External cam filtering – Digi NS9750 User Manual

Page 358

E x t e r n a l C A M f i l t e r i n g

3 3 4

N S 9 7 5 0 H a r d w a r e R e f e r e n c e

External CAM filtering

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

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

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

nibble of data (first nibble is 0

nibble).

Figure 68 shows the timing relationship between the

CAM_REQ

CAM_REJECT

, and MII

receive interface signals when using an MII PHY.