LAN8740A Datasheet, PDF (24/136 Pages) Microchip Technology – Small Footprint MII/RMII 10/100 Energy Efficient Ethernet Transceiver with HP Auto-MDIX and flexPWR® Technology

LAN8740A/LAN8740Ai
There are 4 possible matches of the technology abilities. In the order of priority these are:
• 100M Full Duplex (Highest Priority)
• 100M Half Duplex
• 10M Full Duplex
• 10M Half Duplex (Lowest Priority)
If the full capabilities of the transceiver are advertised (100M, Full Duplex), and if the link partner is capable of 10M and
100M, then auto-negotiation selects 100M as the highest performance mode. If the link partner is capable of half and
full duplex modes, then auto-negotiation selects full duplex as the highest performance operation.
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" for additional information.
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.
3.2.1 PARALLEL DETECTION
If the LAN8740A/LAN8740Ai is connected to a device lacking the ability to auto-negotiate (i.e., 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 Partner
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 LAN8740A/LAN8740Ai will respond by stopping all transmission/receiving operations. Once the break_link_-
timer is completed in the auto-negotiation state-machine (approximately 1250 ms), 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.
DS00001987A-page 24
 2013-2015 Microchip Technology Inc.