lan8720 Standard Microsystems Corp., lan8720 Datasheet - Page 52

• Manufacturer Part Number: lan8720
• Description: Small Footprint Rmii 10/100 Ethernet Transceiver With Hp Auto-mdix Support
• Manufacturer: Standard Microsystems Corp.

Revision 1.0 (04-15-09)
5.3.8.1
5.3.8.2
5.3.8.3
Ethernet
10/100
MAC
Ethernet
10/100
MAC
Near-end Loopback
Near-end loopback is a mode that sends the digital transmit data back out the receive data signals for
testing purposes as indicated by the blue arrows in
by setting bit register 0 bit 14 to logic one.
A large percentage of the digital circuitry is operational near-end loopback mode, because data is
routed through the PCS and PMA layers into the PMD sublayer before it is looped back. The COL
signal will be inactive in this mode, unless collision test (bit 0.7) is active. The transmitters are powered
down, regardless of the state of TXEN.
Far Loopback
Far loopback is a special test mode for MDI (analog) loopback as indicated by the blue arrows in
Figure
data that is received from the link partner on the MDI is looped back out to the link partner. The digital
interface signals on the local MAC interface are isolated.
Connector Loopback
The LAN8720/LAN8720i maintains reliable transmission over very short cables, and can be tested in
a connector loopback as shown in
TXD
5.3. The far loopback mode is enabled by setting bit register 17 bit 9 to logic one. In this mode,
RXD
TXD
X
X
RXD
Digital
Ethernet Transceiver
Figure 5.1 Near-end Loopback Block Diagram
Far-end system
Digital
Ethernet Transceiver
Figure 5.2 Far Loopback Block Diagram
SMSC
SMSC
Analog
DATASHEET
Figure
Analog
52
5.3. An RJ45 loopback cable can be used to route the
X
X
TX
RX
Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Figure
TX
RX
5.1.The near-end loopback mode is enabled
XFMR
XFMR
CAT-5
SMSC LAN8720/LAN8720i
CAT-5
Partner
Link
Datasheet