Experiment with both CC registers for Timer A on the MSP430G2231

timerA.c

#include <msp430g2211.h>


#define LED_0 BIT0
#define LED_1 BIT6
#define LED_OUT P1OUT
#define LED_DIR P1DIR



unsigned int timerCount = 0;
unsigned int timerCount1 = 0;
void main(void)
{
	WDTCTL = WDTPW + WDTHOLD; // Stop watchdog timer
	LED_DIR |= (LED_0 | LED_1); // Set P1.0 and P1.6 to output direction
	LED_OUT &= ~(LED_0 | LED_1)       ; // Set the LEDs off
	TACCTL0 = CCIE;
	TACCTL1 = CCIE;
	TACTL = TASSEL_2 | MC_2; // Set the timer A to SMCLCK, Continuous
	// Clear the timer and enable timer interrupt
	
	__enable_interrupt();
	
	__bis_SR_register(LPM0 + GIE); // LPM0 with interrupts enabled

}


// Timer A0 interrupt service routine
#pragma vector=TIMERA0_VECTOR
__interrupt void Timer_A0 (void)
{
	timerCount = (timerCount + 1) % 8;
	if(timerCount ==0)
		P1OUT ^= (LED_0);
}

// Timer A1 interrupt service routine
#pragma vector=TIMERA1_VECTOR
__interrupt void Timer_A1 (void)
{
	TAIV &= ~(0x02); // CLEAR INTERRUPT
	timerCount1 = (timerCount1 + 1) % 9; //4096;
	if(timerCount1 ==0)
		P1OUT ^= (LED_1);
}

Hi PhirePhly. I'm trying to understand how to use timers in micro-controllers, specifically the msp430g2231. I'm dissecting your code shown in gist 836262 and I understand a lot of it, but I have a few questions. The first one I can think relates to line 34: timerCount = (timerCount + 1) % 8.
What does the %8 have to do with the code. Why is it needed? Same applies for the "%9" in line 44. When I run the code as is, everything works, but when I delete the %8, and %9, I encounter errors. I'm still familiarizing myself with the C/C++ code, so forgive me if this question seems trivial. Thanks

The % is the modulus operator. Think of it as the remainder from dividing by that number.

So line 34 takes the modulus base 8 of (timerCount + 1), so timerCount is going to go 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, ... This is done so that line 35 is true every 8 increments, instead of every 65536 increments, which is what you tried.

I can appreciate that you might be challenged by my code. I tend to use and abuse as many symbols from the language as I can, to produce terse code. Best resource on the matter is K&R C:
http://www.amazon.com/Programming-Language-2nd-Brian-Kernighan/dp/0131103628/

Thanks for your reply. You explained it clearly. I now understand the purpose of the modulus operator and how it is applied in the code.