AM79C978AVCW datasheet(74/256 Pages) AMD

74 / 256 page

Am79C978A

DETAILED FUNCTIONS

1 Mbps HomePNA PHY

The integrated HomePNA transceiver is a physical

layer device supporting the HomePNA specification 1.1

for home phone line networking. It provides all of the

PHY layer functions required to support 1 Mbps data

transfer speeds over common residential phone wiring.

All data bits are encoded into the relative time position

of a pulse with respect to the previous one, the wave-

form on the wire consists of a 7.5 MHz carrier sinusoid

enclosed within an exponential (bell shaped) enve-

lope. The waveform is produced by generating four

7.5 MHz square wave cycles and passing them

through a bandpass filter.

The HomePNA PHY frame consists of a HomePNA

header that replaces the normal Ethernet 64-bit pream-

ble and delimiter and is prepended to a standard Ether-

net packet starting with the source address and ending

with the CRC.

Only the PHY layer and its parameters are modified

from that of the standard Ethernet implementation. The

HomePNA PHY layer is designed to operate with a

standard Ethernet MAC layer controller implementing

all the CSMA/CD protocol features.

The frame begins with a characteristic SYNC interval

that delineates the beginning of a HomePNA frame fol-

lowed by an Access ID (AID) which encodes 8 bits of

Access ID and 4 bits of control word. The Access ID is

used to detect collisions and is dynamically assigned,

while the control word carries speed and power infor-

mation.

The AID is followed by a silence interval, then 32 bits of

data reserved for PHY layer communication. These

bits are accessible via HPR20 and HPR21 and are for

future use.

The data encoding consists of two symbol types: an

AID symbol and a data symbol. The AID symbol is al-

ways transmitted at the same speed and encodes two

bits that determine the pulse position (one of four) rel-

ative to the previous pulse. The access symbol interval

is fixed.

The data symbol interval is variable. The arriving bit

stream is blocked into from 3 to 6 bit blocks according

to a proprietary (RLL25™) algorithm. The bits in each

block are then used to encode a data symbol. Each

symbol consists of a Data Inter Symbol Blanking Inter-

val (DISBI) and then a pulse at one of 25 possible po-

sitions. The bits in the data block determine the pulse

position. Immediately after the pulse a new symbol in-

terval begins. During the DISBI the receiver ignores all

incoming pulses to allow network reflections to die out.

Any station may be programmed to assume the role of

a PHY master and remotely command, via the control

word, the rest of the units on the network to change

their transmit speed or power level.

Many of the framing parameters are programmable in

the HomePNA PHY and will allow future modifications to

both transmission speed as well as noise and reflection

rejection algorithms.

Two default speeds are provided, low at 0.7 Mbps and

high at 1 Mbps. The center frequency is also program-

mable for future use.

HomePNA PHY Medium Interface

Framing

The HomePNA frame on the phone wire network con-

sists of a header generated in the PHY prepended to

an IEEE 802.3 Ethernet data packet received from the

MAC layer. See Figure 38.

When transmitting on the phone wire pair, the

HomePNA PHY first receives an Ethernet MAC frame

from the MAC. The 8 octets of preamble and delimiter

are stripped off and replaced with the HomePNA PHY

header described below, then transmitted on the phone

wire network.

During a receive operation, the reverse process is exe-

cuted. When a HomePNA frame is received by the

PHY, the header is stripped off and replaced with the

four octets of preamble and delimiter of the IEEE 802.3

Ethernet MAC frame specification and then passed on

to the MAC layer.

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