Physical Layer - 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

Physical layer

Differential 1 and 0
The USB uses differential signaling (called D+ and D-) with NRZI (Non-Return
to Zero, Inverted) encoding. A NRZI encoder toggles the output for input source
data bit 0 and maintains current output state for source data bit 1. A differential 1
exists at the driver when the D+ output is at least 2.8 V and the D- output is no
greater than 0.3 V. A differential 0 exists at the driver when D- is at least 2.8 V
and D+ is no greater than 0.3 V. At the receiver, a differential 1 exists when D+
is at least 2 V, and the difference between D+ and D- is greater than 200 mV. A
differential 0 exists when D- is at least 2 V, and the difference between D- and
D+ is at least 200 mV.
Suspend and resume
The USB master signals "suspend" by not sending the periodic 1 ms SOF for a
minimum of 3 ms.
The USB exits from the suspend state in either of these ways:
•
•
Both resume and remote-wakeup generate the resume signaling on the bus.
For more information on USB signal definitions, see the Compaq, Intel,
Microsoft, and NEC Universal Serial Bus Specification.
Suspend and resume
If the USB is not completely powered off and interrupts are still enabled, resume
and suspend interrupts can control the USB's sleep/wakeup model provided that
the host USB driver supports suspend mode and remote wakeup.
USB_SUSPEND and USB_RESUME interrupts are provided in the
USB_INT_STATUS register (for more detail, see the USB XCVR data sheets.)
Full-speed devices send SOF, which is a timing reference the host sends at 1 ms
intervals. The SOF keeps devices from entering the low-power suspend state
when there is no other USB traffic. Low-speed devices do not see the SOF
packet. Instead, the device's hub translates the SOF to a simpler End-of-Packet
signal called the low-speed keep-alive signal. As the SOF does for full-speed
devices, the low-speed keep-alive keeps low-speed devices from entering the
suspend state. Note that the EM is a full-speed device.
The USB controller automatically enters suspend mode when the USB bus has
been idle for 3 ms. No more than 500 µA current can be drawn from the bus
when in the suspend state. This is indicated by the USB_SUSPEND interrupt.
The current suspend status of the USB is checked through the USB_RUNNING
bit of USB_CORE_STATUS. When the USB controller enters the suspend state,
the modem is able to shut down the 48 MHz oscillator to save power.
3 0
Resume is initiated by the USB master, or
The USB slave devices initiate remote-wakeup.
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