Specification
NET2272 USB Peripheral Controller
______________________________________________________________________________
NetChip Technology, Inc., 2003
Patent Pending
335 Pioneer Way, Mountain View, California 94041
TEL (650) 526-1490
FAX (650) 526-1494
http://www.netchip.com
Rev 1.2, October 15, 2003
50
7
Power Management
7.1
Suspend Mode
When there is a three-millisecond period of inactivity on the USB, the USB specification requires a device to enter
into a low-power suspended state. A low power device may not draw more than 500
µA, and a high-power device
may not draw more than 2.5 mA while in this state. This requirement only applies to bus-powered devices. To
facilitate this, the NET2272 provides a Suspend Request Change Interrupt bit and a Suspend Request Interrupt bit.
Additionally, the NET2272 allows local bus hardware to initiate a “device remote wake-up” to the USB.
7.1.1 The Suspend Sequence
The typical sequence of a suspend operation is as follows:
•
During device configuration, the local CPU enables the Suspend Request Change Interrupt bit to generate a
local interrupt.
•
When the USB is idle for three milliseconds, the NET2272 sets the Suspend Request Change Interrupt bit,
generating an interrupt to the local CPU. This interrupt can also occur if the NET2272 is not connected to a
host, and the USB data lines are pulled to the idle state (DP high, DM low), or if the VBUS input is low.
•
The local CPU accepts this interrupt by clearing the Suspend Request Change Interrupt bit, and performs the
tasks required to ensure that no more than 500
µA of current is drawn from the USB power bus.
•
The local CPU writes a 1 to the Suspend Request Interrupt bit to initiate the suspend.
•
The LCLKO output continues to operate for 500
µsec before the NET2272 enters the suspend state. This
allows time for a local CPU that uses LCLKO to power down.
•
A device remote wakeup event will not be recognized during the 500
µsec suspend delay period.
In suspend mode, the NET2272’s oscillator shuts down, and most output pins are tri-stated to conserve power (see
section 3, Pin Description). Note that input pins to the NET2272 should not be allowed to float during suspend
mode. The NET2272 will leave suspend mode by detecting a host initiated wake-up or by a device remote wake-up.
If a device is self-powered, it may ignore the USB suspend request and never write a 1 to the Suspend Request
Interrupt bit.
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