Charging Downstream Port limits

[jlindquist74]

I'm designing a piece of equipment with USB ports that will be used for data, but also to charge the devices plugged into them as rapidly as possible.  I'm stuck trying to understand the behavior of downstream ports that can provide I_{DEV_CHG} charge current.  I started in Chapter 16 of Jan's book, then looked at the USB Battery Charging Spec: http://www.usb.org/developers/devclass_docs/Battery_Charging_V1_2.zip

It's clear that if a device attaches, but does not connect (it does not enumerate,) and it detects that it is connected to a Charging Downstream Port (CDP; host charger or hub charger,) it can draw up to 1500mA.  I can't find anything that says explicitly what the device must do if it connects.  Must it drop to 500mA (900mA for USB 3) after enumeration?  Or can it continue to draw its full charging current?  Figure 3-15 in the spec suggests it might, but I can find no other reference.

[Pat Crowe]

It may continue to draw IDEV_CHG (1.5A). See section 4.6.1. Don't forget that the period between VBUS going on and the PD connecting is a max of one second (except under dead battery conditions), which would not be a useful duration to be allowed to draw the higher current.

You will need to be able to enumerate in the presence of a ground offset relating to cable drops at the higher current.