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FAQs: getting started with USB

Posted: 07 Nov 2008 ?? ?Print Version ?Bookmark and Share

Keywords:USB? FAQ? hub stack?

What are the resource requirements for the USB stack?
Resource requirements vary depending on the compiler, the processor family, the USB operational mode (Host, USB Device, OTG, etc), the USB class/function driver in use, etc. The resource requirements may also vary between release versions of the USB firmware.

How does a USB hub slow down devices?
USB hub has to re-calculate the time left before the end of the frame. The small extra delay added by the hub will decrease the available bandwidth. If several devices are connected on the hub and working in parallel (for example, a webcam + a USB flash drive + a mouse), then the USB bandwidth is shared among the devices usage.

What can be maximum cable length between two hubs or two USB devices?
The maximum length of USB cable is 5 meters.

What is difference between "composite" USB device & "compound" USB device?
A "compound" USB device that has a built in hub, in addition to one or more USB peripherals devices all built into a single product with only one USB cable connecting it to the host.

A "composite" USB device does not use hub silicon. It uses more than one interface in a single peripheral device. Currently, Microchip's Full Speed USB silicon can be used to develop composite USB devices but not compound devices.

What can be maximum capacitance across Vbus pin of a USB device? Why?
The maximum capacitance as seen by the Vbus pin of the USB connector must be less than 10?F.

This is to limit the inrush current that goes into the device when it is plugged into the device. The purpose of limiting the inrush current is to limit the drop in the Vbus voltage due to the charging of the capacitors on the newly attached device. Without limiting the inrush current of hot-plugged devices, a newly attached device may cause other devices to stop working.

If an application requires more capacitance than the spec allows, a soft-start circuit is required to limit the inrush current to the specified limits.

What care should be taken when USB device is self powered?
If the device is self powered, an I/O pin must be used to detect a cable attachment. The D+ or D- line must not be pulled up until USB host drives the Vbus high.

A self powered device must also consistently specify that it is self powered. If the configuration descriptor says that it is self powered then any GET_STATUS requests to the device must also return self-powered.

What should I specify in my configuration descriptor if I want to be either bus powered or self powered, depending on the situation?
If the device wishes to be bus powered at all, even if it self powered part of the time, then it must declare itself as a bus powered device.

The GET_STATUS request should accurately reflect to the host if the USB device is currently running on self power or bus power.

Why do I need INF file while using CDC where as I do not need anything at PC side while using HID class?
The CDC class has many other sub class specifications. The host needs to know which of these device drivers to load for the attached device. This information is contained in the .INF file. This means that the host will require some installation/setup process when a new CDC device is attached for the first time.

Source: Microchip Technology Inc.

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