USB on-the-go full-speed (OTG_FS)
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number and the time remaining until the next SOF are tracked in the host frame number
register (HFNUM).
An SOF pulse signal, generated at any SOF starting token and with a width of 12 system
clock cycles, can be made available externally on the SOF pin using the SOFOUTEN bit in
the global control and configuration register. The SOF pulse is also internally connected to
the input trigger of timer 2 (TIM2), so that the input capture feature, the output compare
feature and the timer can be triggered by the SOF pulse. The TIM2 connection is enabled
through the ITR1_RMP bits of TIM2_OR register.
34.7.2 Peripheral
SOFs
In device mode, the start of frame interrupt is generated each time an SOF token is received
on the USB (SOF bit in OTH_FS_GINTSTS). The corresponding frame number can be read
from the device status register (FNSOF bit in OTG_FS_DSTS). An SOF pulse signal with a
width of 12 system clock cycles is also generated and can be made available externally on
the SOF pin by using the SOF output enable bit in the global control and configuration
register (SOFOUTEN bit in OTG_FS_GCCFG). The SOF pulse signal is also internally
connected to the TIM2 input trigger, so that the input capture feature, the output compare
feature and the timer can be triggered by the SOF pulse. The TIM2 connection is enabled
through the ITR1_RMP bits of the TIM2 option register (TIM2_OR).
The end of periodic frame interrupt (GINTSTS/EOPF) is used to notify the application when
80%, 85%, 90% or 95% of the time frame interval elapsed depending on the periodic frame
interval field in the device configuration register (PFIVL bit in OTG_FS_DCFG). This feature
can be used to determine if all of the isochronous traffic for that frame is complete.
34.8 Power
options
The power consumption of the OTG PHY is controlled by three bits in the general core
configuration register:
•
PHY power down (GCCFG/PWRDWN)
It switches on/off the full-speed transceiver module of the PHY. It must be preliminarily
set to allow any USB operation.
•
A-V
BUS
sensing enable (GCCFG/VBUSASEN)
It switches on/off the V
BUS
comparators associated with A-device operations. It must
be set when in A-device (USB host) mode and during HNP.
•
B-V
BUS
sensing enable (GCCFG/VBUSASEN)
It switches on/off the V
BUS
comparators associated with B-device operations. It must
be set when in B-device (USB peripheral) mode and during HNP.
Power reduction techniques are available while in the USB suspended state, when the USB
session is not yet valid or the device is disconnected.
•
Stop PHY clock (STPPCLK bit in OTG_FS_PCGCCTL)
When setting the stop PHY clock bit in the clock gating control register, most of the
48 MHz clock domain internal to the OTG full-speed core is switched off by clock
