78Q2120C
10/100BASE-TX
Transceiver
SQE TEST
will look for either 10BASE-T idle, 100BASE-TX idle,
or fast link pulses from its link partner. If either idle
pattern is detected, the 78Q2120C configures itself
in half-duplex mode at the appropriate speed. If it
detects fast link pulses, it decodes and analyzes the
link code transmitted by the link partner. When
three identical link code words are received (ignoring
the acknowledge bit) the link code word is stored in
register MR5. Upon receiving three more identical
link code words, with the acknowledge bit set, the
78Q2120C configures itself to the highest priority
technology common to the two link partners. The
technology priorities are, in descending order:
The 78Q2120C supports the Signal Quality Error
(SQE) function detailed in IEEE-802.3.
At an
interval of 1µs after each negative transition of the
TXEN pin in 10BASE-T mode, the COL pin will go
high for a period of 1µs. SQE is not signaled during
transmission after collision is detected. SQE is
automatically disabled when repeater mode is
enabled. This function can be disabled through
register bit MR16.11.
Natural Loopback
100BASE-TX, Full Duplex
100BASE-TX, Half Duplex
10BASE-T, Full Duplex
10BASE-T, Half Duplex
When enabled, whenever the 78Q2120C is
transmitting and not receiving on the twisted pair
media (10BASE-T Half Duplex mode), data on the
TXD[3:0] pins is looped back onto the RXD[3:0] pins.
During a collision, data from the RXI pins is routed to
the RXD[3:0] pins. The natural loopback function is
enabled through register bit MR16.10.
Once auto-negotiation is complete, register bits
MR18.11:10 will reflect the actual speed and duplex
that was chosen.
If auto-negotiation fails to establish a link for any
reason, register bit MR18.12 will reflect this and auto
negotiation will restart from the beginning. Writing a
‘1’ to bit MR0.9(RANEG) will also cause auto-
negotiation to restart.
Repeater Mode
When the RPTR pin is high or register bit MR16.15
is set, the 78Q2120C is placed in repeater mode. In
this mode, full duplex operation is prohibited, CRS
responds only to receive activity and, in 10BASE-T
mode, the SQE test function is disabled.
MEDIA INDEPENDENT INTERFACE
MII Transmit and Receive Operation
AUTO-NEGOTIATION
The 78Q2120C supports the auto-negotiation
functions of Clause 28 of IEEE-802.3 for 10/100
Mbps operation over copper wiring. This function
can be enabled via a pin selection or register
settings. If the ANEGA pin is tied high, the auto-
negotiation function defaults to ON and bit MR0.12
(ANEGEN) is high after reset. Software can disable
the auto-negotiation function by writing to bit
MR0.12. If the ANEGA pin is tied low, the function
defaults to OFF and bit MR0.12 is set low after reset
and cannot be written to.
The MII interface on the 78Q2120C provides
independent transmit and receive paths for both
10Mb/s and 100Mb/s data rates as described in
Clause 22 of the IEEE-802.3 standard.
The transmit clock, TX_CLK, provides the timing
reference for the transfer of TX_EN, TXD[3:0], and
TX_ER signals from the MAC to the 78Q2120C.
TXD[3:0] is captured on the rising edge of TX_CLK
when TX_EN is asserted. TX_ER is also captured
on the rising edge of TX_CLK and is asserted by the
MAC to request that an error code group is to be
transmitted. The assertion of TX_ER is ignored
when the 78Q2120C is operating in 10BASE-T
mode.
The receive clock, RX_CLK, provides the timing
reference to transfer RX_DV, RXD[3:0], and RX_ER
signals from the 78Q2120C to the MAC. RX_DV
transitions synchronously with respect to RX_CLK
and is asserted when the 78Q2120C is presenting
valid data on RXD[3:0]. RX_ER is asserted and is
synchronous to RX_CLK when a code group
violation has been detected in the current receive
packet.
The contents of register MR4 are sent to the
78Q2120C’s link partner during auto-negotiation
using fast link pulse coding. Bits MR4.8:5 reflect the
state of the TECH[2:0] pins after reset. If TECH[2:0]
= ‘111’, then all 4 bits are high. If TECH[2:0] = ‘001’,
then only bit 5 is high. After reset, software can
change any of these bits from a ‘1’ to a ‘0’; but not
from a ‘0’ to a ‘1’. Therefore, a technology permitted
by the setting of the TECH pins can be disabled, but
cannot be enabled through register selection.
With auto-negotiation enabled, the 78Q2120C will
start sending fast link pulses at power on, loss of link
or upon a command to restart. At the same time, it
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© 2009 Teridian Semiconductor Corporation
Rev 1.3
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