ML6691

6

FUNCTIONAL DESCRIPTION

To describe the function of the ML6691, the device is

separated into three sections, the Transmit Section, the

Receive Section, and the Management section. Each

section is discussed below.

TRANSMIT SECTION

The transmit section is responsible for converting transmit

nibble data on the MII lines from the MAC into encoded

and ciphered transmit symbols, as shown in Figure 1. The

nibble input register samples the transmit nibble data on

the MII lines and passes the nibble data onto the encoder.

The encoder then converts the data to 4B/5B code (see

Table 1) under the direction of the state machine. The

state machine detects the leading edges of transmit enable

and impresses the start of frame delimiter — the JK pair —

ignoring the TXD<3:0> during these two symbol times. If,

while transmit enable is asserted, the transmit error is

asserted, the H symbol will be impressed except during

the time the JK pair is emitted. Following the trailing edge

of transmit enable, the end of frame delimiter — the TR

pair — is generated, after which the IDLE symbol is

generated to fill the space between frames. The encoded

data is then enciphered by performing a XOR with the

output of the cipher register.

The cipher register is a linear feedback shift register which

generates the cipher bit stream which is used to scramble

the transmit symbol data. The Boolean algebra expression

is listed below.

TCFR11 = TCFR6

TCFR10 = TCFR5

TCFR9 = TCFR4

TCFR8 = TCFR3

TCFR7 = TCFR2

TCFR6 = NOT (TCFR11 OR TCFR10 OR TCFR9 OR

TCFR8 OR TCFR7 OR TCFR6 OR TCFR5 OR

TCFR4 OR TCFR3 OR TCFR2) OR TCFR1

TCFR5 = TCFR11 XOR TCFR9

TCFR4 = TCFR10 XOR TCFR8

TCFR3 = TCFR9 XOR TCFR7

TCFR2 = TCFR8 XOR TCFR6

TCFR1 = TCFR7 XOR TCFR5

The enciphered data are then passed to the symbol output

register which drives the generated symbol data out to the

PMD transceiver.

Collision detect is implemented by noting the occurrence

of reception during transmission. A linkfail indication at

any time causes an immediate transition to the IDLE state

and supersedes any other transmit operation.

Figure 1. Transmit Section Block Diagram

HEX/4B

SYMBOL/5B

HEX/4B

SYMBOL/5B

0000

11110

J

11000

0001

01001

K

10001

0010

10100

T

01101

0011

10101

R

00111

0100

01010

H

00100

0101

01011

IDLE

11111

0110

01110

0111

01111

1000

10010

1001

10011

1010

10110

1011

10111

1100

11010

1101

11011

1110

11100

1111

11101

Table 1. 4B/5B Encoding Table

TRANSMIT

ERROR

(TXER)

LINK

FAIL

TRANSMIT

STATE MACHINE

ENCODER/

ENCIPHER

SYMBOL OUTPUT

REGISTER

CARRIER AND

COLLISION

DETECT

NIBBLE INPUT

REGISTER

CIPHER

REGISTER

TRANSMIT NIBBLE

CLOCK

(TXCLK)

TRANSMIT

ENABLE

(TXEN)

TRANSMIT SYMBOL CLOCK

(TXC)

TRANSMIT NIBBLE DATA

(TXD[3:0])

TRANSMIT SYMBOL DATA

(TSM[4:0])

RECEIVING

TRANSMITTING

COLLISION

(COL)

CARRIER SENSE

(CRS)