DP83848M_08 Datasheet, PDF (30/80 Pages) National Semiconductor (TI) – PHYTER® Mini - Commercial Temperature Single 10/100 Ethernet Transceiver

4.3.2 Smart Squelch
The smart squelch is responsible for determining when
valid data is present on the differential receive inputs. The
DP83848M implements an intelligent receive squelch to
ensure that impulse noise on the receive inputs will not be
mistaken for a valid signal. Smart squelch operation is
independent of the 10BASE-T operational mode.
The squelch circuitry employs a combination of amplitude
and timing measurements (as specified in the IEEE 802.3
10BSE-T standard) to determine the validity of data on the
twisted pair inputs (refer to Figure 10).
The signal at the start of a packet is checked by the smart
squelch and any pulses not exceeding the squelch level
(either positive or negative, depending upon polarity) will
be rejected. Once this first squelch level is overcome cor-
rectly, the opposite squelch level must then be exceeded
<150 ns
<150 ns
within 150 ns. Finally the signal must again exceed the
original squelch level within a 150 ns to ensure that the
input waveform will not be rejected. This checking proce-
dure results in the loss of typically three preamble bits at
the beginning of each packet.
Only after all these conditions have been satisfied will a
control signal be generated to indicate to the remainder of
the circuitry that valid data is present. At this time, the
smart squelch circuitry is reset.
Valid data is considered to be present until the squelch
level has not been generated for a time longer than 150
ns, indicating the End of Packet. Once good data has
been detected, the squelch levels are reduced to minimize
the effect of noise causing premature End of Packet
detection.
>150 ns
V_SQ+
V_SQ+(reduced)
V_SQ-(reduced)
V_SQ-
start of packet
end of packet
Figure 10. 10BASE-T Twisted Pair Smart Squelch Operation
4.3.3 Collision Detection and SQE
When in Half Duplex, a 10BASE-T collision is detected
when the receive and transmit channels are active simul-
taneously. Collisions are reported by the COL signal on
the MII. Collisions are also reported when a jabber condi-
tion is detected.
The COL signal remains set for the duration of the colli-
sion. If the PHY is receiving when a collision is detected it
is reported immediately (through the COL pin).
When heartbeat is enabled, approximately 1 µs after the
transmission of each packet, a Signal Quality Error (SQE)
signal of approximately 10-bit times is generated to indi-
cate 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.
4.3.4 Carrier Sense
Carrier Sense (CRS) may be asserted due to receive
activity once valid data is detected via the squelch func-
tion.
For 10 Mb/s Half Duplex operation, CRS is asserted dur-
ing 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.
4.3.5 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
nominally 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 the link integrity function is disabled
(FORCE_LINK_10 of the 10BTSCR register), a good link
is forced and the 10BASE-T transceiver will operate
regardless of the presence of link pulses.
www.national.com
30