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 Apple Uses Secret Resistors To Stop Unauthorized Accessory iPod Charging
Old 08-06-2010, 05:39 AM   #1

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Apple Uses Secret Resistors To Stop Unauthorized Accessory iPod Charging

Apple Uses Secret Resistors To Stop Unauthorized Accessory iPod Charging

Apple's plenty protective of their "Made for iPod" revenue stream, so much so that they just went about “>suing a number of unlicensed third-party accessory makers for products of inferior quality that reflected poorly on Apple’s brands.

Of course, part of the reason why third-party accessories work so poorly is because Apple is sneaky and employ various hardware tricks to make sure that only “Made for iPod” accessories work perfectly.

The hackers over at Minty Boost have gotten to the bottom of one such trick Apple deploys to make sure that unauthorized accessories have a hard time charging your iPod or iPhone.

In essence, Apple uses secret resistors that are placed in the connectors for Apple devices: if these resistors aren’t there, your iPhone or iPod Touch won’t deted the 2.8V and 2V signals, and hence won’t charge, coughing up a “Charging is not supported with this accessory” message instead.

The good news for unauthorized accessory makers (and makers) is that once you know the trick Apple employs to stop iPod charging on non-”Made for iPod” devices, it’s pretty easy to work around. Now that the secret of how Apple gimps unauthorized accessories is out in the wild, though, I wouldn’t be surprised to see Apple come up with a new way to stick it to those unwilling or unable to pay their “Made for iPod” licensing fees.


Quick introduction to USB

Every computer has a USB connector on it, and all the connectors are the same, with 4 pins. One pin is Ground, two pins are Data (D+ and D-) and the last pin is 5V power. The Ground and 5V pins are used to provide power to whatever is plugged in - keyboard, mouse, USB key, etc. The two data lines are used to transfer information back and forth - what keys are pressed, saving files to the USB drive, etc.

Shown above, the 4 wires. Red is power, black is ground, and the white and green wires are the data lines

Using USB as a power supply
Some inexpensive USB toys (say a USB fan or mini soda cooler) don't have any data transfer, they just suck the power from the USB port to run. In this case, they do not use or connect to the Data pins (they are left to 'float').

Now technically USB ports can provide up to 500mA of current output, and technically every device is supposed to perform a basic data transfer to the computer (called enumeration) where it says "hey, I'm about to drag 500mA out of the computer, just so you know" and the computer can say "go ahead" or "no can do" (this is called the power negotiation).

However, we've found that every device that does not require to do any data transfer (say, USB fans or USB battery chargers) don't bother to warn the computer and just go ahead and grab up to (or even more!) than 500mA from the USB port. As long as they aren't going thru an unpowered hub, this seems to be just fine. All computers have a resettable fuse on the USB port so that if more than 1000mA is drawn, the power will be disconnected. This protects against short circuits in your $5 USB fan that would take down the entire computer!

Knowing the above, we designed the first Mintyboost to not have anything on the data lines - we assumed that nearly every charger and device would just ignore those pins as they tend not to be used

For example, in the CAD file of the first Mintyboost, the USB connector is the top square. The four ovalish pins in row as the USB wires. Pin #1 and #4 are used for power (blue lines are connected) but there are no traces on the middle two data pins - they are floating.

When we first released the v1.0 of the MintyBoost oh so long ago, we quickly got feedback from people who owned all sorts of Apple brand gadgets. It turned out that older devices worked fine but some of the newer ones, such as the iPod Mini, were not charging. Hmm! Time to experiment!

First attempt

We figured there was something simple that would make the Apple device charge, and it definately had to do with the data lines (the power lines are fixed at ground and 5V). We thought "is there a enumeration chip inside every charger?" but since thats expensive and kind of overkilly we decided instead to read up on the USB protocol (go Jan!) In particular, in her fantastic book there's a part about the low level signaling states. Since you want to get the iPod charging, but NOT make it try to enumerate, we figured that we should see if there was some sort of special state you could put the data lines into that would say "no computer is attached but there is power". Turns out there is! Its called the SEI and occurs when BOTH data lines are at 3V. For mega details, read this chapter

Now to test. We stole an iPod from a friend and cut open a cable so we could mess with the data lines. We tried 3 options each - connected to ground, connected to 3V and not connected (float). At the same time we measured the current draw going thru the power line.

When the pins are floating no charging occurs. The next thing to note is that whether the pins are pulled up or down effects the current draw. Since this was the first Mintyboost, using the MAX756, we wanted to use the lower 100mA rate so one pin pulled down and another pulled up. This would be more efficient for the battery use and keep the chip from getting hot (it could provide 250mA but didn't like it much)

Thus was born Mintyboost v1.1!

Note that because the mintyboost runs from 2 AA batteries, we can get 3V directly from the batteries so we connected the pullup right to the battery input

The iPhone and larger iPods
This worked ok for about a year, then people starting getting the newer iPods and noticed that the Mintyboost didn't work anymore

We made a new version that now had either a pulldown or pullup on the D+ line

This worked for a bit until the iPhones came out. With enormous batteries, the iPhone was not happy charging at 250mA - it wanted 500mA or even 1000mA to charge! We sought out an upgrade to the MAX756 and found the LT1302 which could provide 500mA no problem.

Using 100K pullups on the data lines worked pretty good and all was happy! Then the iPhone 3Gs came out and...

Apple stopped being as 'lax' with the charging interface and started being very picky about having the official chargers. We still doubted that there was an enumeration chip inside each charger - too expensive and complex. So there must be something else going on in those data lines.

Time to sacrifice an official Apple iPhone 3Gs charger!

Taking it apart, desoldering the 4 data line resistors and mearing them on our multmeter, we found the following as shown in the schematic:

The four resistors create a voltage at each of the data lines thats not 3.3V but rather 2.8 and 2.0 (or so) volts. The problem is that when you do this, the iPhone starts to draw as much as 1Amp! Way more than the LT1302 and a couple AA's can provide. We were a bit sad and thought that there was no way to get the Mintyboost working with an iPhone when we took at trip to J&R and found an item called the TuneJuice. The TuneJuice is an iPhone charger that uses 4 AAA batteries. This is very interesting because there is no way to get an Amp out of AAA's - they are just way too small. That means there must be something ELSE going on in that TuneJuice charger to keep the iPhone from gobbling up the batteries. So we took apart the charger!

And found the following! (We substituted the closest 1% resistor values)

This time both voltages on the data lines are = 49.9K / (49.9K+ 75K) * 5.0V = 2.0V

We did some experimenting (see the video up top) and determined that in fact the different voltages/resistances did effect the charging rates! Using the 2.8V&2.0V setup resulted in a 1 Amp charge rate and the 2.0V&2.0V setup resulted in a 500mA charge rate.

This made us very happy, because 500mA is within the capability of the MintyBoost chip. We redesigned the PCB to allow us to have 4 resistors on the datalines and put two 75K and two 49.9K resistors in each kit. So far we have had no problems charging any of the latest Apple devices.

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The Following 7 Users Say Thank You to ddub420 For This Useful Post:
buckster (02-16-2013), kerode (02-14-2013), KnightWolf (02-14-2013), multipazz (08-06-2010), neo_drone (08-06-2010), NorthernOne (02-25-2013), psychozev69 (08-06-2010)

Old 08-06-2010, 10:53 AM   #2
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These guys are awesome vim going to make one of these.
"Well, it took a lot of work to be the ass that I am
And I'm really damn sure that anyone can, equally,
easily, fuck you over." -Modest Mouse

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Old 08-06-2010, 11:41 AM   #3
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this information is not so new...

Apple iPod, iPhone (2g, 3g), iPad Dock connector pinout and wiring @

if you look at pins 25 & 27 in the link above...

To charge an iPhone 3G / iPod Touch 2nd gen or Ipod Classic (6th Gen), usb data- (25) should be at 2.8v, usb data+(27) should be at 2.0v. This can be done with a few simple resistors: 33k to +5v (23) and 22k to gnd(16) to obtain 2v and 33k to +5v and 47k to gnd to obtain 2.8v. This is a notification to the iphone that it is connected to the external charger and may drain amps from the usb.
now if they can reverse engineer the iPad USB Camera Adapter ... then i will be impressed
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Old 02-14-2013, 04:10 PM   #4
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iPod 2.1A charger Foxlink NSW24363LPS advertises D+ = 2.73V, D- = 2.0V3 at 5.1V no-load output.
Short D+ or D- to ground, and you get 120 uA.
Short D+ to gnd and you get 118 uA; to 5.1V and you get 103uA.
This gives a 49.9K/43K for 2.73V.
Likewise for D- to gnd is 68uA, to 5.1V is 103uA, giving 49.9K/75K.
It only cost $5 to cut a USB charger cable, multimeter, and calculator to find this out.

Last edited by NorthernOne; 02-25-2013 at 01:19 PM. Reason: changed mA to uA on one entry.
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Old 02-14-2013, 08:43 PM   #5
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Sorry to answer ... But all this is not true
One huge giant lie , there are no "secret" transistors or resistor or all the buulshit written here

The only pins used for charging are the pins for +v and gnd , the other 2 are used just for USB communication , nothing else

I have a USB cable maden with only 2 wires ( for power only )

If you guys from 1st post had your device connected to a computer you would find in logs
That the device checks the amount of energy or can get ( amperes )
If the amount of energy is not enough the device will not start to charge

It's documented in logs inside your device
If your device fail to charge using a 3rd party charger it means , the charger doesn't provide the minimum needed to charge the device

So if you do tests with batteries , these batteries must be able to provide the amount of energy needed or else your test won't work

iPhone iPods need 5v 0.5 A to run and charge
iPads are more power suckers thus then need 5v 2.1 A ( 4 times what the iPhone needs ) to run and charge

This is also the reason for special USB ports needed in computers for iPhone iPod charging

Just a non standard minimum power consumption , don't mess with the data wires , these are just for data
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Old 02-16-2013, 04:34 AM   #6
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Yea built a few of these with my top secret resistors (radio shack) with small photoelectric cells. Nice weekend project.
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charing, iphone, ipod, mintyboost

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