AVR-Chat

On Thu, Apr 21, 2005 at 12:49:10PM -0700, Dave Hylands wrote:
> 
> Hi David,
> 
> > Then when the interrupt is serviced it is cleared by the CPU back to
> > zero.  BTW, the docs do not say precisely WHEN during the interrupt
> > service routine....  Anyway,  the docs say repeatedly that the flag
> > can be cleared by the user BY WRITING A 1 TO THE FLAG REGISTER.  I
> > have not tried this in real life, but I would have expected that the
> > SW should write a zero to the register.  But apparently writing a 1
> > to the register that already has a 1 will reset the 1.....
> 
> The reason that writing a 1 to it clears it is because if you write a
> 1 to some other bit in the same register, you're writing a zero to all
> of the other bits.

Believe it was Tony who wrote the >> quoted section above.

As for myself we're talking too much in generalities and no enough
specifics. So here is real code:

SIGNAL( SIG_INPUT_CAPTURE3 )
{
	//  Toggle edge sensitivity in preparation for next IRQ.
	//  Manual states this should be done early in this routine.
	//  Note toggling it here inverts the meaning of relay_dir.
	TCCR3B ^= (1<<ICES3);
	
	if( relay_cnt && ((TCCR3B & (1<<ICES3)) == relay_dir) ) {
		RELAY_DRIVE_P |= RELAY_DRIVE_B;	//  drive triac gate
		ETIFR  |= (1<<OCF3B);	//  clear pending, just in case
		ETIMSK |= (1<<OCIE3B);	//  enable OC3B
		TCNT3 = 0;		//  unstick timer
		relay_cnt--;
	} else {
		RELAY_DRIVE_P &= ~RELAY_DRIVE_B; //  make sure its off
		ETIMSK &= ~(1<<OCIE3B);
	}
}

SIGNAL( SIG_OUTPUT_COMPARE3B )
{
	RELAY_DRIVE_P &= ~RELAY_DRIVE_B;	//  clear triac gate
	ETIMSK &= ~(1<<OCIE3B);		//  disable myself (OC3B)
}

void
t3_init(void)
{
	RELAY_DRIVE_DDR |= RELAY_DRIVE_B;	//  enable output
	TCCR3A = 0;				//  no OC's connected
	
	//  mode 4, div by 64
	TCCR3B = (1<<WGM32) | (1<<CS31) | (1<<CS30);

	OCR3B = XTAL/64/1000;		//  1/1000th of 1 second
	ETIFR  |= (1<<ICF3);		//  clear any pending IRQ
	ETIMSK |= (1<<TICIE3);		//  enable input capture IRQ
	AC_XC_UTIL_DDR &= ~AC_XC_UTIL_B; //  make sure its an input
}

void
engage_relay(uint8_t onoff)
{
	//  relay direction must be set first, else lockout IRQ when setting
	if( onoff ) {
		relay_dir = 0;			//  rising edge ???
		ac_relay_state = 0;		//  need to verify value
	} else {
		relay_dir = (1<<ICES3);		//  falling edge ???
		ac_relay_state = 1;		//  need to verify value
	}
	relay_cnt = 3;		//  60 Hz, 3 cycles = 50 mS
}

The above isn't complete enough to compile, some external references and
defines are missing. But I think its complete enough to read. If the
"unstick timer" line is removed then SIG_OUTPUT_COMPARE3B is never
called. I don't know how to let TCNT3 free run. Its apparently stopped
in SIG_INPUT_CAPTURE3 as its contents are zero. But writing a zero
starts it counting so that SIG_OUTPUT_COMPARE3B is called when TCNT3
equals OCR3B. Notice I'm writing to a counter, not a control register.

-- 
David Kelly N4HHE, dkelly@HiWAAY.net
========================================================================
Whom computers would destroy, they must first drive mad.