CS89712 datasheet(61/170 Pages) CIRRUS

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CS89712

DS502PP2

mand is issued, or if the entire frame has been read.

See Section 2.32, “Basic Receive Operation”.

2.33 Receive Frame Address Filtering

The Ethernet port is equipped with a Destination

Address (DA) filter used to determine which re-

ceive frames will be accepted. The DA filter can be

configured to accept the following frame types:

2.33.1

Individual Address Frames

For all Individual Address frames, the first bit of

the DA is a "0" (DA[0] = 0), indicating that the ad-

dress is a Physical Address. The address filter ac-

cepts Individual Address frames whose DA

matches the Individual Address or whose hash-fil-

tered DA matches one of the bits programmed into

the Logical Address Filter (the hash filter is de-

scribed later in this section).

2.33.2

Multicast Frames

For Multicast Frames, the first bit of the DA is a "1"

(DA[0] = 1), indicating that the frame is a Logical

Address. The address filter accepts Multicast

frames whose hash-filtered DA matches one of the

bits programmed into the Logical Address Filter

(the hash filter is described later is this section). As

shown in Table 28, Broadcast Frames can be ac-

cepted as Multicast frames under a very specific set

of conditions.

2.33.3

Broadcast Frames

Frames with DA equal to FFFF.FFFF.FFFFh are

broadcast frames. In addition, the CS89712 can be

configured for Promiscuous Mode, in which case it

will accept all receive frames, irrespective of DA.

2.33.4

Destination Address Filter

The DA filter is configured by five DA filter bits in

the RxCTL register: IAHashA, PromiscuousA,

MulticastA, IndividualA, and BroadcastA. Four of

these bits are associated with four status bits in the

RxEvent register: IAHash, Hashed, IndividualAdr,

and Broadcast. The RxEvent register reports the re-

sults of the DA filter for a given receive frame. See

Section 3.18.4 on page 119 for RxCTL details.

The IAHashA, MulticastA, IndividualA, and

BroadcastA bits are used independently. As a re-

sult, many DA filter combinations are possible. For

example, if MulticastA and IndividualA are set,

then all frames that are either Multicast or Individ-

ual Address frames are accepted. The Promiscu-

ousA bit, when set, overrides the other four DA

bits, and allows all valid frames to be accepted.

Table 29 summarizes the configuration options

available for DA filtering.

It may become necessary for the software to change

the Destination Address (DA) filter criteria without

resetting the Ethernet port. This can be done as fol-

lows:

1) Clear SerRxON (LineCTL register bit 6) to pre-

vent any additional receive frames while the fil-

ter is being changed.

2) Modify the DA filter bits (B, A, 9, 7, and 6) in

the RxCTL register. Modify the Logical Ad-

dress Filter, if necessary.

3) Set SerRxON to re-enable the receiver.

Because the receiver has been disabled, the

CS89712 will ignore frames while the software is

changing the DA filter.

2.33.5

Hash Filter

The hash filter is used to help determine which

Multicast frames and which Individual Address

frames should be accepted by the CS89712.

2.33.5.1

Hash Filter Operation

See Figure 21. The DA of the incoming frame is

passed through the CRC logic, generating a 32-bit

CRC value. The six most-significant bits of the

CRC are latched into the 6-bit hash register (HR).

The contents of the HR are passed through a 6-to-

64-bit decoder, asserting one of the decoder’s out-

puts. The asserted output is compared with a corre-

sponding bit in the 64-bit Logical Address Filter,

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