PIC18F97J60_11 Datasheet, PDF (221/492 Pages) Microchip Technology – 64/80/100-Pin, High-Performance, 1-Mbit Flash Microcontrollers with Ethernet

PIC18F97J60 FAMILY
When the select line is raised high, the A ports of the
switches will connect with the B1 ports, leaving the B0
ports disconnected. This will swap the RX polarity and
route the TPIN+ pin to the signal on RJ-45 Jack Pin 6.
TPIN- will connect to RJ-45 Pin 3. This swapped polar-
ity can correct an incorrectly wired signal generated at
the remote link partner or in the intermediate cabling.
In the MCU, software must be written to toggle the
select line state to the correct level based on the con-
nected link partner. This is best achieved by
periodically toggling the select line at low frequency
(<5 Hz), while watching for a successful packet recep-
tion event. When the correct polarity is found, the select
line should stop toggling and remain static until the
Ethernet link is removed.
In some cases, rather than periodically toggling the
polarity select at a low frequency to receive a packet,
faster determination of the correct polarity may be
determined by transmitting a packet and looking for an
immediate response. This method requires the use of
a protocol that results in a response packet from the
network. In many networks, Dynamic Host Configura-
tion Protocol (DHCP) discovery packets may be used
to resolve the correct TPIN polarity quickly.
Care should be taken when selecting the bus switches
to ensure that they are capable of passing the Ethernet
signals without distortion. The TPIN± pins will weakly
bias the RX common-mode voltage to approximately
VDD/2, and the Ethernet RX waveform will add up to
±1.4V onto this common-mode voltage. Therefore, the
switches must be capable of passing signals of at least
3.05V.
Additionally, the bus switches should have low
capacitance to minimize the signal loss and impedance
discontinuity that may affect the RX signal. The
switches, rated -3dB bandwidth, must be well above
20 MHz.
FIGURE 19-3:
RX POLARITY CORRECTION CIRCUIT (TX CONNECTIONS NOT SHOWN)
PIC18FXXJ6X
GPIO
TPIN+
TPIN-
1
6
2
5
3
4
NC7SB3157
+3.3V
1
6
2
5
3
4
NC7SB3157
+3.3V
100K
RJ-45 and Magnetics
TX
RX
4x
75 ohm
1000 pF 2kV
 2011 Microchip Technology Inc.
DS39762F-page 221