N900 Hardware USB Host
This page is for those working on the implementation of friendly USB host mode.
See also the garage project.
Status
The current status is that USB Host mode is not reliable, even for those involved in development. It can work, see this post by egoshin on talk.maemo.org. Who has succeeded in mounting a USB stick using an externally powered hub.
However, this is using the partially implemented OTG driver, and confusing it into thinking it's talking to a OTG device in device mode.
This remains unreliable, and an implementation of host mode is likely to be much more stable.
Proper host mode would only require the normal cable, and a commonly available adapter. (USB A-A adaptor)
A user friendly implementation that makes it easy to use host mode is some way off.
There are several parts to getting USB host mode working for normal users.
Some of these are quite easy, many are not.
- USB modules for supporting hardware. This is fairly simple for most hardware - some hardware modules will not compile cleanly on ARM, and binary drivers that allow you to use windows drivers with some stuff will never work.
- Understanding and reverse engineering BMEs (Battery managment Entity) interaction with the hardware.
This is in progress.
- Usermode software.
- The current software will not do the right thing if the user plugs in almost all devices - even if hostmode worked. To make a mouse work, you might want to enable a pointer for example.
Ideas for hostmode user-software
When an external power supply is found, the following should be done in sequence.
- If the supply is a charger (D+,D- short, detected by the 1707a), fast charge.
- Attempt to enumerate as a device, if this succeeds (including handling of suspended busses), then enumerate and charge as a device, respecting the enumerated current.
- If enumeration as a device, and charger detection have failed, we may be plugged into a hub or device with reverse power. Attempt to enumerate connected devices. Charge at user-defined maximum current. For example - may be a 500mA power supply also supplying the hub and a keyboard = 2 unit loads - max draw = 300mA.