Digi NS9750 Benutzerhandbuch

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NS9750 supports external Ethernet CAM filtering, which requires an external CAM 
controller to operate in conjunction with the MAC inside NS9750. The interface to the 
CAM controller is provided through GPIO in NS9750. External CAM filtering uses these 
bits:

GPIO[19] configured as an output and for function 0

GPIO[18] configured as an input and for function 0

For MII PHYs, the 

CAM_REQ

 

(GPIO[19])

 signal is driven high by NS9750, to identify the 

beginning of each Ethernet frame being transferred to NS9750. The signal is driven 
high coincident with the 6

th

 nibble of the packet from the frame. The external CAM 

hardware must monitor the MII receive interface between the PHY and the MAC 
waiting for the 

CAM_REQ

 assertion. When 

CAM_REQ 

is asserted, the CAM hardware can 

extract the destination address field from the MII receive bus. As an alternative, the 
external CAM hardware can use the 

RX_DV

 signal from the MII PHY to detect the start 

of a frame.

After performing the necessary destination address lookup, the incoming frame can 
be rejected by CAM filtering hardware by asserting the 

CAM_REJECT

 

(GPIO[20])

 input 

high. This signal must be asserted no later than the 4

th

 nibble from the end of the 

frame. Once it is asserted, it must remain asserted until three 

RX_CLKs

 after the end 

of the frame, to guarantee that the 

RX_WR

 logic has captured it. For example, a 64-

byte frame contains 128 nibbles of data on the MII interface. 

CAM_REJECT

 must be 

valid by the 123

rd

 nibble of data (first nibble is 0

th

 nibble). 

Figure 68 shows the timing relationship between the 

CAM_REQ

, 

CAM_REJECT

, and MII 

receive interface signals when using an MII PHY.