Matlab communication with Arduino and Shift Registers

Projects Interfacing w/ Software on the Computer
1

I am currently doing a project that requires me to use Matlab data to communicate with an Arduino. The Arduino, using this data then lights specific LEDs with 3 linked shift registers. The Arduino setup is very similar to that on the shift register tutorial on the Arduino site [http://arduino.cc/en/tutorial/ShiftOut]. I was able to use serial communication successfully to transmit the data and the Arduino correctly lights up the LEDs. However, after a few seconds, the LEDs will just shut off. I am not really sure what is happening except that I think it is a problem with the serial communication. Here is the code I am using:

MATLAB:

fclose('all');
close all
delete(instrfindall);
s = serial('/dev/tty.usbmodem1411');
s.BaudRate = 115200;
fopen(s);
pause(2);

numSR = 3; %number of Shift Registers
srDataArray = zeros(1, numSR);

data = [1,1,0,0,1,0,0,1,0,0,0,0,0,0,0,0,1,1,1,0,0,0,1,1];
  
for i=1:numSR
    offset = 8*(i-1);
    byte = data((1+offset):(8+offset));
    str_x = num2str(byte);
    str_x(isspace(str_x)) = '';
    srDataArray(i) = bin2dec(str_x); 
end

disp(srDataArray);
dataString = mat2str(srDataArray);
fprintf(s,dataString);
disp('Lights should be on...');
fclose(s);

Arduino:

#include <SPI.h>

const int ssPin = 10;
void setup(){
  pinMode(ssPin,OUTPUT);
  digitalWrite(ssPin,HIGH);
  SPI.begin();
  SPIwrite(0,0,0); //FLUSH SHIFT REGISTERS
  Serial.begin(115200);
}

void loop(){
  if(Serial.available()>0){
    int a = Serial.parseInt();
    int b = Serial.parseInt();
    int c = Serial.parseInt();
    SPIwrite(a,b,c);
  }
}

void SPIwrite(int x, int y, int z){
  digitalWrite(ssPin,LOW);
  SPI.transfer(x);
  SPI.transfer(y);
  SPI.transfer(z);
  digitalWrite(ssPin,HIGH);
}

I should mention that my Arduino code uses the built in SPI library

2

How are you controlling the SRCLR, OE, and RCLK inputs on the shift registers?
Typically SRCLR is tied to +5, OE to GND, and chipselect is used for RCLK

digitalWrite (ssPin, LOW);
SPI.transfer(dataByte);
digitalWrite (ssPin, HIGH); // outputs updated on this rising edge