Hi, I am facing an issue in handling the usart interrupt. I am using two interrupts, external interrupt IRQ0 and usart receive interrupt.For testing purpose, IRQ0 is low all the time. In the below program, when i receive usart data, usart0_handler is executed and then IRQ0_handler also gets automatically executed.
so every time usart data arrives,both interrupt routines usart and irq0 gets executed.
I am using greenhills multi compiler. Can any body please point out error in this code.
Thanks in advance.

#include "project.h"

#define BOARD_MCK 48000000

void PIO_Init (void);
void ext_intr_1pps(void);
void USART0_handler(void);
void uart0_init(void);

AT91PS_USART COM0 = AT91C_BASE_US0;

unsigned char ch;

//esternal interrupt handler
void IRQ0_handler(void)
{
AT91F_US_PutChar (COM0,ch);
}

//usart receive interrupt handler
void USART0_handler (void )
{
unsigned int status_reg = COM0->US_CSR;
if(status_reg & AT91C_US_RXRDY)
{
ch=AT91F_US_GetChar(COM0);
}
}

int main()
{
AT91F_PMC_EnablePeriphClock ( AT91C_BASE_PMC, 1 << AT91C_ID_PIOA ) ;
AT91F_PMC_EnablePeriphClock ( AT91C_BASE_PMC, 1 << AT91C_ID_PIOB ) ;
PIO_Init ();
uart0_init();
AT91F_PIO_ClearOutput( AT91C_BASE_PIOB, AT91C_PIO_PB21 | AT91C_PIO_PB20);
AT91F_AIC_ConfigureIt(AT91C_BASE_AIC,AT91C_ID_IRQ0,AT91C_AIC_PRIOR_HIGHEST,AT91C_AIC_SRCTYPE_POSITIVE_EDGE,IRQ0_handler);
AT91F_AIC_EnableIt (AT91C_BASE_AIC, AT91C_ID_IRQ0);

for (;;);

}

void PIO_Init (void)
{
AT91C_BASE_PIOB->PIO_PPUER = AT91C_PIO_PB21 |AT91C_PIO_PB20;
AT91F_PIO_CfgOutput( AT91C_BASE_PIOB, AT91C_PIO_PB21 | AT91C_PIO_PB20);
}


void uart0_init ( void )
{
AT91F_PIO_CfgPeriph(AT91C_BASE_PIOA,((unsigned int)AT91C_PA0_RXD0)|((unsigned int)AT91C_PA1_TXD0),0);//Peripheral B

// First, enable the clock of the USART
AT91F_PMC_EnablePeriphClock (AT91C_BASE_PMC,1 << AT91C_ID_US0);
AT91F_US_Configure (COM0,AT91B_MCK,AT91C_US_ASYNC_MODE,9600,0);

// Enable usart transmit and receive but receive interrupt only
COM0->US_CR = AT91C_US_RXEN | AT91C_US_TXEN;
AT91F_US_EnableIt(COM0,AT91C_US_RXRDY );

COM0->US_RTOR = 0x1F40; //COUNT FOR TIMEOUT OF 832ms.//for faster queries this time should be less.
COM0->US_TTGR = 0;
// open Usart 0 interrupt
AT91F_AIC_ConfigureIt(AT91C_BASE_AIC,AT91C_ID_US0,AT91C_AIC_PRIOR_LOWEST,AT91C_AIC_SRCTYPE_INT_HIGH_LEVEL,USART0_handler);
AT91F_AIC_EnableIt (AT91C_BASE_AIC, AT91C_ID_US0);
}