Accessing STM32F4 peripherals

Hi all,
on a Fez Cerberus I try to access the TIM3 registers of STM32F4 using GHI.OSHW.Hardware.LowLevel in order to measure a signal period with the input capture feature.
After I wrote some value in the TIM3 registers, when I try to read them I read always 0x00000000.
Is the TIM3 peripheral free to use or it is used by the firmware ?

The timer requires activation probably through some other registers.