This section just serves to refresh the most significant points. Operation The USB protocol has been explained in detail in Chapter 5. USB device communication is based on pipes (logical channels).Ī pipe is a connection from the host controller to a logical entity, found on a device, and named an endpoint.īecause pipes correspond 1-to-1 to endpoints, the terms are sometimes used interchangeably.Ī USB device could have up to 32 endpoints (16 in, 16 out), though it is rare to have so many.Īn endpoint is defined and numbered by the device during initialization (the period after physical connection called enumeration ) and so is relatively permanent, whereas a pipe may be opened and closed. The maximum number of devices, including hub devices, which can be connected to a single host controller is 127.Ī single device may provide several functions for example, a webcam (video device function) with a built-in microphone (audio device function).Īn alternative to this is a compound device, in which the host assigns each logical device a distinctive address and all logical devices connect to a built-in hub that connects to the physical USB cable. Inclusion of hubs allows for branching into a tree structure.Ī USB host may implement multiple host controllers and each host controller may provide one or more USB ports. It consists of a host, one or many downstream USB ports, and peripheral devices connected to ports in a tiered-star topology. This makes differential signaling immune to electrical interference.

The signal at the receiving end is interpreted as the difference between the two lines that make up the differential pair.

In differential signaling, each signal is transmitted using a differential pairthe signal carried by one wire is the same level as the one carried by the other wire, but in opposite polarity. The USB protocol is less prone to interference than the other protocols described earlier in this chapter because it uses differential signaling.ĭifferential Signaling When a signal is transmitted through a single wire, surrounding electrical interference acts on it and can potentially modify the signal levels, causing the signal to be detected incorrectly (a 1 getting detected as 0 and vice versa).

8bit single chanel lvds screen spec