NET2890 datasheet(34/73 Pages) ETC

34 / 73 page

Specification

NET2890 USB Interface Controller

____________________________________________________________________________________

NetChip Technology, Inc., 1999

335 Pioneer Way, Mountain View, California 94041

TEL (650) 526-1490

FAX (650) 526-1494

http://www.netchip.com

Rev 2.0, Draft 9, July 16, 1999

4.10 Root Port Reset

If the SIE in the NET2890 detects a single-ended zero on the root port for greater than 2.5 microseconds, it

is interpreted as a root port reset. The LRESET# output pin is asserted, and the following resources are

reset:

•

SIE

•

USB state machines

•

Local state machines

•

Our Address Register

•

Device Configured bit of Main Control Register.

•

FIFOs and FIFO Valid bits

The remainder of the configuration registers are not affected by the root port reset.The Root Port Reset

Interrupt Status bit is set when a root port reset has been detected, and if enabled, a local interrupt is

generated. The local CPU should take appropriate action when this interrupt occurs.

According the the USB Specification, the width of the USB reset is minimally 10ms and may be longer

depending on the upstream host or hub. There is no specified maximum width for the USB reset. The

LRESET# pin is asserted while the USB reset is active, and is negated a few 48 MHz clocks after the USB

reset condition is removed by the host.

4.11 NET2890 Power Configuration

The USB specification defines both bus-powered and self-powered devices. A bus-powered device is a

peripheral which derives all of its power from the upstream USB connector, while a self-powered device

has an external power supply. The NET2890 is well-suited for both types of applications.

The most significant consideration when deciding whether to build a bus-powered or a self-powered device

is power consumption. The USB specification lays out the following requirements for maximum current

draw:

•

A peripheral not configured by the host can draw only 100 mA from the USB power pins.

•

A device may not draw more than 500 mA from the USB connector’s power pins.

•

In suspend mode, the peripheral may not draw more than 500

µA from the USB connector’s

power pins

If these power considerations can be met without the use of an external power supply, the peripheral can be

bus-powered; otherwise a self-powered design should be implemented.

4.11.1 Bus-Powered Device

If the local bus is powered at 3.3 Volts, the VDD and VDDL pins of the device are connected to a 3.3 Volt

regulated source derived from the USB 5.0 volt power pin. For a 5.0 volt local bus, the VDDL pin may be

connected directly to the USB 5.0 Volt power pin, while the VDD pins must still be connected to 3.3 Volts

through a regulator. The rest of the local-side circuitry is also connected to the USB power pin, either

through a regulator or directly. Therefore, the peripheral’s local circuitry and the NET2890 will all power

up simultaneously, and initialization can occur normally with a power-on reset.

4.11.2 Self-Powered Device

Generally, a peripheral with higher power requirements will be self-powered. In a self-powered device, the

NET2890 VDD and VDDL pins of the NET2890 are powered by the local power supply. This allows the local

bus to continue accessing the NET2890, even when the device is not connected to the USB bus. The USB

connector’s power pin is left unconnected.

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