The latest revision of the shield adds a micro-SD card slot, which
can be used to store files for serving over the network. It is
compatible with the Arduino Duemilanove and Mega (using the Ethernet
library coming in Arduino 0019). An SD card library is not yet included
in the standard Arduino distribution, but the sdfatlib by Bill Greiman works well.
The latest revision of the shield also includes
a reset controller, to ensure that the W5100 Ethernet module is
properly reset on power-up. Previous revisions of the shield were not
compatible with the Mega and need to be manually reset after power-up.
The original revision of the shield contained a full-size SD card slot;
this is not supported.
Arduino communicates with both the W5100 and SD
card using the SPI bus (through the ICSP header). This is on digital
pins 11, 12, and 13 on the Duemilanove and pins 50, 51, and 52 on the
Mega. On both boards, pin 10 is used to select the W5100 and pin 4 for
the SD card. These pins cannot be used for general i/o. On the Mega,
the hardware SS pin, 53, is not used to select either the W5100 or the
SD card, but it must be kept as an output or the SPI interface won't
work.
Note that because the W5100 and SD card share
the SPI bus, only one can be active at a time. If you are using both
peripherals in your program, this should be taken care of by the
corresponding libraries. If you're not using one of the peripherals in
your program, however, you'll need to explicitly deselect it. To do
this with the SD card, set pin 4 as an output and write a high to it.
For the W5100, set digital pin 10 as a high output.
The shield provides a standard RJ45 ethernet jack.
The reset button on the shield resets both the W5100 and the Arduino board.
