AM79C978VCSLASHW datasheet(78/261 Pages) AMD

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Am79C978

Table 12.

Access ID Symbol Pulse Positions and

Encoding

The next two AID symbols (5 and 6) encode four bits of

control word information. The MSB is encoded in AID

Symbol 5. Control word messages are described fur-

ther in the Management Interfaces section.

AID Transmit Timing

The transmitter encodes the Access ID in a pulse posi-

tion in each 128 TIC interval. Each AID symbol interval

must have only one pulse. Pulse transmission must

start in only one of the four possible positions (mea-

sured from the beginning of the Access ID symbol) de-

fined in Table 12.

AID Receive Timing

The receiver allows for jitter by establishing a window

around each legal pulse position. This window is -2 +1

TICS wide on either side of the position. A pulse that ar-

rives outside of the legal AID positions is considered a

COLLISION event.

Collisions

A Collision is detected only during Access ID and silent

intervals (AID symbols 0 through 7). In general during

a collision, a transmitting station will read back an AID

value that does not match its own and recognizes the

event as a collision and alerts other stations with a JAM

signal. Non-transmitting stations may also detect some

collisions by interpreting received non-conforming AID

pulses as collisions.

With two transmitters colliding, each transmitter nor-

mally blanks its receive input immediately after trans-

mitting (and simultaneously receiving) a pulse.

Therefore, only when a transmitting station receives

pulses in a position earlier than the position it transmit-

ted will it recognize it as a pulse transmitted by another

station and signal a collision.

For this reason, guaranteed collision detection is pos-

sible only as long as the spacing between successive

possible pulse positions in an AID symbol (20 TICs or

2.3

µs) is greater than the round trip delay between the

colliding nodes. At approximately 1.5 ns propagation

delay per foot, the maximum distance between two

HomePNA units must not be greater than 500 feet for

collision detection purposes (1.5

µs round trip delay

plus margin).

The following criteria must be met to guarantee reliable

collision detection:

At least one HomePNA station of a colliding group

must always detect a collision when the delay between

the beginning of its transmitted packet and the begin-

ning of the received colliding packet is between -1.5

µs

and +1.5

µs.

In general, any received pulse at a HomePNA station

that does not conform to the pulse position require-

ments of AID symbols 0 through 7 shall indicate a col-

lision on the wire. When a transmitting station senses a

collision, it emits a JAM signal to alert all other stations

to the collision. The following conditions signify a COL-

LISION event:

1. A HomePNA station receives an AID that does not

match the one being sent.

2. A HomePNA station receives a pulse outside the

AID_GUARD INTERVAL in AID intervals 0 to 7.

3. A HomePNA station receives a pulse inside the

SILENT_INTERVAL (AID symbol 7).

As in all cases, pulses received during a blanking inter-

val are ignored.

Passive stations (stations not actively transmitting dur-

ing the collision) cannot reliably detect collisions.

Therefore, once a collision is detected by a transmitting

station, the station must inform the rest of the stations

of the collision with a JAM pattern described below.

Only a transmitting station emits a JAM signal.

Once a collision is detected, the COLLISION signal to

the MAC interface is asserted and is not reset until the

MAC deactivates the TXEN signal.

JAM Signal

A JAM pattern consists of 1 pulse every 32 TICs and

continues until at least the end of the AID intervals.

After the AID interval, the JAM pattern will continue

until TXEN from the MAC is deactivated.

ACCESS ID Values

The access ID values for slave stations are picked by

each individual station randomly from the set of AID

slave numbers described in the management section.

During operation, each HomePNA station monitors

HomePNA frames received on the wire. If it detects an-

other HomePNA station using the same AID, it will se-

lect a new random AID.

Silence Interval (AID symbol 7)

The Access ID symbols are followed by a fixed silence

interval of 129 TICs. The receive blanking interval is

the same as that of the AID symbols (1 through 6).

Any pulses detected in the silence interval are consid-

ered a COLLISION event for transmitting stations and

are handled as described in the Collisions section.

Pulse

Position

TICs from Beginning of AID

Symbol

Bit Encoding

166

286

106

126

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