Apple iPad charging modes analysis

The voltages on USB Data+ and Data– pins tell the iDevice how much current the USB port can supply. Let me sum up my research and all the available information on this topic.
[UPDATE: 2013]

USB pinout

Pin	Wire color	Description
1	Red	+5V
2	White	Data–
3	Green	Data+
4	Black	Ground

Apple charger specs

charger	voltage	max. current	max. power
USB 2.0	5V	500mA	2.5 Watt
iPhone charger (A1205, A1265, A1300)	5V	1A	5 Watt
iPad charger (A1357)	5.1V	2A	10 Watt
iPad 4 charger (A1401)	5.1V	2.4A	12 Watt

Charging modes

charging mode	Data–	Data+	actual current drawn by my iPhone4	actual current drawn by my original iPad	actual current drawn by my iPad3
500mA mode	2V	2V	100mA before handshake, 240mA, 350mA, 440mA on my desktop on different USBs (wtf?), 550mA when connected to Macbook Air (incl. running from battery)	100mA before handshake, 480mA desktop PC, 920mA Macbook Air USB	100mA before handshake, 480mA desktop PC, 920mA Macbook Air USB
1A mode	2.75V	2V	520mA	800-900mA	950mA
2A mode	2V	2.75V	550mA	800-900mA	990mA
2.4A mode	2.66V	2.66V	?	?	?

The current drawn by a device remains constant even you unlock it and start some power hungry app.
You can notice that the currents seem a bit strange, but I'm looking into it. Maybe the devices need to be discharched completely – I'll post an update later [april 2013]. EDIT: A completely discharched iPhone draws the same power.

Resistor configurations you can use in your project

Required Data+/– voltages can be set with a simple voltage divider circuit. You can select different resistors based on what you may have in stock. Data+ and Data– voltages probably don't need to be precise but approximate. There's some examples I found on the internet which should work:

resistor	500mA mode	1A mode	1A mode (genuine iPhone charger)	1A mode	2A mode	2A mode (genuine iPad charger)	2.4A mode (could work)
R1	75k	33k	75k	75k 1%	33k	43k2 1%	? 51k
R2	50k	22k	51k	49k9 1%	22k	49k9 1%	? 49k9
R3	75k	33k	39k	43k2 1%	33k	75k 1%	? 51k
R4	50k	47k	51k	49k9 1%	47k	49k9 1%	? 49k9
tested by me	–	ok	–	ok	ok	ok	–

device	500mA mode	1A mode	1A mode	1A mode	2A mode	2A mode	2.4A mode
iPhone 3GS	?	240mA?	?	?	?	?	?
iPhone 4	?	380mA	?	?	620mA	?	?
iPad 3	?	620mA	?	?	1.1A	?	?

Detailed table for 1A mode (33/22/33/47k set):

voltage	iPhone 4	iPad 3
≤ 4.3V	0mA nothing happens	80mA nothing happens
4.35V	0mA nothing happens	80mA nothing happens when voltage slowly drops from higher voltage, 'Charging is not supported with this accessory.' alert screen pops up and 180mA is drawn
4.5V	180mA	280mA 'not charging' in status bar
4.66V	180mA 'not charging' screen	380mA 'not charging' in status bar
4.7V	230mA	380mA 'not charging' in status bar
4.85V	?	480mA 'not charging' in status bar
5V	380mA	620mA 'not charging' in status bar
5.2V	420mA	660mA 'not charging' in status bar
≥ 5.25V	?	0.7A 'not charging' in status bar

I'll try the remaining configs later this month (april 2013). Stay tuned!

Voltage divider circuit

Useful links

Connect and charge iPhone/iPad with your old Kenwood headunit
ricpaul's posts on ipadforums.net forum
timothyb's DIY iPad 2 USB Charger

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