Interfacing Arduino with mBed LPC1768 via SPI- seems to be unreliable?

I am trying to send some data between an Arduino UNO (master) and an mBed LPC1768 microcontroller board (slave). To test that this will work I have written a simple Arduino sketch that sends 255 bytes (just ints, 1-255), waits for one second, repeats forever. The program on my mBed simply prints the recieved values to the serial port which I’m viewing with putty, and also increments a counter every time a byte is recieved. This way the mBed slave knows if it’s recieved the correct byte. But my problem is that it doesn’t seem to work- at least not very well. usually 3 or 4 bytes are recieved before it reports a mismatch, and I have tried at every possible clock speed, even tried with a 1ms delay between each byte.
I tried this same test program with 2 mBed boards and it worked just fine- for some reason with an Arduino as the master it is unreliable. Can anybody shed any light on this?


#include <SPI.h>

void setup (void)

  digitalWrite(SS, HIGH);  // ensure SS stays high for now
  SPI.begin ();


void loop (void)
  for (int i = 1; i < 256; i++){  //send 255 bytes
    digitalWrite(SS, LOW);
    SPI.transfer (i);
  digitalWrite(SS, HIGH);
  delay (1000);  // 1 second delay between the next 255 bytes
}  // end of loop


#include "mbed.h"

Serial pc(USBTX, USBRX);
SPISlave spi(p5, p6, p7, p8); //mosi, miso, sclk

int main() {
int counter = 0;
int x;
    while(1) {
        x =;
        pc.printf("%d", x);
        pc.printf("  ");
    if(counter != x){
        pc.printf("mismatch at byte no. ");
        pc.printf("%d", counter);
        pc.printf(", counter is at ");
        pc.printf("%d", counter);
        pc.printf(" and the value received is ");
        pc.printf("%d\n\r", x);
    if(counter == 255){
    x = 0;
            counter = 0;