What would be the total GPIO pins available.

Good day,

I am thinking of employing 1 or 2 Rpi's into my model train layout to remote manage my turnouts and other specialty items on my layout.

I know through GPIO-P1 I have 17 available GPIO pins. From some of my other reading I have discovered on the new model B's there was a P5 added that gives 4 more GPIO pins for usage, but it also says in the same document that those pins are used by the system. The available pins on P5 are GPIO28-GPIO31, but those are used as follows: GPIO28-31 are used by the board ID and are connected to resistors R3 to R10.

I know that some are also avabile via S5 which from my understanding will eventually be used for the upcoming camera board.

So if I read correctly, theorietically I could have 25 possible GPIO pins available or would it only be 21 if I take and use the 4 from S5.

Any input on this from others would be great.

Thanks

Terry

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finian826 over 13 years ago

I think the MCP23017 is going to be my best bet. I am thinking of placing a minimum of 4 chips on a card connected to the Pi. The only issue I am seeing is how to isolate the Pi from the 5V returm signals through the chip. With having 4 chips on the board I was thinking of powering the chips with an external 5V power supply. Will the MCP23017 use 5V to return the signal back to the Pi or will it use what every voltage is powering the chip?

Or would I be best to have a regulator circuit on the board to drop a 5V input down to 3.3V to power the chips?

For higher voltage interfacing, like what is present on a model train layout, I plan to use opto-isolator chips to keep the 5V or 3.3V side away from the 12V to 24V side.

Terry
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finian826 over 13 years ago

I think the MCP23017 is going to be my best bet. I am thinking of placing a minimum of 4 chips on a card connected to the Pi. The only issue I am seeing is how to isolate the Pi from the 5V returm signals through the chip. With having 4 chips on the board I was thinking of powering the chips with an external 5V power supply. Will the MCP23017 use 5V to return the signal back to the Pi or will it use what every voltage is powering the chip?

Or would I be best to have a regulator circuit on the board to drop a 5V input down to 3.3V to power the chips?

For higher voltage interfacing, like what is present on a model train layout, I plan to use opto-isolator chips to keep the 5V or 3.3V side away from the 12V to 24V side.

Terry
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