Proprietary add-on board, how to detect its presence?

[Bosse_B]

We have designed a proprietary PC board to plug into the 40-pin connector.
The board contains the following subsystems:
1) An I2C bus where we have connected the following devices
- An EEPROM for storage of config data
- An AWS encryption chip
- An RTC circuit with battery to keep accurate time in the absence of Internet connection
2) The second I2C bus for the HAT EEPROM where we have connected an EEPROM (same type as above)
3) An RS232 level shifter that connects the RPi serial port at 3V level to a board connector with RS232 levels
4) Two relays connected to the GPIO output port
5) A couple of opto-isolated outputs connected to GPIO output pins
6) A few optocouplers hooked to the GPIO input port and driven from an external connector

Ideally I would like to set up the HAT functionality so that on startup the system will detect what is available and use it, but it seems like a really uphill battle trying to figure out how that would be done...

So my question is:
How can I best detect that our board is available so it can be used from Linux?

[trejan]

You've already got the HAT EEPROM which does this. Look in /proc/device-tree/hat/

[klricks]

Maybe attempt to read from the eeprom or RTC. I would think that if they were not present an error of some sort would be generated.

[Bosse_B]

You've already got the HAT EEPROM which does this. Look in /proc/device-tree/hat/

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ls -la /proc/device-tree/hat/
ls: cannot access '/proc/device-tree/hat/': No such file or directory

Using HAT was the plan, but I am not sure what to put into it in order to get it working.
The RPi HAT specification is rather dense and I did not get my head around it enough to find out how to actually configure the board via HAT.
So this EEPROM is currently empty....

I have verified that all of the peripherals including the HAT I2C bus works on the board.

The RTC is actually recognized by Raspbian after I added this to the /boot/config.txt file:

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dtoverlay=i2c-rtc,ds3231

But as AFAIU the HAT system should in some manner be able to configure the system automagically to use the provided peripherals on a board, right? It is controlled by what one writes to the HAT EEPROM on manufacture of the board.

So I want:
- The RTC to be recognized and used
- The RPi serial port assigned to this board (and not to Bluetooth for example)
- The I/O pins for the relays and and optical I/O reserved for the board (or at least set up properly)
- Some kind of presence marker for the board noted in the system

Is that possible?

[trejan]

But as AFAIU the HAT system should in some manner be able to configure the system automagically to use the provided peripherals on a board, right? It is controlled by what one writes to the HAT EEPROM on manufacture of the board.

You need to program the HAT EEPROM with the correct data for your board. Use https://github.com/raspberrypi/hats/blo ... ttings.txt as a template. You'll probably need a DT fragment for the RTC.

[Bosse_B]

Thanks!
I can use a clarification on the settings file format:

When setting up the GPIO map in the eeprom_settings.txt file, what do I do with pins used for:
- The serial port (GPIO14/15)
- The I2C bus (GPIO2/3)
Either I do not remove the comment on those lines or I set the pins as OUTPUT/INPUT as needed.
This causes no problem for the serial port, but the I2C SDA line is bidirectional....

If I leave these pins commented out will they then be handled appropriately by the Serial and I2C handlers on the system?
With all used pins configured in the file, the GPIO section looks like this:

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#         GPIO  FUNCTION  PULL
#         ----  --------  ----
setgpio  2      INPUT     DEFAULT
setgpio  3     OUTPUT     DEFAULT
#setgpio  4     INPUT     DEFAULT
setgpio  5      INPUT     DEFAULT
#setgpio  6     INPUT     DEFAULT
#setgpio  7     INPUT     DEFAULT
#setgpio  8     INPUT     DEFAULT
#setgpio  9     INPUT     DEFAULT
#setgpio  10    INPUT     DEFAULT
setgpio  11     INPUT     DEFAULT
setgpio  12     INPUT     DEFAULT
setgpio  13    OUTPUT     DEFAULT
setgpio  14    OUTPUT     DEFAULT
setgpio  15     INPUT     DEFAULT
#setgpio  16    INPUT     DEFAULT
#setgpio  17    INPUT     DEFAULT
#setgpio  18    INPUT     DEFAULT
setgpio  19    OUTPUT     DEFAULT
setgpio  20     INPUT     DEFAULT
setgpio  21     INPUT     DEFAULT
#setgpio  22    INPUT     DEFAULT
#setgpio  23    INPUT     DEFAULT
#setgpio  24    INPUT     DEFAULT
#setgpio  25    INPUT     DEFAULT
setgpio  26     INPUT     DEFAULT
#setgpio  27    INPUT     DEFAULT

Oh, another thing:
Can I add a comment on the pin usage on each line like this:

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setgpio  13    OUTPUT     DEFAULT  # START relay
setgpio  19    OUTPUT     DEFAULT  # POWER relay