USB System Architecture

Figure 5-10: Reset Signaling States

' '

' '

PV

USB devices have two signaling states, which are opposite for lowand fullspeed devices:

•J state

•K state

When a device is initially attached to the USB, one of its data lines is near Vcc and the other is near ground. This state is known as the “J” signaling state and is also the idle state for the device. When a signal transition occurs, the two data lines switch, resulting in a transition to the “K” state.

Differential Signaling

USB transfers data packets using differential signaling to reduce various sources of signal noise. These sources include:

•Amplifier noise — the noise introduced when a signal is amplified by both the driver and receiver of a signal.

•Cable noise — the noise picked up by the cable due to electromagnetic fields.

104

Chapter 5: LS/FS Signaling Environment

Figure 5-11 on page 105 illustrates the data signaling between a USB hub and device. Note that the differential signaling is implemented in a half-duplex fashion. That is, the device on either end of the cable can both transmit and receive data, but in only one direction at a time. The half-duplex implementation requires that the drivers be placed into the high impedance state when they are not transmitting data.

A differential “1” is signaled when D+ is greater than D- and a differential “0” is signaled when D- is greater than D+. The actual voltages specified for differential drivers are:

•Differential “1” = D+ > VOH (min) and D- < VOL (max)

•Differential “0” = D- > VOH (min) and D+ < VOL (max)

The voltage levels that the receiver must detect are:

•Differential “1” = (D+) - (D-) > 200mV and D+ > VIH (min)

•Differential “0” = (D-) - (D+) > 200mV and D- > VIH (min)

Figure 5-11: Signaling Interface Between Hub and Device

.:

105