unsigned int period;
unsigned int pit_frequency;
period=100000;
pit_frequency=48;
AT91C_BASE_PITC->PITC_PIMR = period? (period * pit_frequency + 8) >> 4 : 0;
AT91F_AIC_ConfigureIt( AT91C_BASE_AIC, AT91C_ID_SYS, AT91C_AIC_SRCTYPE_INT_LEVEL_SENSITIVE,0, pit_irq_handler);
AT91C_BASE_PITC->PITC_PIMR |= AT91C_PITC_PITIEN | AT91C_PITC_PITEN;
AT91F_AIC_EnableIt(AT91C_BASE_AIC, AT91C_ID_SYS);

//прерывание
void pit_irq_handler(void)
{
unsigned int temp;      
temp = AT91C_BASE_PITC->PITC_PIVR;
}

#define NOFRAME    __arm __task
.........
void os_clock_interrupt (void) NOFRAME {
   /* Do task switch to clock demon: entered upon a clock interrupt. */