319537-003US Datasheet, PDF (218/450 Pages) Intel Corporation – Intel System Controller Hub

UHCI Host Controller (D29:F0, F1, F2)
12.3.6
SOFMOD—Start of Frame Modify Register
I/O Offset:
Default Value:
Base + (0Ch)
40h
Attribute:
Size:
R/W
8 bits
This register is used to modify the value used in the generation of SOF timing on the
USB. When a new value is written to bits 7:0, the SOF timing of the next frame will be
adjusted. This feature can be used to adjust out any offset from the clock source that
generates the clock that drives the SOF counter. This register can also be used to
maintain real time synchronization with the rest of the system so that all devices have
the same sense of real time. Using this register, the frame length can be adjusted
across the full range required by the USB specification. The initial programmed value is
system dependent based on the accuracy of hardware USB clock and is initialized by
system BIOS. It may be reprogrammed by USB system software at any time. The value
will take effect from the beginning of the next frame. This register is reset upon a host
controller reset or global reset. Software must maintain a copy of the value for
reprogramming if necessary.
Default
Bit
and
Access
Description
7
0, RO Reserved
SOF Timing Value: Guidelines for the modification of frame time are
contained in Chapter 7 of the USB Specification. The SOF cycle time
(number of SOF counter clock periods to generate a SOF frame length) is
equal to 11936 + value in this field. The default value is decimal 64 which
gives a SOF cycle time of 12000. For a 12-MHz SOF counter clock input, this
produces a 1-ms Frame period. The following table indicates what SOF
Timing Value to program into this field for a certain frame period.
6:0
40h
R/W
Frame Length (# 12 MHz
Clocks) (decimal)
11936
11937
—
11999
12000
—
12063
SOF Timing Value (this
register) (decimal)
0
1
—
63
64
—
127
218
Datasheet