This is only the beginning of a long list of associated standards for USB. There are USB Class Standards such as the HID Class Specification which details the common operation of devices (keyboards, mice etc) falling under the HID (Human Interface Devices) Class - only another 97 pages.
If you are designing a USB Host, then you have three Host Controller Interface Standards to choose from. None of these are detailed in the USB 2.0 Spec.
Why we use USB?
- Setup takes a long time and is confusing
- Frequent PC crashes
- Cables clutter with devices conflicts
- Inaccessible Connectors
- Poor hot-plug support
The USB Solution
- Easy of Use
- One interface for many devices
- Automatic Configuration
- Easy to connect
- Easy Cables
- Hot Pluggable
- No User and Jumper Setting
- No power supply required (sometime)
USB Speed And Standard
USB 1.0 (Released in January 1996) and USB 1.1 (Released in September 1998) are old standards / protocols , USB 1.1 can switch over 2 speeds: Low speed & Full speed.
USB 2.0 (Release in April 2000) is the actual standard (not "high speed"). USB 2.0 provides compatibility with USB 1.1 . USB 2.0 can switch over 3 speeds:
Low sped 1.5 Mbps, app. 192 Kilobyte/sec
Full speed 12 Mbps, app. 1,5 Megabyte/sec
High speed 480 Mbps, app. 61 Megabyte/sec
USB VS Other Systems
Architecture Of The USB
The topology of USB is based on “tiered star topology” , single host controller and up to 127 'slave' devices.
The host controller is connected to a hub, untegrated within the PC, which allows a number of attachment points.
A device can be plugged into a hub, and that hub can be plugged into another hub and so on. However the maximum number of tiers permitted is six.
The length of any cable is limited to 5 metres. This limitation is expressed in the specification in terms of cable delays etc, but 5 metres can be taken as the practical consequence of the specification.
Typical 4-Port USB Hub
Host Is A Master
All communications on this bus are initiated by the host. This means, for example, that there can be no communication directly between USB devices.
A device cannot initiate a transfer, but must wait to be asked to transfer data by the host. The only exception to this is when a device has been put into 'suspend' (a low power state) by the host then the device can signal a 'remote wakeup'.
Cables - Electrical
USB cables have been designed to ensure correct connections are always made.
- USB requires a shielded cable containing 4 wires.
- Two of these, D+ and D-, form a twisted pair responsible for carrying a differential data signal, as well as some single-ended signal states. (For low speed the data lines may not be twisted.)
- The signals on these two wires are referenced to the (third) GND wire.
- The fourth wire is called VBUS, and carries a nominal 5V supply, which may be used by a device for power
Connectors
"A" receptacles point downstream from a Host or Hub, while "B" receptacles point upstream from a USB device or hub.
Series A plugs mate with A receptacles, and B plugs mate with B receptacles.
USB is a BUS
What we need to remember!,
At any point in time, only the host OR one device can be transmitting at a time. When the host is transmitting a packet of data, it is sent to every device connected to an enabled port. Only one device, the addressed one, actually accepts the data. (The others all receive it but the address is wrong for them.)
Tidak ada komentar:
Posting Komentar