dp83846a National Semiconductor Corporation, dp83846a Datasheet - Page 23

• Manufacturer Part Number: dp83846a
• Description: Dsphyter ? Single 10/100 Ethernet Transceiver
• Manufacturer: National Semiconductor Corporation

3.3.9 4B/5B Decoder
The code-group decoder functions as a look up table that
translates incoming 5B code-groups into 4B nibbles. The
code-group decoder first detects the J/K code-group pair
preceded by IDLE code-groups and replaces the J/K with
MAC preamble. Specifically, the J/K 10-bit code-group pair
is replaced by the nibble pair (0101 0101). All subsequent
5B code-groups are converted to the corresponding 4B
nibbles for the duration of the entire packet. This conver-
sion ceases upon the detection of the T/R code-group pair
denoting the End of Stream Delimiter (ESD) or with the
reception of a minimum of two IDLE code-groups.
3.3.10 100BASE-TX Link Integrity Monitor
The 100 Base TX Link monitor ensures that a valid and sta-
ble link is established before enabling both the Transmit
and Receive PCS layer.
Signal detect must be valid for 395us to allow the link mon-
itor to enter the 'Link Up' state, and enable the transmit and
receive functions.
3.3.11 Bad SSD Detection
A Bad Start of Stream Delimiter (Bad SSD) is any transition
from consecutive idle code-groups to non-idle code-groups
which is not prefixed by the code-group pair /J/K.
If this condition is detected, the DP83846A will assert
RX_ER and present RXD[3:0] = 1110 to the MII for the
cycles that correspond to received 5B code-groups until at
least two IDLE code groups are detected. In addition, the
False Carrier Sense Counter register (FCSCR) will be
incremented by one.
Once at least two IDLE code groups are detected, RX_ER
and CRS become de-asserted.
3.4 10BASE-T TRANSCEIVER MODULE
The 10BASE-T Transceiver Module is IEEE 802.3 compli-
ant. It includes the receiver, transmitter, collision, heart-
beat, loopback, jabber, and link integrity functions, as
defined in the standard. An external filter is not required on
the 10BASE-T interface since this is integrated inside the
DP83846A. This section focuses on the general 10BASE-T
system level operation.
3.4.1 Operational Modes
The DP83846A has two basic 10BASE-T operational
modes:
— Half Duplex mode
— Full Duplex mode
Half Duplex Mode
In Half Duplex mode the DP83846A functions as a stan-
dard IEEE 802.3 10BASE-T transceiver supporting the
CSMA/CD protocol.
Full Duplex Mode
In Full Duplex mode the DP83846A is capable of simulta-
neously transmitting and receiving without asserting the
collision signal. The DP83846A's 10 Mb/s ENDEC is
designed to encode and decode simultaneously.
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3.4.2 Collision Detection and SQE
When in Half Duplex, a 10BASE-T collision is detected
when the receive and transmit channels are active simulta-
neously. Collisions are reported by the COL signal on the
MII. Collisions are also reported when a jabber condition is
detected.
The COL signal remains set for the duration of the collision.
If the ENDEC is receiving when a collision is detected it is
reported immediately (through the COL pin).
When heartbeat is enabled, approximately 1 s after the
transmission of each packet, a Signal Quality Error (SQE)
signal of approximately 10-bit times is generated to indicate
successful transmission. SQE is reported as a pulse on the
COL signal of the MII.
The SQE test is inhibited when the PHY is set in full duplex
mode. SQE can also be inhibited by setting the
HEARTBEAT_DIS bit in the 10BTSCR register.
3.4.3 Carrier Sense
Carrier Sense (CRS) may be asserted due to receive activ-
ity once valid data is detected via the squelch function.
For 10 Mb/s Half Duplex operation, CRS is asserted during
either packet transmission or reception.
For 10 Mb/s Full Duplex operation, CRS is asserted only
during receive activity.
CRS is deasserted following an end of packet.
3.4.4 Normal Link Pulse Detection/Generation
The link pulse generator produces pulses as defined in the
IEEE 802.3 10BASE-T standard. Each link pulse is nomi-
nally 100 ns in duration and transmitted every 16 ms in the
absence of transmit data.
Link pulses are used to check the integrity of the connec-
tion with the remote end. If valid link pulses are not
received, the link detector disables the 10BASE-T twisted
pair transmitter, receiver and collision detection functions.
When
(FORCE_LINK_10 of the 10BTSCR register), good link is
forced and the 10BASE-T transceiver will operate regard-
less of the presence of link pulses.
3.4.5 Jabber Function
The jabber function monitors the DP83846A's output and
disables the transmitter if it attempts to transmit a packet of
longer than legal size. A jabber timer monitors the transmit-
ter and disables the transmission if the transmitter is active
beyond the Jab time (20-150 ms).
Once disabled by the Jabber function, the transmitter stays
disabled for the entire time that the ENDEC module's inter-
nal transmit enable is asserted. This signal has to be de-
asserted for approximately 250-750 ms (the “unjab” time)
before the Jabber function re-enables the transmit outputs.
The Jabber function is only relevant in 10BASE-T mode.
3.4.6 Automatic Link Polarity Detection and Correction
The DP83846A's 10BASE-T transceiver module incorpo-
rates an automatic link polarity detection circuit. When
seven consecutive inverted link pulses are received,
inverted polarity is reported.
the
link
integrity
function
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disabled