Universal serial bus full-speed device interface (USB) RM0008
627/1128 DocID13902 Rev 15
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 Table 172, 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.
A device may require to exit from suspend mode as an answer to particular events not
directly related to the USB protocol (e.g. a mouse movement wakes up the whole system).
In this case, the resume sequence can be started by setting the RESUME bit in the
USB_CNTR register to ‘1 and resetting it to 0 after an interval between 1 mS and 15 mS
(this interval can be timed using ESOF interrupts, occurring with a 1mS period when the
system clock is running at nominal frequency). Once the RESUME bit is clear, the resume
sequence will be completed by the host PC and its end can be monitored again using the
RXDP and RXDM bits in the USB_FNR register.
Note: The RESUME bit must be anyway used only after the USB peripheral has been put in
suspend mode, setting the FSUSP bit in USB_CNTR register to 1.
Table 172. 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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