Turn your Arduino into a Magnetic Card Reader!

Step 6 Detect the Card Leaving the Reader

Detect when a card has gone

Formally, one would sample the /CARD_PRESENT pin to see if it's gone HIGH again, but we don't need no steenkin' /CARD_PRESENT taking up another I/O port. This is where those timers come in.

Every time the interrupt is called because we've detected a falling edge on /STROBE, we stop a timer, clear the timer value and start reading. When we've finished reading we start the timer again. Repeat ad nauseum, or until the timer reaches a certain value. That means that the last interrupt has been called and no more data has come in, so we assume that's it and start processing the data we've collected.

For timers, we use TIMER1, ie the 16-bit timer. I'm using a 16 Mhz resonator externally to my AVR. If you're using an arduino, then you probably are, too. So, I've chosen a prescaler value of 1024 which means every (16,000,000 / 1024) times the timer will increment. That is to say, it will 'tick' 15,625 times a second. The /CARD_PRESENT will go HIGH indicating the card has left the reader about 150ms after the last data bit. Knowing this, I just decided to check about every 1/4 of a second. That would look something like this:

    ( ((F_CPU) / PRESCALER) / 4 )

which turns out to be around 3900. So, when the timer counter TCNT1 reaches 3900, then I know it's been about 300ms and I can pretty safely conclude that the card has left the reader. Easy.

#define PRESCALER	     1024#define CHECK_TIME     ( (F_CPU / PRESCALER) / 4 )		// 250 ms#define StartTimer()     BSET(TCCR1B,CS10), BSET(TCCR1B,CS12)	// 1024 prescaler#define StopTimer()      BCLR(TCCR1B,CS10), BCLR(TCCR1B,CS12)#define ClearTimer()     (TCNT1 = 0)

You've seen in the ISR where the timer is started, stopped, and cleared on each interrupt. Now, in the main loop we just check to see if the timer counter has reached our target value, and if so, start the data processing.

for (;;){	if( TCNT1 >= CHECK_TIME)	{			
StopTimer();		
ClearTimer();		
ProcessData();		
ReadData();					
idx = 0;		
bit = 6;		
bDataPresent = 0;		
memset(&buff,0,MAX_BUFF_SZ1);	
}	
}

Now it's safe to process the data.