Universal serial bus full-speed device interface (USB) RM0008
612/1096 Doc ID 13902 Rev 12
The actual procedure used to suspend the USB peripheral is device dependent since
according to the device composition, different actions may be required to reduce the total
consumption.
A brief description of a typical suspend procedure is provided below, focused on the USB-
related aspects of the application software routine responding to the SUSP notification of
the USB peripheral:
1. Set the FSUSP bit in the USB_CNTR register to 1. This action activates the suspend
mode within the USB peripheral. As soon as the suspend mode is activated, the check
on SOF reception is disabled to avoid any further SUSP interrupts being issued while
the USB is suspended.
2. Remove or reduce any static power consumption in blocks different from the USB
peripheral.
3. Set LP_MODE bit in USB_CNTR register to 1 to remove static power consumption in
the analog USB transceivers but keeping them able to detect resume activity.
4. Optionally turn off external oscillator and device PLL to stop any activity inside the
device.
When an USB event occurs while the device is in SUSPEND mode, the RESUME procedure
must be invoked to restore nominal clocks and regain normal USB behavior. Particular care
must be taken to insure that this process does not take more than 10mS when the wakening
event is an USB reset sequence (See “Universal Serial Bus Specification” for more details).
The start of a resume or reset sequence, while the USB peripheral is suspended, clears the
LP_MODE bit in USB_CNTR register asynchronously. Even if this event can trigger an
WKUP interrupt if enabled, the use of an interrupt response routine must be carefully
evaluated because of the long latency due to system clock restart; to have the shorter
latency before re-activating the nominal clock it is suggested to put the resume procedure
just after the end of the suspend one, so its code is immediately executed as soon as the
system clock restarts. To prevent ESD discharges or any other kind of noise from waking-up
the system (the exit from suspend mode is an asynchronous event), a suitable analog filter
on data line status is activated during suspend; the filter width is about 70ns.
The following is a list of actions a resume procedure should address:
1. Optionally turn on external oscillator and/or device PLL.
2. Clear FSUSP bit of USB_CNTR register.
3. If the resume triggering event has to be identified, bits RXDP and RXDM in the
USB_FNR register can be used according to Tab l e 17 1, which also lists the intended
software action in all the cases. If required, the end of resume or reset sequence can
be detected monitoring the status of the above mentioned bits by checking when they
reach the “10” configuration, which represent the Idle bus state; moreover at the end of
a reset sequence the RESET bit in USB_ISTR register is set to 1, issuing an interrupt if
enabled, which should be handled as usual.
Table 171. Resume event detection
[RXDP,RXDM] status Wakeup event Required resume software action
“00” Root reset None
“10” None (noise on bus) Go back in Suspend mode
“01” Root resume None
“11” Not allowed (noise on bus) Go back in Suspend mode
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