STR720 datasheet(278/401 Pages) STMICROELECTRONICS

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STR720 - USB SLAVE INTERFACE (USB)

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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 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

in all the cases. If required, the end of resume or reset sequence can be detected moni-

toring the status of the above mentioned bits by checking when they reach the “10” config-

uration, 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

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 61. 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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