am79c901a Advanced Micro Devices, am79c901a Datasheet - Page 39

• Manufacturer Part Number: am79c901a
• Description: Homephy Single-chip 1/10 Mbps Home Networking Phy
• Manufacturer: Advanced Micro Devices

Twisted Pair Interface Status
The Am79C901A 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± inputs
will cause the PCS Control block to assert the Carrier
Sense (CRS) signal at the MII interface. A collision
would cause the PCS Control block to assert Carrier
Sense (CRS) and Collision (COL) signals at the MII. In
the Link Fail state, this block would cause the PCS
Control block to deassert 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 MII and
allow the PCS Control block to assert or deassert the
CRS pin to indicate the current state of the RX± pair. If
there is no receive activity on RX±, this block would
cause the PCS Control block to assert only the COL pin
at the MII. If there is RX± activity, this block would
cause the PCS Control block to assert both COL and
CRS at the 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 MII.
Jabber Function
The Jabber function inhibits the 10BASE-T twisted pair
transmit function of the Am79C901A 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
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 deassert 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 deassert the CRS signal
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 MII.
If there is RX± activity, this block will cause the PCS
Control block to assert both COL and CRS on the MII.
P R E L I M I N A R Y
Am79C901A
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
ignored, but it will set the polarity to the correct state.
The reception of two consecutive packets will cause
the polarity to be locked, based on the polarity of the
End of Transmit Data (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 de-
termine the abilities of the devices sharing a link.
After exchanging abilities, the Am79C901A device
and remote link partner device acknowledge each
other and make a choice of which advertised abilities
to support. The Auto-Negotiation function facilitates
an ordered resolution between exchanged abilities.
This exchange allows both devices at either end of
the link to take maximum advantage of their respec-
tive shared abilities.
The Auto-Negotiation algorithm uses a burst of link
pulses called Fast Link Pulses (FLPs). The burst of link
pulses are spaced between 55 and 140 s so as to be
ignored by the standard 10BASE-T algorithm. The FLP
burst conveys information about the abilities of the
sending device. The receiver can accept and decode
an FLP burst to learn the abilities of the sending device.
The link pulses transmitted conform to the standard
10BASE-T template. The device can perform Auto-Ne-
gotiation with reverse polarity link pulses.
The Am79C901A device uses the Auto-Negotiation al-
gorithm to select the type connection to be established
according to the following priority: 10BASE-T full du-
plex, then 10BASE-T half-duplex. See Table 10.
The Auto-Negotiation algorithm is initiated by the fol-
lowing events: Auto-Negotiation enable bit is set, hard-
ware reset, soft reset, transition to link fail state (when
Auto-Negotiation enable bit is set), or Auto-Negotiation
restart bit is set. The result of the Auto-Negotiation pro-
cess can be read from the status register (Summary
Status Register, TBR24).
By default, the link partner must be at least 10BASE-T
half-duplex capable. The Am79C901A PHY can auto-
matically negotiate with the network and yield the high-
est performance possible without software support.
Network Speed
Table 10. Auto-Negotiation Capabilities
20 Mbps
10 Mbps
Physical Network Type
10BASE-T, Half Duplex
10BASE-T, Full Duplex
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