Difference between revisions of "USB reference"
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Revision as of 07:37, 24 April 2011
|It has been proposed that this page be moved to "USB FAQ". (Discuss)|
- 1 Readme First!
- 2 USB FAQ
- 3 Pandora as USB slave
- 4 Hosting USB devices
- 5 OTG host mode
- 6 Links
If you have a question to ask, that isn't covered in this FAQ already, please ask over on the Forums.
How many USB ports does Pandora have?
Pandora has two USB ports:
- 1x USB 2.0 host port (USB standard-A receptacle)
- 1x USB 1.1/2.0 OTG port (USB mini-AB receptacle)
What does “host port” mean?
A USB host port “hosts” any device that you plug in, just like the USB ports on your computer. Pandora’s full size USB A port can host USB 2.0 devices (only if they are high speed compliant!), and provide up to 500mA (the maximum allowed according to USB standards).
What does “OTG” mean?
OTG stands for On The Go. USB OTG ports can operate in two different modes. When connected through this port, the Pandora can act as a device, for example when connected to a computer for charging & data transfer. For this mode, you need a standard-A to mini-B cable. However, the Pandora can also act as a host port for USB 1.1 and USB 2.0 devices. For details on how to make the necessary cable connection, see the OTG host mode section. This port can supply a maximum of 100mA in host mode. Although this port implements On The Go functionality, it can not get an official On The Go certification because the OTG specification requires a device to have exactly one USB port. (The Pandora has two.) Also, the mini-AB connector used on the Pandora has been taken out of the specification in 2007 (but is still widely adopted).
What’s all this stuff about USB 1.1 and USB 2.0? Isn’t USB, uh, universal?
USB 1.1 is a subset of the USB 2.0 protocol. Among many other things, the USB 2.0 standard introduced high-speed data transmission, which requires a special controller on the host side. Most USB ports (eg. on your computer’s motherboard, in USB hubs) actually contain two chips to allow universal compatibility – one that takes care of the Low-Speed (1.0 Mbit/s) and Full-Speed (12 Mbit/s) devices, and another one for the Hi-Speed (480 Mbit/s) devices. The reason is that Low-Speed and Full-Speed are essentially the same protocols operated at different clock frequencies. Hi-Speed is a completely new development and nothing like the other two.
For reasons beyond our control, Pandora’s USB standard-A host port does not contain this second chip. Only USB 2.0 devices bearing the “USB HI-SPEED” logo can be plugged directly into this port. However, USB 1.1 devices can be connected to this port through a USB hub, provided the hub bears the “USB HI-SPEED” logo.
Pandora’s USB OTG (mini-AB) port is fully compatible with both USB 1.1 and USB 2.0, allowing it to host either type of device.
So which devices are which?
There are no set rules. As a very general guide:
- Common Low-Speed and Full-Speed devices include keyboards, mice, headsets, older USB devices.
- Common Hi-Speed devices include cameras, PMPs, HDDs, hubs, card readers, thumb drives.
This is just a rough representation of what’s out there in the wild, and you will find exceptions. Devices like HSDPA modems and GPS units could fall into either category. The only reliable indication of true Hi-Speed compatibility is the “USB Hi-Speed Certified” logo (right).
Arrgh, USB used to be simple. What can I actually connect to Pandora?
Almost anything, as long as it's supported by Linux. Think of a USB device that everybody uses (mouse, keyboard, thumb drive...) and chances are it will be a simple matter of plug and play. But this might not be the case for more specialised USB devices (modems, ethernet adapters, USB displays...), many of which will only function with Windows specific drivers.
If you intend to purchase USB accessories for use with Pandora, you should first research Linux support for that type of device. Devices already tested on Pandora will be added to the USB compatibility list.
The various USB connection scenarios are as follows:
Pandora as USB slave
Using a USB standard-A to mini-B cable (the kind you probably own several of), you can:
- Transfer data to/from Pandora
- Charge Pandora's battery.
- Charging Pandora from your computer's USB port is best done with Pandora in standby mode. The current provided by your computer will probably not be sufficient to provide a quick charge while Pandora is being used.
- There are USB standard-A to mini-B cables that can be used to charge your Pandora but don't work to transfer data to/ from the Pandora! MSWindows will complain abount "An unknown device connected". Try using a different cable.
Hosting USB devices
- Directly plug in a Hi-Speed USB 2.0 device such as a thumb drive, or a Huawei E160 modem.
- Using a USB standard-A to USB standard-B (or mini-B, micro-B, depending on what's on the device) cable, connect Hi-Speed USB 2.0 devices such as cameras, PMPs, and HDDS.
- Connect a USB hub, which will enable you to to connect other non-Hi-Speed USB devices such as keyboards, mice, or a Huawei E220 modem.
- This port can supply a maximum of 500mA. If you plan to use a USB HDD, note that very few of these can draw sufficient power from a single USB port. A powered HDD caddy or powered USB hub will be necessary in most cases.
OTG host mode
How you connect a device to the mini-AB OTG port depends on the device's USB connector.
- standard-B receptacle: Use a mini-A to standard-B cable.
- mini-B receptacle: Use a mini-A to mini-B cable.
- micro-B receptacle: Use a mini-A plug to standard-A receptacle adapter and a standard-A to micro-B cable.
- standard-A plug: Use a mini-A plug to standard-A receptacle adapter.
- mini-A plug: No extra cable or adapter needed.
- micro-A plug: No standardized solution exists.
This port allows you to connect any USB 2.0-compliant device (Hi-Speed, Full-Speed, Low-Speed, including USB 1.1 ones) directly, without the use of a hub.
- mini-A and mini-AB connectors have been removed from the present USB standard. This means that the above connectors may vanish from the market at some point (although there will probably be demand for them for some time). There are a lot of connectors available that were never standardized. It is recommended that you do not use them as they may lead to non-working connections. Stick to the cables and adapters mentioned above and look for a mention of "OTG" or "On The Go" and you should be on the safe side.
- In host mode, this port can supply a maximum of 100mA. USB 1.1 devices that draw more than 100mA will need to connect to Pandora's other USB port via a hub, as depicted in the previous example. The Huawei E220 is one such device.
- If you have trouble connecting any device to the small OTG port, check if 'Setting->Startup->Enable/Disable services on boot->Start USB Network on bootup' is checked! This needs to be set in order to work. Default setting is unchecked (better naming of this option
is planned for Hotfix5has not been included in HotFix 5, see ).
Host mode connector
Shown here are a Mini-B connector (left, black) and a Mini-A connector (right, white). The mini-A connector should be the one you want in order to use the OTG port in host mode. Click on the image for a larger version.
With the right connector, it is not necessary to switch the mode of the port, it will be done automatically. It may also be possible to force a mode switch, but this has not been clarified by the devs, and could result in two devices both attempting to power the port (which is not recommended)