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Controlling an iPod with an Atmega32

Introduction

Have you ever imagined, "What does that cable I plug into my iPod every day actually do, and how do I take advantage of it for myself?" We did too, and that's what we aimed to do with our 476 project. The iPod is, of course, the popular digital music player developed by Apple. The heart of the project is to use the Mega32 microcontroller to transmit and receive commands from the iPod using the built-in Dock Connector, as opposed to cracking the iPod open and wiring it up. The Dock Connector is the little port at the bottom of the iPod that you would normally connect a white USB or FireWire sync cable into. Commands that are currently implemented include play, stop, fast forward and other basic commands that allow the user to enjoy music from the iPod, as well as the ability to display the current track's Title, Artist, and Album on an external LCD display.

This document is written for, primarily, the 476 graders! Just as importantly though, this document is intended for future 476 classes looking for interesting project ideas to tackle, because as far as we could tell, no one had attempted a project dealing with the iPod before. This document is also intended for those searching on the Internet for help on hacking their iPod.
High-Level Design

At first we were worried about the feasibility of the project. We had decided early on that we wanted a project with obvious user feedback. After speaking to Bruce Land about the concept of an iPod dock, we decided that it would be our final project -- what is more obvious than music starting or stopping?

A huge amount of our time involved researching the iPod communication protocol. The iPod's communication protocol through the Dock Connector is not actually publicly available from Apple. However, thanks to the reverse engineering efforts by many hobbyists, there is a lot of information about the protocol (termed the "Apple Accessory Protocol") available on the Internet.

It is possible that the protocol isn't public due to the "Made for iPod" licensing program, which lets accessory makers in the "iPod ecosystem" connect to the iPod and display certain logos on their packaging after licensing. This could present certain legal issues, as it may be considered some form of protected intellectual property. However, as the protocol has been obtained through reverse engineering methods and not through a breach of contract, we do not anticipate this to be a big factor. Additionally, this is only an educational exercise and not in any way intended to be a commercially viable product.

The structure of our project is to have the Atmel Mega32 controller talk to the iPod in both sending and receive mode through the iPod's Dock Connector. We would send commands over the serial pins on the Mega32 and receive on the appropriate iPod pins as well. We can verify that the commands worked by simply observing the iPod and checking to see that the iPod responded appropriately, e.g. with a volume change, skipped track, etc.

The only relevant standards that we use are 8-N-1 serial communication for talking to the iPod. The actual Apple Accessory Protocol is an Apple creation, so not quite a "standard" in a usual sense. Otherwise, we do not make any use of any other standards, such as IEEE or ISO standards. The iPod sends out responses for strings in ASCII format.
State Machine diagram

Research

A large amount of work went into determining whether this idea was actually feasible. No one appears to have made any similar efforts in 476 final projects before, and though there is a similar idea in ECE 313, that course was designed around a standard CD player and controlled the player by shorting the buttons.

We initially found the iPod Dock Connector pin-out, which is available from pinouts.ru After that, we eventually found the Apple Accessory Protocol, which is detailed best on the iPod Linux wiki.\\
Important Pins on the iPod\\
|Pin| 	Use|
|1  |	Ground|
|11 |	Serial Ground|
|12 |	iPod RX; microcontroller transmits on this pin|
|13 |	iPod TX; microcontroller receives from this pin|
|21 |	iPod Accessory resistor: We use 500k|

It's important to note which pins are the receive and transmit pins. On the pinouts.ru page, pin 13 is "Serial RxD," which means serial receive from the iPod's point of view. Mixing it up could confuse you unnecessarily for a few hours.

Once we had a pinout and what looked like usable, if unofficial, documentation of the protocol, we started looking at actual connectivity to the iPod. There were a few efforts done by other people to create their own docks, but these were not quite what we were looking for.

After a bit of searching, we managed to find a hobbyist's website that had the actual iPod connectors available for sale. Even better, they also had a custom PCB available for sale to break out all of the individual signals from each of the pins, which was very useful because the pins themselves are rather small.

A short while later, we ended up discovering a second source for our PCB, which cost about the same as the original source but came with the dock connector pre-soldered, which was very useful as it was probably going to be a better soldering job than we could hope to accomplish by ourselves. Additionally, unlike the other source, it would ship within the United States. Using First Class Mail, we received our component in about 2 days (shipped on Thursday from CO, received on Saturday).

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