Tiny ARTX on AT91M42800A using PIT

Hello
Has anybody tried to get it work on the 42800 cpu?
i experience problems in adapting the artxconfig.c
I wanted to use the cpu's PIT to generate the os_clock_interrupt, but didn't get it work.
So i wrote a simple programm to test the PIT and got it work.

the source:

void os_clock_interrupt_2 (void) __irq
{
	PIOB_SODR = LED_MASK;
	if(bActiveLed == 0x80) bActiveLed = 1;
	bActiveLed <<= 1;
	(ST_SR & 0x00000001); //without this it won't work, but i don't know why?!

	AIC_EOICR = 0xffff; //signal end of ISR
}

void main (void)
{
	PIOB_PER = LED_MASK;
	PIOB_OER = LED_MASK;
	PIOB_SODR = LED_MASK;

	//PIT INIT
	ST_PIMR = 0x00007500;//Set PIT Reload Value (SCK Cycles)
	ST_IMR = 1;	//Enable PIT_IRQ
	ST_IER = 1;
//AIC Init
  AIC_SPU = (DWORD)os_def_interrupt_2;
  AIC_SVR12  = (DWORD)os_clock_interrupt_2;
  AIC_SMR12  = AIC_SRCTYPE_INT_EDGE_TRIGGERED | 0;
  AIC_IECR = (1<<12);

//Generate 1st IRQ to test
	AIC_ISCR = (1<<12);

	while(1)
	{
		PIOB_CODR = (dword)(bActiveLed<<8);
	}
}

but it won't work an Tiny ARTX.
The Sys is running, but the scheduler doesn't change the running task.

i changed the atrx config defines to this:

#define OS_TIM        (1 << 12)             // Interrupt Mask
#define OS_TRV        ((DWORD)(((double)OS_CLOCK*(double)OS_TICK)/1E6)-1)	//Reload Value in Clock Cycles

#define OS_TVAL       (0) // Timer Value --> n/a
#define OS_TOVF       (ST_SR && 0x00000001) // Overflow Flag
#define OS_TREL()     0;// Timer Reload --> n/a
#define OS_TSOVF()    AIC_ISCR  = OS_TIM;      //  Set Overflow

//Interrupt Ack
#define OS_TIACK()    (ST_SR & 0x00000001);
//System Timer Init
#define OS_TINIT()
ST_PIMR = OS_TRV;ST_IMR = 1;ST_IER = 1;	AIC_SPU = (DWORD)os_def_interrupt; AIC_SVR12  = (DWORD)os_clock_interrupt; AIC_SMR12  = AIC_SRCTYPE_INT_EDGE_TRIGGERED | 0;  AIC_IECR = OS_TIM;	AIC_ISCR = OS_TIM;

#define OS_LOCK()       AIC_IECR = 0;

#define OS_UNLOCK()     AIC_IECR = OS_TIM;

the actual situation: the os_clock_interrupt is only called once, after timer initialization.
Anybody an idea what's the problem with this? maybe i'll use a normal Timer instead of the PIT?