Texas Instruments OMAP5912 Reference Manual page 1379

Table 41. Start of Frame Register (SOF)
Bit Name
15:13 Reserved
12 FT_LOCK
11 TS_OK
10:0 TS
SPRU761A
Description
Reserved
Frame timer locked bit: the USB host sends out a start of frame (SOF) packet
every millisecond. When the device does not receive a SOF packet because of a
bus error, a local start of-frame that is used for ISO FIFO switch is generated.
Once the core receives two valid SOF, separated by TF (time frame), it sets
SOF.FT_LOCK to 1, only if TF is in frame interval IF allowed by the USB 1.1
specification (IF = [11964:12036] USB bit time). If TF is out of this interval,
SOF.FT_LOCK value remains 0, and a local SOF is generated by the core.
When SOF.FT_LOCK is set and the frame timer is locked to the timing TF, a local
SOF is generated, if no valid SOF has been received in an interval of TF since
the last valid SOF. The SOF.FT_LOCK bit is cleared if a valid SOF is received out
of the interval IF. If the core receives a valid SOF in this interval, the frame timer
locks to the new frame time. If the core does not receive a valid SOF, the frame
timer remains locked to TF.
When SOF.FT_LOCK is cleared, a local SOF is generated after the 12036 USB
bit time, if no valid SOF has been received, and SOF.FT_LOCK remains 0.
0: Frame timer is not locked.
1: Frame timer is locked.
The value after MPU or USB reset is low.
Note: Each time the core receives a valid SOF out of the allowed interval IF, a
local SOF is generated and the ISO FIFO switches.
The timestamp OK bit indicates that the timestamp in SOF.TS is valid for the
current frame. It returns a 1 if a valid SOF packet was received from the USB host
and a 0 otherwise.
0: Timestamp is invalid.
1: Timestamp is valid.
Value after MPU or USB reset is low.
The timestamp number field returns the timestamp from the last USB host-valid
SOF packet. The frame number is valid if SOF.TS_OK is 1. In case of an SOF
miss, this value is not updated and TS_OK is cleared.
The value after MPU or USB reset is low (all 11 bits).
This read-only register provides a frame timer status for use in ISO
communications. A write to this register is denied.
USB Device Controller
Universal Serial Bus (USB) 87