USB Charging Detection

This is an example of why I wanted to get my oscilloscope. Ok, honestly the specifics of how a USB charging chip detects iOS, Android, or something else isn't too important since the chip already does it, but from a diagnostic perspective it is important if something doesn't work right.Above is how an Android device (in this case a Kindle Fire from last year) negotiates for a high-current charge. The trigger point (orange "T" on top) is where the charger was connected.The actual connection looks like that. Blue = D+, Magenta = D-, Yellow = Vcc. Both Android and iOS look the same way. Each line is in turn pulled to ground for a moment before what looks like an RC circuit recharges.What happens after that is different though.In the case of Android:D+ moves from the iOS spec of 2.7V up to 3V on the nose for around 3ms before D+ and D- get shorted out and tied to each other. I'm still not 100% sure why the voltage shift before shorting.In case of iOS...The device in question starts to draw a bit of current from D+ and D- it seems. I'm not sure, but it seems that the voltage detection  on the device side isn't as high impedance as Android.Part way through, this bit of curious squiggle happens:I have no idea. It looks like it's re-probing the voltages.I also tried a current-limited version of iOS as well:In this case I artificially limited the current to 1A. Once the iPad starts drawing current there's a huge voltage sag as the power supply drops the voltage to keep the current at 1A. The iPad recovers and started to charge.500mA wasn't as kind:It started to try, but it just gave up.It's interesting to look at how different iOS and Android are with something as simple as charging.