This isn't my biggest project, but it is most complicated and related to too much unknown to me things. It is my first Python code and my first Arduino project.
Audi Gamma CC Bose (HU) is my auto cassette player. It has ability to rule a CD Changer, not directly, but via specific Audi Interface (AI).
Even though communication with VAG's CD Changers is pretty well documented, I couldn't find any information about communication between my HU and the AI. This is what I surveyed.
The HU sends I2C/TWI signals to AI on 960 Hz (or sort of) to the address 0x40. ON is two bytes 0xA1 and 0x21, OFF is same bytes back order. Other signals what I riddle are from '^', 'v', '>>' and '<<' buttons hold and release. There are two more signals, one when HU doesn't like communication - I called "Cancel", and one more, what HU sends it from time to time instead of OFF.
- (Start)
- 1000000 (7bit addrtess 0x40)
- 0 (Write)
- 0 (Ack)
- 00100001 (8bit data 0x21)
- 0 (Ack)
- 10100001 (8bit data 0xA1)
- 0 (Ack)
- (Stop)
- (Start)
- 1000000 (7bit addrtess 0x40)
- 0 (Write)
- 0 (Ack)
- 10100001 (8bit data 0xA1)
- 0 (Ack)
- 00100001 (8bit data 0x21)
- 0 (Ack)
- (Stop)
Communication on the other direction is pretty ... incoherent. It looks like address and 3 bytes via I2C, but it is not. And there is no Ack signal in response. It should be around 30 ms after HU signal and after direction button release it repeats twice. And ditto any time when on display should appears something new.
In order to display CD01TR01 it will looks like:
Down side I colored what it would look like if it was I2C. But it is:
- (Start)
- 1000000 (7bit address 0x40, same as I2C)
- 01 (2 bits, similar to I2C Write & Ack, but there is no Ack level from the HU
------------------------------ 1st data chunk
- 0 (1 bit, not used)
- 00 (2 bits, ???)
- 0 (1 bit, 1 means NO DISK)
- 0 (1 bit, ???)
- 0001 (4 bits, CD #)
------------------------------ 2nd data chunk
- 0 (1 bit, not used)
- 00000001 (8bit data TR #)
------------------------------ 3rd data chunk
- 0 (1 bit, not used)
- 00000011 (8bit data CRC value)
- (Stop)
Check sum is CD + TR + 1, sometimes minus 0x10 in order not to overflow 0xFF.
I implemented the emulator from Arduino to replace the AI and Raspberry Pi to replace the CD Changer. I use separate DAC because that build in RPi is ... not good enough.
Communication between the Arduino and RPi is serial UART similar (but not same) to standard CDC communication.
Communication between the RPi and the DAC what I chose, indeed PCB5102A, is via I2S.
Power is based on LM2575-5.
Note: There is a trick. First hand shake of the HU comes around 0.4 seconds after HU power on. The Arduino takes more time for boot. Thats why I add two NPN transistors - one in order to lower acknowledge bit of I2C/TWI communication, and another one to cancel this after Arduino's boot.
I've made a special PCB to hold everything. The connector to the HU is hand made 10 pins similar to common ISO connector.
- VAG CDC Faker
- Nick Gammon's page about I2C / How to use I2C-bus
- Raspberry Pi rev2 template with mounting holes
- efficient 12V to 5V convertor LM2575
- DAC PCM5102A (Beyond ES9023 PCM1794)
- ALOHAta
- Lars Frerichs
- grizzly@audibg (George)