Volvo head units (radios...)

Intro

I decided, that I'm tired of using CDs in my car. However it's got a quite old unit (2002 I think) and it cannot play mp3 and it doesn't have any aux or line-in input. And no, I don't want to replace it as it has astonishing sound quality (6 speakers + "central" sub-woofer) and great design and radio works in an unbelievable way. And integrates with the rest of the car nicely.
So I decided to make line input or a player that can connect to HU. 

HU story

I have HU-605 which has cassette player, AM/FM radio with all what can radio have and a CD. There are number of HU-xxx units and they all share the (more or less) same design. All parts are modules, and (I suppose) communicate with master controller through some sort of a bus. How do I know? Well, most units have DIN-8 connector a the back that allows you to connect something. And that something usually meant CD-changer. But if you look closely to the LCD (in test mode you can light-up all all segments) you will notice that there is a number of input sources possible:

(note, this is HU-650 display, note the lack of MD-CHGR channel)

And my HU has FM, AM, TAPE and CD channels "lit" during the normal operation. When I disassemble it (it's quite modular) and I remove tape deck and power it on - TAPE disappears! That's why I think all the modules share same bus and controller piece only, surprisingly, controls them. 

So we have a HU unit supporting many external (and internal) devices  - and a 8-pin DIN connector on the back to connect something.

Ready made devices

In general we have two classes of devices:

• iMIV - created by smart guy called mrg_Ed from swedespeed.com forums - this adapter has... all (iPOD support, video support, song names etc., all); and is costly ;)
• chinese/other adapters emulating CD-changer from folders on SD card and are not that cheap (300PLN)

So I decided I'll try to make my own. Starting from simple "line in". 

HU protocol

is not public. There are not that many people around the globe that know it or have some documentation. It's known as "melbus". 

Hardware layer

There are 3 lines involved - data, clock and busy. Signal level is 0-5V, so it can be almost directly connected to microcontroller. Note that inside HU itself there are specialized drivers based on complementary pair of transistors (push-pull). But I guess if you don't want lines having long meters you can safely use typical cmos microcontroller pin. As far as I managed to discover clock is generated by master (HU) and both data and busy is bi-directional. Busy is mainly pulled low by devices to inform master of presence. Data is bi-directional data bus.

DIN8 pinout:

1. Clock 
2.  Signal ground
3.  Run - +12 when radio is on? Can be used to power-up accessory?
4.  Data
5.  Busy
6.  Left analog audio
7.  Right analog audio
8. Mono audio (navigation, phone etc.)

This will be continued...