RFID (SoftwareSerial) influences/breaks connection with SD (SPI) - arduino

Question

I'm creating a RFID logger with Arduino. I connected a RFID scanner, RTC module and a SD card reader. All parts are working fine together but when i started combining different sketches a problem occured.
Reading files and writing to files on the SD card is no problem as long as i don't scan a RFID card. When input is received from the RFID scanner it is no longer possible to read or write to the sd card. The connection seems to be "disconnected" as soon as RFID input is received.
I tried using different pins for the RFID scanner, another sequence of initializing in the setup but it doesn't make a difference.
Is this a limitation of the Arduino or am I doing something wrong?
I'm using a ATS125KRW RFID shield and a Catalex MicroSD card adpater in combination with a Arduino Mega.
// SD
#include <SD.h>
File myFile;
char idArray[100][11];
char nameArray[100][11];
// RTC
#include <Wire.h>
#include "RTClib.h"
RTC_DS1307 rtc;
// RFID
#include <SoftwareSerial.h>
SoftwareSerial rfid = SoftwareSerial(A8 , A9); //(RX,TX)
String cardID; //string to store card id
char c;
char cardArray[11];
int incomingByte = 0; // for incoming serial data
String rfidInput;
boolean logtosd;
void setup()
{
Serial.begin(9600);
initializeRFID();
initializeSD();
initializeRTC();
readfromSD();
}
void loop()
{
while(rfid.available()>0)
{
c = rfid.read();
rfidInput += c;
}
if(rfidInput.length() >=12)
{
Serial.print("SCanned: ");
Serial.println(rfidInput);
//writetoSD(rfidInput);
writetoSD("kaart");
rfidInput = "";
}
while(Serial.available()>0)
{
c = Serial.read();
cardID += c;
}
if(cardID.length() >= 2)
{
writetoSD(cardID);
cardID = "";
}
}
void initializeSD()
{
// GND en VCC aansluiting via pin 48 en 49
pinMode(48, OUTPUT); // set pin to output
digitalWrite(48, LOW); // GND pin dus LOW
pinMode(49, OUTPUT); // set pin to output
digitalWrite(49, HIGH); // VCC pin dus HIGH
pinMode(53, OUTPUT);
if (!SD.begin(53))
{
Serial.println("SD initialization failed");
return;
}
Serial.println("SD initialized");
}
void readfromSD()
{
//open the file for reading:
myFile = SD.open("db.txt");
if (myFile)
{
char line[25]; //Array to store entire line
int linenumber = 0;
int arrayPlace = 0;
while (myFile.available())
{
char ch = myFile.read();
if(ch != '\n')
{
line[arrayPlace] = ch;
arrayPlace ++;
}
else
{
char id[11];
char name[11];
//get ID from entire line
for(int x = 0; x <= 10 ; x++)
{
id[x] = line[x];
}
//Get NAME from entire line
for(int x = 11; x <= 19 ; x++)
{
if (line[x] != ';')
{
name[x-11] = line[x];
}
else
{
// NULL TERMINATE THE ARRAY
name[x-11] = '\0';
//STOP
x = 20;
}
}
// save name to nameArray
for(int x = 0; x <= 11 ; x++)
{
nameArray[linenumber][x] = name[x];
}
// NULL TERMINATE THE ARRAY
id[10] = '\0';
// save id to idArray
for(int x = 0; x <= 11 ; x++)
{
idArray[linenumber][x] = id[x];
}
linenumber +=1;
arrayPlace = 0;
} //else
} //while
// close the file:
myFile.close();
}
else
{
// if the file didn't open, print an error:
Serial.println("error opening db.txt");
}
}
void writetoSD(String cardID)
{
//open file for writing
myFile = SD.open("test.txt", FILE_WRITE);
// if the file opened okay, write to it:
if (myFile)
{
Serial.println("Writing time to test.txt...");
DateTime now = rtc.now();
myFile.print(now.day(), DEC);
myFile.print('/');
myFile.print(now.month(), DEC);
myFile.print('/');
myFile.print(now.year(), DEC);
myFile.print(' ');
myFile.print(now.hour(), DEC);
myFile.print(':');
myFile.print(now.minute(), DEC);
myFile.print(':');
myFile.print(now.second(), DEC);
myFile.print('\t');
Serial.println("Writing string to test.txt...");
myFile.println(cardID);
// close the file:
myFile.flush();
Serial.println("done.");
}
else
{
// if the file didn't open, print an error:
Serial.println("error opening test.txt");
}
}
void initializeRTC()
{
// GND en VCC aansluiting via pin 18 en 19
pinMode(18, OUTPUT); // set pin to output
digitalWrite(18, LOW); // GND pin dus LOW
pinMode(19, OUTPUT); // set pin to output
digitalWrite(19, HIGH); // VCC pin dus HIGH
#ifdef AVR
Wire.begin();
#else
Wire1.begin(); // Shield I2C pins connect to alt I2C bus on Arduino Due
#endif
rtc.begin();
Serial.print("RTC Initialized: ");
DateTime now = rtc.now();
Serial.print(now.day(), DEC);
Serial.print('/');
Serial.print(now.month(), DEC);
Serial.print('/');
Serial.print(now.year(), DEC);
Serial.print(' ');
Serial.print(now.hour(), DEC);
Serial.print(':');
Serial.print(now.minute(), DEC);
Serial.print(':');
Serial.print(now.second(), DEC);
Serial.println();
}
void initializeRFID()
{
rfid.begin(9600);
Serial.println("RFID initialized");
}

Answer 1

I think you are running out of RAM ,i have done a lot of data logging on SD card with Arduino and its a very resources taking job for the minial 2kb ram on the UNO (assuming u using UNO).
Try using MemoryFree() library before every place you see there might be problem to see if you are running outta memory?