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

Look at What GPIO pins are used by the Piface CAD shield? as well as PiFace Control and Display | Raspberry Alpha Omega

Hi Clem,

thank you for the answer. I took a look and except what was on the E14 site the other I already know. The problem is that I don't find a connecto schemarics of the specific PiFaceCAD. As a matter of fact it is not difficult to replicate the GPIO connector but it seems that the only documentation available is for the setting of other stackable PiFace boards, all using the same I2C bus so things are simple. What I am trying to understand is if these devices ONLY use the I2C + power and gnd or there are other GPIO pin used for some reason.

Enrico

By implication the GPIO pins seem all used based on comment at the end of the second link. But I do not know for sure.

Exactly what I thoug, but in the meantime, how is it possible to stack up to five different addressable boards (or multiple identical) on the PiFace rack only changing the I2C addressing bits? This seems confictual.

Enrico

No problem since all boards listen and react only if they see their own address on the line

So you mena that the I2C addressing is not barely to manage a chip but the entire board that "works" only when the righe bus commands are sent ?

So you mena that the I2C addressing is not barely to manage a chip but the entire board that "works" only when the righe bus commands are sent ?

The data address is required to pick which chip will respond to the stream that follows. It is data/software driven and not electrical. This works as long as each electrical board has a unique address set.

Clem

You are really very helpful Clem ! Your suggestions are raising an idea. Please can you tell me me if I am not wrong:

the entire board expect - to work instead ignoring signals - to be "activated" in some way through a I2C addressing. This - if so - seems reasonable. But what kind of chip can be used to address the entire board?

Thank you. Enrico

P.S. I was aware that it was a problem of logic signals instead of electric as all the stackable boards are always connected and powered

Rather than trying to redo some good tutorials, here is a couple that woirked for me: http://www.best-microcontroller-projects.com/i2c-tutorial.html and http://tronixstuff.com/2010/10/20/tutorial-arduino-and-the-i2c-bus/

Of course let me know where the details are muddy and I will get water to sprain off that area (give help of course).

Clem

Got the links Clem.

I think I have explained in a bad form. I know almost well what the I2C bus is and I have used it a in past. When I mare from scratch the Mill Machine controller based on the Arduino Duemilanove I added several pieces with too less PINs As a matter of fact the g-code engine, preset, settings (e.g. zero point and limits, measure units, working space coords etc.) was done by Arduino. Only a high level 3D commands was sent via Maxx232 serial from the PC on a Processing rendering engine. So, I have used I2C EEPROMs to manage a sql-like database, two shift-out registers, a shift-in register and other IC based on I2C (including a real-time clock).

All these things was single IC that the micro controller board was properly addressing on the single bus. What instead I am searching for - if exists, but I suppose yes - and that I don't know what it is if it is possible to condition the entire board with a sort of X-I2C IC. When I address this chip, that as a matter of fact do nothing more than - for example - setting a bit or something like this, my board react and as the address is different than the one of the PiFaceCAD (or in general any other PIFace board) these boards do nothing. As least this is the idea.

Enrico

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)

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.

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.

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