20

06/19/2000

GL600USB/GL600USB-A/GL600USB-B

Revision 1.3

external 1.5-k

Ω pull-up resistor connected between a data line and a voltage source between 3.0 V and 3.6

V. Since the power provided by the USB cable is specified to be between 4.4V and 5.0V, an on-chip

regulator is used to drop the voltage to the appropriate level for sourcing the USB transceiver and external

pull-up resistor. An output pin driven by the regulator is provided to source the 1.5-k

Ω external resistor.

4.7.2

USB Transceiver

The USB transceiver provides the physical interface to the USB D+ and D- data lines. The transceiver is

composed of two parts: an output driver circuit and a receiver.

The USB transceiver uses a differential output driver to driver the USB data signal onto the USB cable. The

differential high and low state are well balanced to minimize signal skew. Slew rate control on the driver is

used to minimize the radiated noise and cross talk. The driver’s outputs support 3-state operation to achieve

bi-directional half-duplex operation. The driver can tolerate a voltage on the signal pins of –0.5V to 3.8V

with respect to local ground reference without damage.

The rise and fall time of the signals on this cable are greater than 75ns to keep RFI (radio frequency

interference) emissions under FCC (Federal Communications Commission) class B limits and less than

300ns to limit timing delays, signaling skews, and distortions. The driver reaches the specified static signal

levels with smooth rise and fall times, and minimal reflections and ringing when driving the cable. This

driver is used only on segments between low-speed devices and the ports to which they are connected.

USB data transmission is done with differential signals. A differential input receiver is used to accept the

USB data signal. A differential 1 on the bus is represented by D+ being at least 200mV more positive than

D- as seen at the receiver, and a differential 0 is represented by D- being at least 200mV more positive than

D+ as seen at the receiver. The signal cross over point must be between 1.3V and 2.0V.

The receiver features an input sensitivity of 200mV when both differential data inputs are in the range of

0.8V and 2.5V with respect to the local ground reference. This is called the common mode input voltage

range. Proper data reception also is achieved when the differential data lines are outside the common mode

range. The receiver can tolerate static input voltage between –0.5V to 3.8V with respect to its local ground

reference without damage. In addition to the differential receiver, there is a single-ended receiver for each

of the two data lines.

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

Common Mode Input Voltage (volts)

Figure 4-3 Differential Input Sensitivity over Entire Common Mode Range