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PiFace CAD: What are the really used PINs on the PI

balearicdynamics

I have investigated here and there without success until now. There is a lot of (good) documentation about the usage of the PiFace CAD display but I can't find what are the really used PINs on the Raspberry PI. In fact I see that this add-on board can be stacked (PiFace – PiFace Rack) but despite a wide software documentation it seems there is nothing about hardware connections. Maybe I ave searched in the wrong direction ....

 

Thanks in advance. Enrico

  • 0 in reply to balearicdynamics

    First up, the PI Face boards are not using I2C, there using SPI (Look up MCP23S17 which is the chip used on both boards)

     

    Within the length of wires on the boards being stacked there should not be an issue regarding the basic electrical load

     

    I2C can have up to 127 different addressable devices on one line (Logically, not necessarily electrically due to loading etc). Some devices have a common address as well as a unique address allowing them to be talked to in parallel for mass updates or individually (DAC8574 Digital to Analog convertor for example)

     

    Some manufactures components have devices that have the same address as other manufactures so you need to pay attention to the data sheets

     

    the chips used on the PI Face boards come in two types, an I2C version (MCP23017) and a SPI Version (MCP23S17) http://ww1.microchip.com/downloads/en/DeviceDoc/21952b.pdf. The PIFAce boards only seem to use the SPI version

     

    PiFace Digital Boards only expose two of the address setting lines on the 16 bit expander (23S17) limiting the usable boards to only 4, not 8. Also the Piface Digital board using SPI, NOT I2C and uses CE0 to enable the board from the PI.

    The PI Face CAD is also using the MCP23S17 and is also therefore SPI and limited to 4 assuming it is using the same 2 jumpers on the address. this board uses CE1 to enable tote board though so does not conflict with the Digital Board

     

    If you have other SPI connected devices you will run into issues as the PI only supports 2 SPI devices logically. It is a feature of the MCP23S17 chips that allows having up to 8 technically on one SPI but the physical design of the PIFace Digital and Possibly CAD limit this to 4 on for each type (To Be confirmed for the CAD, but definitely true for the Digital)

  • 0 in reply to Robert Peter Oakes

    Many thanks Peter, things are more clear. So, why the address setting on the Digital? What is the sense as they uses the SPI ? I supposed that SPI was also used in some way as the PiFace rack includes - as in their specifications - both I2C and SPI address settings.

     

    Then the question now moves on how to set a custom board with the SPI address instead of I2C. Or I am wrong and the address settings via jumpers are for the SPI and the I2C (why mentioned everywhere? Boh) is to be ignored.

  • 0 in reply to balearicdynamics

    As I explained, the PIFace boards use a chip (23S17) that can support upto 8 on the same chip select (Limited to 4 on the PiFace) Think of it as a sub address. when the command is sent to the SPI using CE0 for example, all the chips connected to CE0 listen to the command byte, it contains an address in it as well as the command, only the chips that have the hardware address pins set the same way as the address bits will respond to the command effectively increasing the available IO pins in multiples of 16 with a single Chip Enable line. Note that not all SPI chips support this feature, the MCP23S17 does.

  • 0 in reply to Robert Peter Oakes

    Yep, got it. I have rad in detail the datasheet and I think that - thinking in the right direction, i.e. SPI usage - I can reach what I have in mind also with the PiFaceCAD installed.

     

    Thank you Peter.

     

    Enrico