Usb Handshaking; Suspending; Resume - Sierra Wireless MC5720 Hardware Integration Manual

P C I E x p r e s s M i n i C a r d H a r d wa r e I n t e g r a t i o n G u i d e

USB handshaking

The module behaves as a non-host device supporting full-speed USB.
Consequently, the host that interfaces to the module must act as a USB host
device. The module does not use discrete hardware signals for handshaking;
instead the USB standard Resume and Suspend functions are used to control the
sleep and wakeup states. Detailed specifications for the Resume and Suspend
functions can be found in the USB specification in the Compaq, Intel, Microsoft,
and NEC, Universal Bus Specification.
EM modules are either equipped with an on-board USB transceiver, or provide
the interface to a USB transceiver. This section covers both types of EM
modules.

Suspending

A USB controller automatically enters suspend mode when the USB bus has
been idle for 3 ms. The USB controller triggers the USB_SUSPEND interrupt to
the module's processor and the module shuts down the 48 MHz clock to save
power.
While in the suspend state, the module provides power to the D+/- interface to
signal to the host device of its current state. During the Suspend state, the host
must maintain the VCC_3V3 voltage. The SOFTCON interface signal is also
maintained during suspend.
Critical timing parameters for the suspend state can be found in the Compaq,
Intel, Microsoft, and NEC Universal Serial Bus Specification.

Resume

USB activity may be resumed by either the USB host or by the module. If the
host initiates USB activity the USB transceiver detects the change in bus activity
and triggers the USB_RESUME interrupt to the module's processor. The module
then enables its USB clock and responds to the host.
If the module initiates USB communication, it must first enable its USB clock,
enable the USB transceiver, and then send the resume signal for at least 20 ms.
(See the Compaq, Intel, Microsoft, and NEC Universal Serial Bus Specification.)
Critical timing parameters for the resume state can be found in the Compaq,
Intel, Microsoft, and NEC Universal Serial Bus Specification.
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NOTE: The host USB driver must not send start-of-frames when
the modem or host have no data to send. This causes modem
wake-ups and adversely impacts current consumption of the
module.
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