AM79C901AVCT datasheet(34/90 Pages) AMD

34 / 90 page

Am79C901A

PR ELI M IN AR Y

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

further in the Mode Interface section.

AID Transmit Timing: The transmitter encodes the Ac-

cess ID in a pulse position in each 129 TIC interval.

Each AID symbol interval must have only one pulse.

Pulse transmission must start in only one of the four

possible positions (measured from the beginning of the

Access ID symbol) defined in Table 7.

AID Receive Timing: The receiver allows for jitter by

establishing a window around each legal pulse posi-

tion. This asymmetrical window is two TICS wide on

one side of the position and one TIC wide on the other.

A pulse that arrives 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, recognize the event

as a collision, and alert other stations with a JAM sig-

nal. 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 roundtrip 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 roundtrip 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 beginning 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 sta-

tions to the collision. The following conditions signify

a COLLISION 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

interval 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 stations are randomly picked

by each individual station from the set of AID numbers

described in the management section. During opera-

tion, each HomePNA station monitors HomePNA

frames received on the wire. If it detects another Home-

PNA station using the same AID, it will select 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.

Data Symbols

Data symbols encode data for a much higher transmis-

sion rate, and they do not allow collision detection.

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