RM0367 Datasheet, PDF (909/1009 Pages) STMicroelectronics – This reference manual targets application developers

Universal serial bus full-speed device interface (USB)
RM0367
32.5.5
As it happens with double-buffered bulk endpoints, the USB_EPnR registers used to
implement Isochronous endpoints are forced to be used as unidirectional ones. In case it is
required to have Isochronous endpoints enabled both for reception and transmission, two
USB_EPnR registers must be used.
The application software is responsible for the DTOG bit initialization according to the first
buffer to be used; this has to be done considering the special toggle-only property that these
two bits have. At the end of each transaction, the CTR_RX or CTR_TX bit of the addressed
endpoint USB_EPnR register is set, depending on the enabled direction. At the same time,
the affected DTOG bit in the USB_EPnR register is hardware toggled making buffer
swapping completely software independent. STAT bit pair is not affected by transaction
completion; since no flow control is possible for Isochronous transfers due to the lack of
handshake phase, the endpoint remains always ‘11 (Valid). CRC errors or buffer-overrun
conditions occurring during Isochronous OUT transfers are anyway considered as correct
transactions and they always trigger an CTR_RX event. However, CRC errors will anyway
set the ERR bit in the USB_ISTR register to notify the software of the possible data
corruption.
Suspend/Resume events
The USB standard defines a special peripheral state, called SUSPEND, in which the
average current drawn from the USB bus must not be greater than 2.5 mA. This
requirement is of fundamental importance for bus-powered devices, while self-powered
devices are not required to comply to this strict power consumption constraint. In suspend
mode, the host PC sends the notification by not sending any traffic on the USB bus for more
than 3 ms: since a SOF packet must be sent every 1 ms during normal operations, the USB
peripheral detects the lack of 3 consecutive SOF packets as a suspend request from the
host PC and set the SUSP bit to ‘1 in USB_ISTR register, causing an interrupt if enabled.
Once the device is suspended, its normal operation can be restored by a so called
RESUME sequence, which can be started from the host PC or directly from the peripheral
itself, but it is always terminated by the host PC. The suspended USB peripheral must be
anyway able to detect a RESET sequence, reacting to this event as a normal USB reset
event.
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.
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