AM79C978A AMD [Advanced Micro Devices], AM79C978A Datasheet - Page 72

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

has internal filtering and does not require external filter
modules or common mode chokes.
Signals appearing at the RX± differential input pair are
routed to the internal decoder. The receiver function
meets the propagation delays and jitter requirements
specified by the 10BASE-T standard. The receiver
squelch level drops to half its threshold value after un-
squelch to allow reception of minimum amplitude sig-
nals and to mitigate carrier fade in the event of worst
case signal attenuation and crosstalk noise conditions.
Twisted Pair Interface Status
The Am79C978A device will power up in the Link Fail
state. The Auto-Negotiation algorithm will apply to
allow it to enter the Link Pass state.
In the Link Pass state, receive activity which passes
the pulse width/amplitude requirements of the RX± in-
puts will cause the PCS Control block to assert Carrier
Sense (CRS) signal at the internal MII interface. A col-
lision would cause the PCS Control block to assert Car-
rier Sense (CRS) and Collision (COL) signals at the
internal MII. In the Link Fail state, this block would
cause the PCS Control block to de-assert Carrier
Sense (CRS) and Collision (COL).
In jabber detect mode, this block would cause the PCS
Control block to assert the COL signal at the internal
MII and allow the PCS Control block to assert or de-as-
sert the CRS pin to indicate the current state of the RX±
pair. If there is no receive activity on RX±, this block
72
Figure 37. 10BASE-T Transmit and Receive Data
Clock
Manchester
Encoder
TX Driver
TX±
Data
Paths
Clock
Manchester
RX Driver
Decoder
Squelch
Circuit
RX±
Data
22399A-40
Am79C978A
would cause the PCS Control block to assert only the
COL pin at the internal MII. If there is RX± activity, this
block would cause the PCS Control block to assert
both COL and CRS at the internal MII.
Collision Detect Function
Simultaneous activity (presence of valid data signals)
from both the internal encoder transmit function and
the twisted pair RX± pins constitutes a collision,
thereby causing the PCS Control block to assert the
COL pin at the internal MII.
Jabber Function
The Jabber function inhibits the 10BASE-T twisted pair
transmit function of the Am79C978A device if the TX±
circuits are active for an excessive period (20-150 ms).
This prevents one port from disrupting the network due
to a stuck-on or faulty transmitter condition. If the max-
imum transmit time is exceeded, the data path through
the 10BASE-T transmitter circuitry is disabled (al-
though Link Test pulses will continue to be sent). The
PCS Control block also asserts the COL signal at the
internal MII and sets the Jabber Detect bit in Register 1
of the active PHY. Once the internal transmit data
stream from the MENDEC stops, an unjab time of 250-
750 ms will elapse before this block causes the PCS
Control block to de-assert the COL indication and re-
enable the transmit circuitry.
When jabber is detected, this block will cause the PCS
Control block to assert the COL signal and allow the
PCS Control block to assert or de-assert the CRS sig-
nal to indicate the current state of the RX± pair. If there
is no receive activity on RX±, this block causes the
PCS Control block to assert only the COL signal at the
internal MII. If there is RX± activity, this block will cause
the PCS Control block to assert both COL and CRS on
the internal MII.
Reverse Polarity Detect
The polarity for 10BASE-T signals is set by reception of
Normal Link Pulses (NLP) or packets. Polarity is
locked, however, by incoming packets only. The first
NLP received when trying to bring the link up will be ig-
nored, but it will set the polarity to the correct state. The
reception of two consecutive packets will cause the po-
larity to be locked, based on the polarity of the ETD. In
order to change the polarity once it has been locked,
the link must be brought down and back up again.
Auto-Negotiation
The object of the Auto-Negotiation function is to deter-
mine the abilities of the devices sharing a link. After
exchanging abilities, the Am79C978A device and re-
mote link partner device acknowledge each other and
make a choice of which advertised abilities to support.
The Auto-Negotiation function facilitates an ordered
r es olu tio n be twee n ex cha nge d ab il iti es. This