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FAQs: USB embedded host

Posted: 11 Nov 2008 ?? ?Print Version ?Bookmark and Share

Keywords:FAQ? USB? embedded?

What is the difference between a Host, Embedded Host, limited host and mini-host?
Hosts are almost always referred in context to PCs and laptops where any USB peripheral can be plugged. Full host must source 500mA of current on the Vbus to power the peripheral devices connected to it.

Embedded hosts are always found in small form factor and portable devices like a set top box, PDA, etc. It has to source current of minimum 8mA on the Vbus. It has limited memory space to store limited drivers, thus the connectivity to peripheral is also limited. Unlike a full host the embedded host is not required to load device drivers for a device it does not support. It is required, however, to notify the user that an unsupported device was attached.

Embedded hosts are often called "mini-Hosts" or "limited hosts" but they refer to the same type of device. The terms embedded host, mini-host, or limited host are not referred to in the USB specification or the OTG supplement. The certification procedural walkthrough refers to these devices as Embedded hosts.

Can a high speed peripheral, like a hard drive, be connected to a full speed host?
Yes. The USB protocol requires all full speed and high speed communication to initiate as full speed, and then scale up to high speed if both devices support it. In the case that one of the devices supports full speed only, the communication would be limited to full speed, 12Mbit/s.

So any hard drive would work with any host?
No. Hard drives are mass storage devices, and as such have some form of data format provisions. For the devices to work, as opposed to just recognize each other, the file and interface protocols must also match. For example, a Thumb drive is a basic mass storage application. But for it to function, the USB class drivers, the SCSI interface, and the FAT16 format must all be present.

Currently only the FAT16 file format is supported in the USB embedded host firmware release versions. FAT32 support is being developed. NTFS and other file systems are not supported.

If I am an embedded host then do I need to support Session Request Protocol (SRP) and Host Negotiation Protocol (HNP)?
For an embedded host SRP is an optional feature but is not required. In most cases it is probably not desired either. For more information about what SRP is please see the OTG section of this FAQ.

Since an embedded host is only capable of being a host and never a USB device, HNP should not be supported. For more information about what HNP is please see the OTG section of this FAQ.

Source: Microchip Technology Inc.





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