My usart interrupt occure only one time!!

Hi.
who can help me !!
my interrupt occure only one time. I dont know why?? can any body help me why my program dosent work correctly?
my startup is SAM7.s

thise is my code:

#include "at91sam7x256.h"
#include "lib_at91sam7x256.h"
#include "delay.h"
void Usart0IrqHandler(void);
int main (void)
{

int rx; unsigned int rxok; int errorcounter;

*AT91C_PMC_PCER=0xC0000040; //Peripheral Clock Enable *AT91C_PIOA_ASR=0X0000003; *AT91C_PIOA_BSR=0X0000000; *AT91C_PIOA_PDR=0x00000003; //Disable I/O,PA5=RXD0,PA6=TXD0 *AT91C_US0_IDR=0xFFFFFFFF; //ALL OFF USART INTERRUPT DISABLE *AT91C_US0_CR=0x000000AC; //RESET RX & TX *AT91C_US0_BRGR=0x139; //BARD RATE:9600 *AT91C_US0_TTGR=0x00000100; //SET TIME GARD=0

AT91F_PDC_Open(AT91C_BASE_PDC_US0); *AT91C_US0_MR=0X8C0; //8bit,No,1Bit Stop,Asaynchron mode *AT91C_US0_CR=0x50; //ENABLE RX & ENABLE tx *AT91C_US0_IER =0x01; //rxbufer full interrupt is set *AT91C_AIC_IDCR=0x40; //DISABLE THE INTERRUOT ON THE INTERRUPT CONTROLLER AT91C_AIC_SVR[AT91C_ID_US0] = (unsigned int)Usart0IrqHandler; // Set the USART0 IRQ handler address in AIC AT91C_AIC_SMR[AT91C_ID_US0] =(0x00000007); *AT91C_AIC_IECR=0x40; *AT91C_AIC_FFDR=0XFFFFFFFF;

while(1){ *AT91C_US0_CR=0x100; rxok=*AT91C_US0_CSR;

*AT91C_US0_IER =0x4; while(rxok==0x1a1b){ rxok=*AT91C_US0_CSR; delay_ms(10); errorcounter=errorcounter+1; if (errorcounter==100) { *AT91C_US0_CR=0xc; // resettx/resetrx errorcounter=0; } }
} return 0; } void Usart0IrqHandler(void){

int rx; int i;

unsigned int rxok; int errorcounter; errorcounter=0; kk=kk+1; rxok=*AT91C_US0_CSR; if (rxok && 0x1){ rx=(*AT91C_US0_RHR & 0xff); *AT91C_US0_THR=kk;//rx&0xff; } else while(rxok==0x1a1b){ rxok=*AT91C_US0_CSR; delay_ms(10); errorcounter=errorcounter+1; if (errorcounter==1000) { *AT91C_US0_CR=0xc; // resettx/resetrx errorcounter=0; } };

*AT91C_US0_CR=0x2100; *AT91C_US0_IDR =0x4; rxok=*AT91C_US0_CSR;

for (i=0; i < 8; i++) { AT91C_BASE_AIC->AIC_EOICR = 0; }

return; }