Arrow DECA User Manual page 103

In the MAC Options tab (See
DP83620. The MDIO Module should be included, as it is used to generate a 2.5MHz MDC clock
for the PHY chip from the controller's source clock(here a 100MHz clock source is expected) to
divide the MAC control register interface clock to produce the MDC clock output on the MDIO
interface. The MAC control register interface clock frequency is 100MHz and the desired MDC
clock frequency is 2.5MHz, so a host clock divisor of 40 should be used.
DECA User Manual
Figure 7-10 MII interface MAC Configuration
Figure 7-11), users should set up proper values for the PHY chip
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