SMSC FlexPWR LAN8720 - Manual (Page 52)

Small Footprint RMII 10/100 Ethernet Transceiver with HP Auto-MDIX Support
Datasheet
Revision 1.0 (05-28-09) 52 SMSC LAN8720/LAN8720i
DATASHEET
5.3.8.1 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
Figure 5.1 .The near-end loopback mode is enabled
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.
5.3.8.2 Far Loopback
Far loopback is a special test mode for MDI (analog) loopback as indicated by the blue arrows in
Figure 5.3 . The far loopback mode is enabled by setting bit register 17 bit 9 to logic one. In this mode,
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.
5.3.8.3 Connector Loopback
The LAN8720/LAN8720i maintains reliable transmission over very short cables, and can be tested in
a connector loopback as shown in
Figure 5.3 . An RJ45 loopback cable can be used to route the
Figure 5.1 Near-end Loopback Block Diagram
Figure 5.2 Far Loopback Block Diagram
SMSC
Ethernet Transceiver
10/100
Ethernet
MAC
CAT-5
XFMR
Digital
RXD
TXD
Analog
RX
TX
X
X
Far-end system
SMSC
Ethernet Transceiver
10/100
Ethernet
MAC
CAT-5
XFMR
Digital
RXD
TXD
Analog
RX
TX
Link
Partner
X
X