DP83848C_07 datasheet(31/82 Pages) NSC

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4.3.2 Smart Squelch

The smart squelch is responsible for determining when

valid data is present on the differential receive inputs. The

DP83848C 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

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.

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 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 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 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.

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 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

the

link

integrity

function

disabled

(FORCE_LINK_10 of the 10BTSCR register), a good link is

forced and the 10BASE-T transceiver will operate regard-

less of the presence of link pulses.

end of packet

start of packet

VSQ-(reduced)

VSQ-

VSQ+(reduced)

VSQ+

<150 ns

>150 ns

Figure 10. 10BASE-T Twisted Pair Smart Squelch Operation

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