iPod classic; pullup values and voltages explained!

[Wreck Rider]

Opening
I have a charger for use with iPod classic and the whole story about different value of pullup resistors confused me a little.

Success Story and Question
My charger works now for my iPod classic and that's great, but it didn't make any sense to me why a 15K, 100K, or 560K pullup resistor should make a difference, if its really acting as a pullup resistor. As it turns out, its actually acting more as a bias resistor and the resulting voltages on the D+ and D- pins seems to be the more important part.

Investigation with pullup resistors
With any of the three recommended resistor values as pullup, the voltage on D+ and D- is approx. 5 V as long as nothing is plugged in. When my iPod is connected it draws some juice out of the D+ and D- lines, and changes the resulting voltage on those lines.

The D- line voltage is tabulated here, the D+ in each case is about 0.1 V higher.
with 15 K, iPod draws the voltage down to 3.7 V - doesn't charge
with 100 K, iPod draws the voltage down to 3.4 V - doesn't charge
with 560 K, iPod draws the voltage down to 2.0 V - Charging!!!
The charging icon is the one you expect, it looks like a lighting bolt on the battery indicator.

Theory
Looks like the voltage on the data lines may be the important thing. So, I made a 2.5 V reference point by connecting two same value resistors in a bridge across the +5 VDC and ground connections, then connect both data lines to the 2.5 V point. Plugging the iPod in still changes the voltages.

15 K as bridge, iPod doesn't change the voltage, 2.5 V - doesn't charge
100 K as bridge, iPod drops the voltage to......... 2.0 V - Charging!!!
560 K as bridge, iPod increases the voltage....... 3.0 V - doesn't charge

Hypothesis
Voltage on the D- / D+ lines is important. To initiate charging, it must be about 2.0 V.

Final Check
To check this I make a very rugged 2.0 V reference point with a 10K / 15K bridge, connected to both D+ and D-.

Result and Conclusion
The voltage on both data lines is 2.0V, with or without the iPod plugged in. The iPod charges.

Suggestion for Further Work
In the case of the iPod classic, choice of 560K pullup resistor results in 2.0 V on the D- line, due to a balancing pulldown by the iPod itself, and initiates charging. Smaller resistor values do not initiate charging, and the voltage on D- line is higher than 2.0 V. A different method if creating a 2.0 V level on both data lines also resulted in charging.

The different pullup values needed to initiate charging on other models of iPod may reflect that those other iPod models draw more current on their data lines and so a smaller pullup resistor is needed, to be sure the final voltage is about 2.0 V. It is interesting that more modern devices require higher value pullup resistors, suggesting more modern devices draw less current on their data lines - this is sensible.

I propose that applying a rugged 2.0 reference voltage to D+ and D- lines will initiate charging in any model or generation of iPod, and that would allow a truly universal MintyBooster to be built. I have the ability to build this device but not to test it since I don't have access to lots of different models of iPod.

I hope some other users will try this and post the results.

[Zash]

I can confirm that 560kΩ works with an itouch and iphone. With a 22kΩ resistor it works with 2g shuffle.

Edit: I modified a 1A usb wall wart charger as suggested by this post but with resistor and cap values from this thread.

[Wreck Rider]

The thing I'm interested is whether configuring the data lines at a fixed voltage of 2V will be accepted by many iPod devices rather than having to customize the charger by using pull-up and pull-down resistors. I think it will

Here's the data line biasing strategy I suggest.

http://www.flickr.com/photos/ontarioasparagusdiver/2468147724/

[adafruit]

there is a perfect configuration of resistors that will work will iphones, ipods, etc but it requires 4 resistors and the pcb only has space for 2

[olbetsy]

I did a few voltage measurements with my 1G nano and iPhone. The 1G nano isn't ideal, since it doesn't need any pullup or pulldown to work, but that's what I've got on hand.

I built several dividers according to this schematic:

One with 4 x 100k, another with 4 x 47k and I then made a third that only has 1 100k divider, connected to D+ and D- by a 1k each. This idea came from here: http://cre.ations.net/creation/cozy-faux-leather-universal-usb--iphone-charger

All were powered by a power supply set at 5V, both nano and iPhone were about half charge to ensure they tried to draw current.

My results are:

[Resistor Value] - (charge y/n) - (D+ no load / load) - (D- no load / load)

Nano:

[100k] - (y) - (2.475/2.13) - (2.495/2.13)
[47k] - (y) - (2.482/2.30) - (2.543/2.17)
[100k - 1k] - (y) - (2.497/2.28) - (2.497/2.15)

iPhone:

[100k] - (n) - (2.497/2.23) - (2.497/2.24)
[47k] - (n) - (2.497/2.35) - (2.497/2.40)
[100k - 1k] - (n) - (2.497/2.002) - (2.497/2.002)

So the iPhone wouldn't charge at all, no matter what I did and the nano, unsurprisingly, charged like an angry rhino.

I'm not sure why the third design worked for natetrue and not me.

I tested the voltage on the D (D+/D-) lines on the iPhone and the nano when connected to a charger (not a computer) and they read while charging:

nano - (2.2/2.6) (this remained at a steady voltage)
iPhone - the voltage switched back and forth rapidly on both D lines, D+ (1.74/1.82) and D- (2.4/2.5). I suppose this was the communication between the two?

Oh, and I'm not sure I'd do this again, after this measurement the iPhone it started acting weird and the screen wouldn't respond properly and I had to restart it. Seems fine now, but be warned.

As for what this explains about the voltage situation on the D lines, I'm not really sure.

Any ideas?

edit - I also tried the technique from this thread on the iPhone and it was also unable to charge the iPhone. http://www.ladyada.net/forums/viewtopic.php?p=27262#27262

[WildSwan]

there is a perfect configuration of resistors that will work will iphones, ipods, etc but it requires 4 resistors and the pcb only has space for 2

For the sake of those who are exploring, can you please disclose this perfect configuration of resistors?

[adafruit]

i dont remember, but i think it was ~65K impedence into a ~2V reference so do the math

[WildSwan]

i dont remember, but i think it was ~65K impedence into a ~2V reference so do the math

There we have it guys... lets do the math. Thanks Lady.

[olbetsy]

Well, I'll do some 'spearmintin (get it ) this weekend and let people know what I find.

Although to be honest I don't have a lot of faith, been stymied quite a bit so far.

[magician13134]

Well, I'll do some 'spearmintin

Hee hee! That's pretty clever!

I'm looking forward to someone coming up with a perfect solution! Good luck!

[JPL]

If the bridge does indeed work, will this make it happen on the existing board?

http://www.flickr.com/photos/27343531@N05/2551997974/

My first flicker pic, I hope it works.

[PredatorCoder]

What if you short D+/D- together and do not tie them up or down to anything? That would make it a USB Battery Charging Specification compliant charger (well, you're supposed to source 1.5A current too). See page 13 of the specification. It's quite possible that new Apple products are adhering to this spec and is simply not working properly with an out-of-spec charger. Try removing R6/R7 and instead run a wire from D- to D+. Two of the old resistor holes will work for this (see schematic).

[homuel]

This is how I solved the charging issue with my iPod classic:

http://www.flickr.com/photos/30158432@N05/2827596228/in/pool-adafruit

With this solution I get ~2V on both D+ and D- with ~50K impedance.
Unfortunately it uses 4 resistors and is not directly useable with the v2 pcb. I also do not know how this works on the other iPod models and the iPhone.

Beside this the 10k to gound, 15k to VCC and shorting D+ and D- also works with my iPod classic (6.G).

Holger

[yeoj4151]

Why are you posting this here, this is a fourm for talking about charging an iPod and other devices, not about the iPod's themselves. I would suggest you post on the offical Apple Fourm's, and/or call in and ask someone yourself.