[ethmac] Divided clock

[zou.yixin@zte.com.cn]

Hi,
   The ethernet mac  support 10/100Mbps data rate. 
It is said in the spec that MTxClk and MRxClk are provided by PHY ,which operate at 25MHz(100Bps) and 2.5M(10Mbps).

My question is:
If the clock to the MAC and PHY are provided by an external clock and the frequecy is 25MHz,
suppose they can determine the speed by auto-negotiation,then  how can they work at the rate of 10Mbps?
Is divided clock  necessary? Can you give me some advise on divided clock?

Thanks,
yxzhou