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I'm considering purchasing a RTC extension card (This one for instance).

But this card would be in contact with many GPIO pins.

There are probably unused pins, so I assume I still could use the others with other stacked components. But that's not the first time I see other extension cards connected to many pins, but that still permit to stack many extension cards, using the same pins.

Aren't there conflicts sometimes or is there something that permits to share a GPIO pin with many equipments?

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You can use any spare GPIO.

In your case spare means

  • You can physically connect to the GPIO
  • Nothing else is manipulating the same GPIO at the same time

There is no conflict simply because another device is also physically connected to the same GPIO.

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As long as those pins are really only used by one extension card, there shouldn't be a problem.

Imaginge those unused pins and plugs as simple wire connections (which they basically are) - if the connection is not interrupted (e.g. when the pins and plugs do not quite fit) or the line becomes very long (rather unlikely) there shouldn't be a problem in passing through 0, 3 or 8 more short wires (other extension modules) in between.

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I just found out about I2C.

If I understand well, I2C permits the use of several components on the same BUSes.

  • Yes, that's true. So in some cases, you can even share GPIO pins between several extension modules. But for GPIO pins, which you might use to switch stuff on or off or to convey simple state information (on = HIGH, off = LOW) you should try to use different pins for different information to be transmitted (and therefore usually for different extension modules). – Fantilein1990 Nov 9 '16 at 16:51
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    As well as I2C the Pi supports (two) SPI buses. – joan Nov 9 '16 at 16:53
  • @joan: Is UART a form of SPI? – Fantilein1990 Nov 9 '16 at 17:07
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    @Fantilein1990 No. UART, SPI, and I2C are all serial links, i.e. data is transmitted one bit at a time. But UART is point-to-point asynchronous, whereas I2C and SPI are both master to multiple slave synchronous. I2C uses 2 wires (SDA, SCL). SPI uses 4 wires (MISO, MOSI, SCLK, slave select). – joan Nov 9 '16 at 17:25

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