DP83256 datasheet(11/146 Pages) TI1

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30 Functional Description (Continued)

Framing may be temporarily suspended (ie framing hold)

in order to maintain data integrity

Detecting JK

The JK symbol pair can be used to detect the beginning of a

frame during Active Line State (ALS) and Idle Line State

(ILS) conditions

While the Line State Detector indicates Idle Line State the

receiver ‘‘reframes’’ upon detecting a JK symbol pair and

enters the Active Line State

During Active Line State acceptance of a JK symbol (re-

framing) is allowed for any on-boundary JK which is detect-

ed at least 15 byte times after the previous JK

During Active Line State once reframed on a JK a subse-

quent off-boundary JK is ignored even if it is detected be-

yond 15 byte times after the previous JK

During Active Line State an Idle or Ending Delimiter (T)

symbol will allow reframing on any subsequent JK if a JK is

detected at least 15 byte times after the previous JK

Detecting HALT-HALT AND HALT-QUIET

During Idle Line State the detection of a Halt-Halt or Halt-

Quiet symbol pair will still allow the reframing of any subse-

quent on-boundary JK

Once a JK is detected during Active Line State off-bounda-

ry Halt-Halt or Halt-Quiet symbol pairs are ignored until the

Elasticity Buffer (EB) has an opportunity to recenter They

are treated as violations

After recentering on a Halt-Halt or Halt-Quiet symbol pair

all off boundary Halt-Halt or Halt-Quiet symbol pairs are ig-

nored until the EB has a chance to recenter during a line

state other than Active Line State (which may be as long as

28 byte times)

SYMBOL DECODER

The Symbol Decoder is a two level system The first level is

a 5-bit to 4-bit converter and the second level is a 4-bit

symbol pair to byte-wide code converter

The first level latches the received 5-bit symbols and de-

codes them into 4-bit symbols Symbols are decoded into

two types data and control The 4-bit symbols are sent to

the Line State Detector and the second level of the Symbol

Decoder See Table 3-1 for the 5B4B Symbol Decoding

list

The second level translates two symbols from the 5B4B

converter and the line state information from the Line State

Detector into the National byte-wide code

LINE STATE DETECTOR

The ANSI X3T95 FDDI Physical Layer (PHY) standard

specifies eight Line States that the Physical Layer can

transmit These Line States are used in the Connection

Management process They are also used to indicate data

within a frame during normal operation

The Line States are reported through the Current Receive

State Register (CRSR)

Receive Condition Register A

(RCRA) and Receive Condition Register B (RCRB)

TABLE 3-1 5B4B Symbol Decoding

Symbol

Incoming 5B

Decoded 4B

11110

0000

01001

0001

10100

0010

10101

0011

01010

0100

01011

0101

01110

0110

01111

0111

10010

1000

10011

1001

10110

1010

10111

1011

11010

1100

11011

1101

11100

1110

11101

1111

I (Idle)

11111

1010

H (Halt)

00100

0001

JK (Starting

11000 and

1101

Delimiter)

10001

T (Ending

01101

0101

Delimiter)

R (Reset)

00111

0110

S (Set)

11001

0111

Q (Quiet)

00000

0010

V (Violation)

00001

0010

00010

0010

00011

0010

00101

0010

00110

0010

01000

0010

01100

0010

10000

0010

Note

V denotes PHY Invalid or an Elasticity Buffer stuff byte

I denotes Idle symbol in ILS or an Elasticity Buffer stuff byte

LINE STATES DESCRIPTION

Active Line State

The Line State Detector recognizes the incoming data to be

in the Active Line State upon the reception of the Starting

Delimiter (JK symbol pair)

The Line State Detector continues to indicate Active Line

State while receiving data symbols Ending Delimiter (T

symbols) and Frame Status symbols (R and S) after the JK

symbol pair

Idle Line State

The Line State Detector recognizes the incoming data to be

in the Idle Line State upon the reception of 2 Idle symbol

pairs nominally (plus up to 9 bits of 1 in start up cases)

Idle Line State indicates the preamble of a frame or the lack

of frame transmission during normal operation Idle Line

State is also used in the handshake sequence of the PHY

Connection Management process

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