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[on:]

In the reference for the Ethernet board it says: (http://arduino.cc/en/Main/ArduinoBoardEthernet)

NB: Pins 10, 11, 12 and 13 are reserved for interfacing with the Ethernet module and should not be used otherwise. ]

Then down the page a bit it says:

Each of the 14 digital pins on the Ethernet board can be used as an input or output ...

Which is correct?

[Reply #1 on:]

It's even more confusing as the answer is different depending on whether you'll connect the Ethernet Shield to an Uno or Mega.

So, on the Uno.....Pins 11, 12 & 13 are the SPI pins and so CAN be used with other SPI devices. Pin 10 is used for the Select line to select the Ethernet SPI. In fact the shield also has an SDCard socket which also uses SPI, with Pin 4 as it's Select line.

But..the newer shield doesn't connect directly to 11, 12 & 13, but by the 6 pin ICSP header. The reason for that is Mega compatibility...

On the Mega,SPI is on different pins, so the shield connectors wouldn't work. The ICSP header is present and connects to the Mega SPI pins.

The ICSP header is the common format but the connections are through to different pins on the Uno and Mega.

I think the Mega SPI is on something like 51, 52, 53. You need to remember this if interfacing other SPI devices on the Mega.

So...you can share the SPI pins. Each device using SPI just needs a unique Select pin so that the relevant can choose which device is active on SPI at any particular moment.

[Reply #2 on:]

It's even more confusing as the answer is different depending on whether you'll connect the Ethernet Shield to an Uno or Mega.

Becareful... this isn't the Ethernet Shield.  It is the "Ethernet Arduino."  It does not connect to a Uno or Mega.  Its Standalone.

Each of the 14 digital pins on the Ethernet board can be used as an input or output ...

Which is correct?

Technically they are both correct.  The generic I/O section was probably copied and pasted from the Uno board description.  If you aren't doing any Ethernet operations, you might be able to use pins 10,11,12, and 13.  However, it would be advisable not to since they are connected to something else on this particular board.