ENC28J60-CSLASHML datasheet(53/96 Pages) MICROCHIP

53 / 96 page

Preliminary

DS39662B-page 51

ENC28J60

8.1

Unicast Filter

The unicast receive filter checks the destination

address of all incoming packets. If the destination

address exactly matches the contents of the MAADR

registers, the packet will meet the unicast filter criteria.

8.2

Pattern Match Filter

The pattern match filter selects up to 64 bytes from the

incoming packet and calculates an IP checksum of the

bytes. The checksum is then compared to the EPMCS

registers. The packet meets the pattern match filter

criteria if the calculated checksum matches the EPMCS

registers. The pattern match filter may be useful for

filtering packets which have expected data inside them.

To use the pattern match filter, the host controller must

program the pattern match offset (EPMOH:EPMOL), all

of the pattern match mask bytes (EPMM7:EPMM0) and

the

pattern

match

checksum

pair

(EPMCSH:EPMCSL). The pattern match offset should

be loaded with the offset from the beginning of the des-

tination address field to the 64-byte window which will

be used for the checksum computation. Within the

64-byte window, each individual byte can be selectively

included or excluded from the checksum computation

by setting or clearing the respective bit in the pattern

match mask. If a packet is received which would cause

the 64 byte window to extend past the end of the CRC,

the filter criteria will immediately not be met, even if the

corresponding mask bits are all ‘0’. The pattern match

checksum registers should be programmed to the

checksum which is expected for the selected bytes.

The checksum is calculated in the same manner that

the

DMA

module

calculates

checksums

(see

Section 13.2 “Checksum Calculations”). Data bytes

which have corresponding mask bits programmed to ‘0’

are completely removed for purposes of calculating the

checksum, as opposed to treating the data bytes as

zero.

As an example, if the application wished to filter all

packets having a particular source MAC address of

00-04-A3-FF-FF-FF, it could program the pattern match

offset to 0000h and then set bits 6 and 7 of EPMM0 and

bits 0, 1, 2 and 3 of EPMM1 (assuming all other mask

bits are ‘0’). The proper checksum to program into the

EPMCS registers would be 0x5BFC. As an alternative

configuration, it could program the offset to 0006h and

set bits 0, 1, 2, 3, 4 and 5 of EPMM0. The checksum

would still be 5BFCh. However, the second case would

be less desirable as packets less than 70 bytes long

could never meet the pattern match criteria, even if

they would generate the proper checksum given the

mask configuration.

Another example of a pattern matching filter is

illustrated in Figure 8-3.

FIGURE 8-3:

SAMPLE PATTERN MATCH FORMAT

EMPOH:EPMOL = 0006h

FCS

Type/Length

Data

Bytes used for

Checksum Computation

EPMM7:EPMM0 = 0000000000001F0Ah

11 22 33 44 55 66 77 88 99 AA BB CC 00 5A

09 0A 0B 0C 0D . . . 40 . . . FE 45 23 01

Received

Data

Field

0 1 2 3 4 5

6 7 8 9 10 11

12 13

14 15 16 17 18 . . . 70 . . .

Byte #

64-byte Window used

for Pattern Match

Input Configuration:

Values used for Checksum Computation = {88h, AAh, 09h, 0Ah, 0Bh, 0Ch, 0Dh, 00h}

EPMCSH:EPMCSL = 563Fh

Note:

Received data is shown in hexadecimal. Byte numbers are shown in decimal format.

(00h padding byte added by hardware)

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