RGMII clock source for K66

[geovarghese]

We are using K66 in one of our projects. The clock source of K66 is external oscillator with 25MHz input. We need to enable the Ethernet interface in RGMII mode. It seems like the RGMII reference clock is tied to EXTAL and it requires 50Mhz for its operation. Can any one please tell me how to make the Ethernet module work in my board?

[xiangjun_rong]

Hi, Geo,

You are right, if you use GMII, each clock transfers 4 bits, the required clock is 25MHz, the ethernet line speed is 4*25MHz=100M bps. If you use RGMII, each clock transfers 2 bits, the required clock is 50MHz, the ethernet line speed is 2*50MHz=100M bps. 

I attach the schematics of FRDM-K64F, The PHY is KSZ8081, on the FRDM-K64F board, the PHY is connected to 25MHz crystal, but the PHY can generate 50MHz clock based on the 25MHz crystal and can output the 50MHz clock to the RMII_RXCLK pin, which is connected to EXTAL pin of K64, the K64 can use PLL to get high clock frequency for core/system/peripheral based on the 50MHz clock so that the core can synchronize with the ethernet module.

Hope it can help you

BR

Xiangjun rong