Datasheet

 2016 Microchip Technology Inc. DS00002165B-page 21
LAN8720A/LAN8720AI
Once a capability match has been determined, the link code words are repeated with the acknowledge bit set. Any dif-
ference in the main content of the link code words at this time will cause auto-negotiation to re-start. Auto-negotiation
will also re-start if not all of the required FLP bursts are received.
The capabilities advertised during auto-negotiation by the transceiver are initially determined by the logic levels latched
on the MODE[2:0] configuration straps after reset completes. These configuration straps can also be used to disable
auto-negotiation on power-up. Refer to
Section 3.7.2, "MODE[2:0]: Mode Configuration," on page 30 for additional infor-
mation.
Writing the bits 8 through 5 of the Auto Negotiation Advertisement Register allows software control of the capabilities
advertised by the transceiver. Writing the Auto Negotiation Advertisement Register does not automatically re-start auto-
negotiation. The Restart Auto-Negotiate bit of the Basic Control Register must be set before the new abilities will be
advertised. Auto-negotiation can also be disabled via software by clearing the Auto-Negotiation Enable bit of the Basic
Control Register.
Note: The device does not support “Next Page” capability.
3.2.1 PARALLEL DETECTION
If the LAN8720A/LAN8720Ai is connected to a device lacking the ability to auto-negotiate (for example, no FLPs are
detected), it is able to determine the speed of the link based on either 100M MLT-3 symbols or 10M Normal Link Pulses.
In this case the link is presumed to be half duplex per the IEEE standard. This ability is known as “Parallel Detection.”
This feature ensures interoperability with legacy link partners. If a link is formed via parallel detection, then the
Link Part-
ner Auto-Negotiation Able bit of the Auto Negotiation Expansion Register is cleared to indicate that the Link Partner is
not capable of auto-negotiation. The controller has access to this information via the management interface. If a fault
occurs during parallel detection, the Parallel Detection Fault bit of Link Partner Auto-Negotiation Able is set.
Auto Negotiation Link Partner Ability Register is used to store the link partner ability information, which is coded in the
received FLPs. If the link partner is not auto-negotiation capable, then the Auto Negotiation Link Partner Ability Register
is updated after completion of parallel detection to reflect the speed capability of the link partner.
3.2.2 RESTARTING AUTO-NEGOTIATION
Auto-negotiation can be restarted at any time by setting the Restart Auto-Negotiate bit of the Basic Control Register.
Auto-negotiation will also restart if the link is broken at any time. A broken link is caused by signal loss. This may occur
because of a cable break, or because of an interruption in the signal transmitted by the link partner. Auto-negotiation
resumes in an attempt to determine the new link configuration.
If the management entity re-starts auto-negotiation by setting the Restart Auto-Negotiate bit of the Basic Control Reg-
ister, the LAN8720A/LAN8720Ai will respond by stopping all transmission/receiving operations. Once the break_link_-
timer is completed in the Auto-negotiation state-machine (approximately 1200ms), auto-negotiation will re-start. In this
case, the link partner will have also dropped the link due to lack of a received signal, so it too will resume auto-negoti
-
ation.
3.2.3 DISABLING AUTO-NEGOTIATION
Auto-negotiation can be disabled by setting the Auto-Negotiation Enable bit of the Basic Control Register to zero. The
device will then force its speed of operation to reflect the information in the Basic Control Register (Speed Select bit and
Duplex Mode bit). These bits should be ignored when auto-negotiation is enabled.
3.2.4 HALF VS. FULL DUPLEX
Half duplex operation relies on the CSMA/CD (Carrier Sense Multiple Access / Collision Detect) protocol to handle net-
work traffic and collisions. In this mode, the carrier sense signal, CRS, responds to both transmit and receive activity. If
data is received while the transceiver is transmitting, a collision results.
In full duplex mode, the transceiver is able to transmit and receive data simultaneously. In this mode, CRS responds
only to receive activity. The CSMA/CD protocol does not apply and collision detection is disabled.
3.3 HP Auto-MDIX Support
HP Auto-MDIX facilitates the use of CAT-3 (10BASE-T) or CAT-5 (100BASE-T) media UTP interconnect cable without
consideration of interface wiring scheme. If a user plugs in either a direct connect LAN cable, or a cross-over patch
cable, as shown in
Figure 3-4, the device’s Auto-MDIX transceiver is capable of configuring the TXP/TXN and RXP/RXN
pins for correct transceiver operation.