AM79C978A AMD [Advanced Micro Devices], AM79C978A Datasheet - Page 77

• Manufacturer Part Number: AM79C978A
• Description: Single-Chip 1/10 Mbps PCI Home Networking Controller
• Manufacturer: AMD [Advanced Micro Devices]

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 Management Interfaces section.
AID Transmit Timing
The transmitter encodes the Access 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 be-
ginning of the Access ID symbol) defined in Table 14.
AID Receive Timing
The receiver allows for jitter by establishing a window
around each legal pulse position. This asymmetrical win-
dow 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 in-
tervals (AID symbols 0 through 7). In general during a col-
lision, 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 normally
blanks its receive input immediately after transmitting
(and simultaneously receiving) a pulse. Therefore, only
when a transmitting station receives pulses in a position
earlier than the position it transmitted will it recognize it as
a pulse transmitted by another station and signal a colli-
sion.
For this reason, guaranteed collision detection is possible
only as long as the spacing between successive possible
pulse positions in an AID symbol (20 TICs or 2.3 s) 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 detec-
tion purposes (1.5 s round trip delay plus margin).
Table 14. Access ID Symbol Pulse Positions and
Position
Pulse
1
2
3
4
TICs from Beginning of AID
Encoding
Symbol
106
126
66
86
Bit Encoding
00
01
10
11
Am79C978A
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 be-
ginning of its transmitted packet and the beginning of the
received colliding packet is between -1.5 s and +1.5 s.
In general, any received pulse at a HomePNA station that
does not conform to the pulse position requirements of
AID symbols 0 through 7 shall indicate a collision on the
wire. When a transmitting station senses a collision, it
emits a JAM signal to alert all other stations to the colli-
sion. The following conditions signify a COLLISION
event:
1. A HomePNA station receives an AID that does not
2. A HomePNA station receives a pulse outside the
3. A HomePNA station receives a pulse inside the
As in all cases, pulses received during a blanking interval
are ignored.
Passive stations (stations not actively transmitting during
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 one 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 select
a new random AID.
Silence Interval (AID symbol 7)
The Access ID symbols are followed by a fixed silence in-
terval 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.
match the one being sent.
AID_GUARD INTERVAL in AID intervals 0 to 7.
SILENT_INTERVAL (AID symbol 7).
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