Timer interrupt mode in qsys

Hi all,

when i am trying to configure IOWR_ALTERA_AVALON_TIMER_CONTROL register

IOWR_ALTERA_AVALON_TIMER_CONTROL(TIMER_0_BASE,0X7);

then control register is not getting written..

what to do...kindly help how to configure timer....configured in custom mode....20ms....timer....IRQ enabled