4.3.6 Jabber Function
4.3.8 Transmit and Receive Filtering
The jabber function monitors the DP83848I's output and External 10BASE-T filters are not required when using the
disables the transmitter if it attempts to transmit a packet of DP83848I, as the required signal conditioning is integrated
longer than legal size. A jabber timer monitors the transmit- into the device.
ter and disables the transmission if the transmitter is active
for approximately 85 ms.
Only isolation transformers and impedance matching resis-
tors are required for the 10BASE-T transmit and receive
Once disabled by the Jabber function, the transmitter stays interface. The internal transmit filtering ensures that all the
disabled for the entire time that the ENDEC module's inter- harmonics in the transmit signal are attenuated by at least
nal transmit enable is asserted. This signal has to be de- 30 dB.
asserted for approximately 500 ms (the “unjab” time)
before the Jabber function re-enables the transmit outputs.
The Jabber function is only relevant in 10BASE-T mode.
4.3.9 Transmitter
The encoder begins operation when the Transmit Enable
input (TX_EN) goes high and converts NRZ data to pre-
emphasized Manchester data for the transceiver. For the
duration of TX_EN, the serialized Transmit Data (TXD) is
encoded for the transmit-driver pair (PMD Output Pair).
TXD must be valid on the rising edge of Transmit Clock
(TX_CLK). Transmission ends when TX_EN deasserts.
The last transition is always positive; it occurs at the center
of the bit cell if the last bit is a one, or at the end of the bit
cell if the last bit is a zero.
4.3.7 Automatic Link Polarity Detection and Correction
The DP83848I's 10BASE-T transceiver module incorpo-
rates an automatic link polarity detection circuit. When
three consecutive inverted link pulses are received, bad
polarity is reported.
A polarity reversal can be caused by a wiring error at either
end of the cable, usually at the Main Distribution Frame
(MDF) or patch panel in the wiring closet.
The bad polarity condition is latched in the 10BTSCR regis-
ter. The DP83848I's 10BASE-T transceiver module cor-
rects for this error internally and will continue to decode
received data correctly. This eliminates the need to correct
the wiring error immediately.
4.3.10 Receiver
The decoder detects the end of a frame when no additional
mid-bit transitions are detected. Within one and a half bit
times after the last bit, carrier sense is de-asserted.
Receive clock stays active for five more bit times after CRS
goes low, to guarantee the receive timings of the controller.
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