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I got an Ethermega, and learned that Pin 13 already has a LED.

Are there other digital pins with special circuitry or already allocated to a specific function? (like usb,serial,ethernet,sd, etc)

I need about 30 inputs so I can skip some if that would avoid headaches.

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D4: ChipSelect for the SD card reader (can be used if no SD is present)

D0 and D1 are Serial RX and TX (just like any other arduino)

D10: Ethernet enable line

D13: led. Led is connected via a mosfet, so it doesn't interfer (like on the uno)

D50, D51, D52: SPI for the ethernet chip and SD card

All this was found in the schematic

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Don't worry about the led on pin 13, it just sits there but doesn't block anything.

You can still use the pin as a normal IO pin.

You didn't mention whether your 30 inputs are digital or analog, so let's have a look at digital input first.

I have never used the MCP23017 mentioned by Martynas, but the linked article looks very interesting and I'll definitely order one of these amazing chips!

Another option might be using 8 bit PISO (parallel in - serial out shift registers, such as the 74HC165. Consider this as the complement to the 75HC595 for output. These shift registers are dirt cheap (around 0.3 €), are available as DIP (important for breadboarding) and can be run via SPI.

Advantages:

  • it's fast to read in 8 digital lines into a byte (why would one want to use more than a bit for the state of a digital input?)

  • the shift registers can be daisychained, subsequent calls of ShiftIn allows to read all (digital) input states into a corresponding data structure (array of bytes, etc.) using just three pins on your Arduino.

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    Klaus i wanted to mention 74HC165 :) but MCP23017 link was open already so i just copied it to give the main idea:) +1 – Martynas Dec 4 '14 at 8:17
  • @Martynas I think it was a splendid idea to mention the MCP23017! As written by John Boxall in the article that you linked: "...I think the I2C bus is underappreciated". – Klaus-Dieter Warzecha Dec 4 '14 at 8:20

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