I'm working on an STM32 (ARM Cortex-M3) addon board, called "ARMinARM". It's breaking out all Raspberry Pi B+ GPIO pins, as well as all STM32 pins. They could possibly be all connected or disconnected, depending on the usecase of the end user. Originally for the "old" model B, but I'm reworking it for the B+, see topic here: http://www.raspberrypi.org/forums/viewt ... 45&t=80726
I'm planning on breaking out the eeprom I2C pins to a seperate header on the board. Playing with the idea that the enduser (or myself) could design an addon board for the ARMinARM that would contain the eeprom that identifies the "addon board for the addon board" to the Raspberry Pi. In fact, identifies the two together as one entity. First I simply pass them through. Save them for later. Leaving them floating.
A distinction is made in the spec between "New add-on boards basic requirements" and "B+ HAT requirements". The ARMinARM board is not going to meet the requirements to be called a HAT (no eeprom, no camera slot, no display slot) and I'm perfectly comfortable with that. However,
If you are designing a new add-on board that takes advantage of the pins on the B+ GPIO header other than the original 26 then you must follow the basic requirements:
1. The header must cover the ID_SD and ID_SC pins and either a valid ID EEPROM must be used on these pins, OR the ID_SD pin must be shorted to GND (so the Pi can detect at least that a board is plugged in) and the ID_SC pin must be left unconnected. Do not use ID_SC and ID_SD pins for anything except an ID_EEPROM, or shorting ID_SD to GND on non-EEPROM boards.
A general question, why must the Pi be able to "detect at least that a board is plugged in"? What's it going to do without further info from a missing eeprom?
And specifically, would RPF be at least open to the idea of supporting beforementioned "addon board for addon board that contain an eeprom"? Because also:
Of course users are free to put an ID EEPROM on boards that don't otherwise conform to the remainder of the specifications - in fact we strongly encourage this; we just want things called HATs to be a known and well-specified entity to make life easier for customers, particularly the less technical ones.
And only for the sake of discussion (nitpicking maybe), would an addon board that contains an eeprom for the ARMinARM board (still without camera and display slot) suddenly make the two boards together a 'proper' HAT? No conflicts?
Basically I'm asking: What happens if I initially do leave those pins floating, against all instructions?