System Clock issue on the PPC405GP

Hi,
my board is based on the PPC405GP / 266MHz.
We have just adopted IPL and Startup from Walnut BSP source.
The sleep command waits:

33,3 sec. for [sleep 1000]
3,3 sec. for [sleep 100]

From which PPC405 register the system clock is taken ?

Regards,
Robert