help! - serial communication to an ipod, mode4-AiR

Forum 2005-2010 (read only) Hardware Interfacing
1

I have a 2nd generation iPod nano that I'm trying to control with my arduino.

I'm using the Apple_Accessory_Protocol found on ipodlinux.org:

I can get all the mode 2 commands working, but when i try to send mode 4 commands (after switching to mode 4) I get either no response at all (on some commands) or i get an error response (0x04 0x00 0x01 0x04). which means: 4=you exceeded the limit of whatever you were requesting/wrong parameter-count.

example, if i send: 0xFF, 0x55, 0x03, 0x04, 0x00, 0x1E, 0xDB i get the response: 0xFF, 0x55, 0x06, 0x04, 0x00, 0x01, 0x04, 0x00, 0x1E, 0xD3.

but I have tried this for quite a few commands, either the information on what parameters to send is wrong, or mode4 commands doesn't work on a 2g nano, or somehow I'm sending it wrong.

Does anyone here have any clues to what's wrong?

here is how i connected it and some code.
(signal goes from tx on arduino through 1kohm resistor to rx on ipod and through 2kohm more to ground, pin21 connected to ground with a 500ohm resistor (have tried some different resistors, including 500kohm))
Rx (input 0) --------------------iPod pin12 Tx

Tx (output 1) ---[1k]-------------iPod pin13 Rx
|
| ----iPod pin21 Accessory
| |
| |

2k 500

| |
| |
Gnd-------------------------------iPod pin11 Ground

code in arduino:
byte switchMode4[] = {0xFF, 0x55, 0x03, 0x00, 0x01, 0x04, 0xF8};
byte getPosition4[] = {0xFF, 0x55, 0x03, 0x04, 0x00, 0x1E, 0xDB};

void setup ()
{
Serial.begin(19200);
for (int i=0;i<8;i++){
Serial.print(switchMode4*,BYTE);*
}
}
void loop ()
{
/*for (int i=0;i<7;i++){ // i have experimented with different lengths.
Serial.print(getPosition4*,BYTE);*
}
delay(2000);
}

2 
void loop ()
{
/*for (int i=0;i<7;i++){ // i have experimented with different lengths.
Serial.print(getPosition4[i],BYTE);
}
delay(2000);
}

You have an opening comment symbol there, but no closing comment symbol. The C-style /* and */ comment symbols must match up. They do not care about the end of a line, like the new C++ // coments do.

Does this sketch compile without errors, with a mismatched comment?

3

Not sure about the protocol here but are your checksums formed correctly?

On this link, do you add the checksum with FF or do you AND it with FF?

http://www.ipodlinux.org/wiki/Apple_Accessory_Protocol

How are you reading the response? Are you leaving enough time for the data to appear on the serial port?

Does not sound like a hardwar eissue f you got a rsponse on the other mode.

4

thnx for the replies.

I actually just copy pasted a few lines from the code, leaving out the function for listening incomming transmissions, but yes the code does compile.

I have a very simple function for reading the incoming data:

if(Serial.available()>0){
incomming=Serial.read();
if(DEBUG){
Serial.print(incomming);
}
there is a delay(2000) after each command sent, so there is enough time for a response and the reading part works, I can read the error messages that the iPod sends back.

I actually calculate the checksums manually with a calculator, adding the sum of all the bytes after the 0xFF, 0x55 and then subtracting it from 100(hex), so:
{0xFF, 0x55, 0x03, 0x04, 0x00, 0x1E, 0xDB};
would be :3+4+0+1E=25
100-25 = DB.

so no, I'm not AND'ing it with FF, but that's not necessary unless the first part of the equation would be greater then 100, right? (which it isn't). I also used the same method for creating the checksums for all the mode 2 commands that work.

I've been taking a few days off for another project so I haven't done any more experiments, next one is going to be trying it with another iPod and some different commands.

So I'll report back with my results in a few days.