UM0306 USB full speed device interface (USB)
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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 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 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 register.
3. If the resume triggering event has to be identified, bits RXDP and RXDM in the register
can be used according to Tabl e 58, 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 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 register
to ‘1’ and resetting it to 0 after an interval between 1mS and 15mS (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
Table 58.
Resume event detection
[RXDP,RXDM] Status Wake-up 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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