only problem I've ever had with SPI devices.