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What would be the total GPIO pins available.

finian826

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

Parents

  • Terry,

     

    The MCP23017 chips draw almost no current. 4 of them together are much less than a single LED so they can be powered by 3.3v on the Pi.

     

    The optoisoator chips are a nice design but 64 LEDs are quite a load and you may not find opto-isolators that work with only 3-5 mA current available from the 23017 output directly (do let us know). If not, you are likely to need Darlington transistor arrays (e.g.: ULN 2803 A) between your IO expanders and the optoisolators. The Darlington array-connected LEDs can be powered from a separate supply or from the 5V supply of the Pi as long as the Pi supply itself has enough power.

     

    Finally, once you use the Darlington array, I'm not entirely sure the opto-isolators are neccessary. That depends more on the grounds and whether you have inductive loads (motors, solenoids) that could introduce spikes into the Pi power bus.

     

    The PiFace uses MCP23017 and ULN2803's to drive LEDs. I haven't seen a nice schematic for this board but there could be one out there.

     

    Adrian

Reply

  • Terry,

     

    The MCP23017 chips draw almost no current. 4 of them together are much less than a single LED so they can be powered by 3.3v on the Pi.

     

    The optoisoator chips are a nice design but 64 LEDs are quite a load and you may not find opto-isolators that work with only 3-5 mA current available from the 23017 output directly (do let us know). If not, you are likely to need Darlington transistor arrays (e.g.: ULN 2803 A) between your IO expanders and the optoisolators. The Darlington array-connected LEDs can be powered from a separate supply or from the 5V supply of the Pi as long as the Pi supply itself has enough power.

     

    Finally, once you use the Darlington array, I'm not entirely sure the opto-isolators are neccessary. That depends more on the grounds and whether you have inductive loads (motors, solenoids) that could introduce spikes into the Pi power bus.

     

    The PiFace uses MCP23017 and ULN2803's to drive LEDs. I haven't seen a nice schematic for this board but there could be one out there.

     

    Adrian

Children
  • in reply to e14 Contributor

    Adrian,

     

    I was wondering that to, what I might do is use the Darlington arrays on the outputs controlling devices on the layout. I will be using the opto-isolators on the switch machine circuits for sure. Some will be slow moving tortoise switch motors and some will be solenoids, thse devices I wish to try and keep isolated from the Pi circuits.

     

    I am still planning the circuits out so that I can figure out how many units I will need to employ.

     

    Terry

  • in reply to finian826

    Terry,

     

    be aware if you want total isolation from the Pi, and if you use for example the SPI interface and the MCP23S17 (SPI) you can put in the middle a digital isolator like the ISO7241 that can support the high speed of the SPI interface and has separate power supplies for each side.

     

    Silicon Labs also has similar products, and as far as I remember you can get samples from both.

     

    -J