Well back then (~10 years ago?) it was just a standard three signal minijack (left out, right out, mic in)
I think the audio out was open to software, but the incoming mic data required an MFI chip to be able to open/read the channel in software.
I never did get my hands on an MFI chip, so I never got far enough to figure out what it interfaced with. I assumed at the time it must somehow talk to the phone over the mic line, otherwise how could you build a microphone that used the standard mic jack? I'm sure it's encrypted, I know they were like 10¢ or so in volume, and once Apple approved/blessed your idea you could get tech docs and samples. That's about all I know, as I couldn't get them to work with us, as my company refused to hand over and project plans, market data etc to them. It was going to take multiple months before we got anywhere, and we had weeks to get a demo running. Spent about 3x time and money on that project as it should have taken because of their user hostility. So f*ck em.
Maybe some one else here can add some details on how MFI works on microphones.
Edit: should follow up saying that once I figured out that they weren't locking out WiFi, we added a small WiFi hotspot module to the PCB. So you plugged it into the phone, it drew power from the phone to run a hotspot. You then had to go into the phone and join the hotspot. Then our app would open a port on the local wireless LAN to tunnel a few hundred bytes of data a second. Blew my mind that I could only get around Apple blocks by sticking another radio on the phone.
I don’t know what special chip you’re talking about.
I wrote firmware for both wired USB and wireless IAP2 for external hardware support on iPhones, and it didn’t require anything more than the standard developer account.
Shipping and getting the device certified by Apple, with a literal stamp on the box that says “Made for Apple Devices” or whatever, is where MFi comes into play.
MFi is an account you sign up for to get spec sheets, support, and access to order custom components like lightning connector modules. But that’s all optional for hobby programming.
Things might be different now, but for sure back when I was doing this there was an "authentication chip" or "authentication coprocessor" needed to unlock almost all available I/O functionality.
There's this [0] which in the 2nd paragraph says that you need an authentication chip.
Here's a few articles from back in the day [1] [2] talking about how you can't use the serial port without the MFI authentication chip. Many sites recommended jailbreaking your phone to use the serial port, which of course doesn't help me with a commercial solution.
There are people complaining about needing a chip to use audio in/out features. [3] [4] Apparently this irritated a bunch of car audio buffs when they locked this down.
We tried using the standard Bluetooth SPP protocol (serial port protocol) as a bypass. Found out that Apple won't support SPP without the chip. [5] [6] For those that don't know, Bluetooth was originally designed to be a wireless serial port to replace RS-232 cables. [7] So support for this should be standard behavior out of the box. But of course Apple wanted to tax this feature as well.
I can't seem to find articles about people talking about using line-in software modem as a bypass, but this stuff is from a long time ago and forums come and go. I certainly didn't come up with the idea on my own, I only attempted it because other people had done this to bypass the serial port lockdown.
Like I said, maybe it's different now, but it wasn't back then. After my bad experience I vowed never to purchase an Apple product again.
How does this work? We are talking about the now-deceased analogue minijack, right?