Datasheet

15
Board Design Theory
Automatic Parallel Interface Pinout Changes
U1, U2, U3, U5, U13 and U14 are bus switches and glue logic for the purpose of changing
the pinout of the parallel PICtail Plus board edge contacts. They are unnecessary and
should be omitted in ordinary application circuits as the appropriate SPI or PSP interface
pins, between the host processor and ENC424J600/624J600, can be directly connected.
Power Supply and Shutdown
U7 and its surrounding components form a 3.3V regulated power supply with shutdown
capability. When plugged into a PICDEM.net 2, only a single 5V power supply is available.
The MCP1825 LDO regulator supplies the needed 3.3V power for the ENC624J600,
associated pull up resistors and the Ethernet magnetics. Because the ENCX24J600 draws
considerable current in Sleep mode, it is beneficial in some applications to completely dis-
connect power from the ENCX24J600 and magnetics instead of using the ENCX24J600
Sleep feature.
Disconnecting power can be achieved by means of the SHDN
signal. Care must be taken in
software to also tri-state or drive all SPI/PSP interface signals to GND when power is
removed to avoid having current enter the ENCX24J600 through the I/O pins.
If an application circuit already has a regulated 3.3V supply available, the MCP1825 can be
replaced with a MOSFET to achieve the same shutdown functionality at lower cost, as
shown in the following diagram. When using a MOSFET, the C6 and C7 capacitors would be
unnecessary.
Replacing MCP1825 with a MOSFET
If using a circuit like this, be sure to power all ENCX24J600 V
DD pins and the center tap of
the Ethernet TX transformer through this power switch. It is invalid to disconnect V
DD without
removing power from the magnetics.
Ethernet Auto-Crossover
U8 is a quad 2:1 multiplexer bus switch capable of passing analog signals bidirectionally. Its
purpose is to swap the TPOUT+/- interface pins with the TPIN+/- interface pins to the RJ-45
connector at run time under software control. If the user plugs in a crossover Ethernet cable
when a straight through cable is required, or vice versa, the software can automatically swap
the two data pairs via the MDIX signal. This feature permits a true plug-and-play experience
for users who may not be aware of the internal differences between a crossover cable and a
straight cable.
When MDIX is held low (MDI state), the TPOUT+/- signals are routed to Pins 1 and 2 at the
RJ-45 connector, while the TPIN+/- signals are routed to Pins 3 and 6, respectively. These
connections are the ordinary default required for end devices. When MDIX is pulled high
(MDIX state), the TPOUT+/- signals are routed to Pins 3 and 6, while the TPIN+/- signals
appear on Pins 1 and 2, respectively, of the RJ-45 connector. In this state, the wires are
crossed as a crossover cable would do externally.
S
D
G
3V3ETH
SHDN
+3.3V
100kO
Si1307DL
100 kΩ