GL602USB datasheet(19/35 Pages) ETC1

19 / 35 page

Revision 1.6

-19-

02/28/2000

4.6 USB ENGINE

The USB module contains three functional blocks: a 3.3-volt regulator, a low-speed USB transceiver, and the Serial

Interface Engine (SIE). The USB module is only enabled under USB mode. The following description is the function

of the regulator, transceiver, and SIE.

4.6.1 Voltage Regulator

The USB data lines are required by the USB specification to have a maximum output voltage between 2.8V and 3.6V.

Because the GL602USB is a low speed USB device, the D- lines also are required to have an 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.

4.6.2 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 drive the USB data signal onto the USB cable. The static

output swing of the driver in its low state is below the VOL of 0.3V with 1.5-kΩ load to 3.6V and in its high state is

above the VOH of 2.8V with 15-kΩ load to ground. The output swings between 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.

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