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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