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Intel® 460GX Chipset Software Developer’s Manual 13-11
Universal Serial Bus (USB) Configuration
13.3.4 FRNUM–Frame Number Register (I/O)
I/O Address: Base + (06-07h)
Default Value: 0000h
Attribute: Read/Write (Writes must be Word Writes)
Bits [10:0] of this register contain the current frame number which is included in the frame SOF
packet. This register reflects the count value of the internal frame number counter. Bits [9:0] are
used to select a particular entry in the Frame List during schedule execution. This register is
updated at the end of each frame time.
This register must be written as a word. Byte writes are not supported. This register cannot be
written unless the Host Controller is in the STOPPED state as indicated by the HCHalted bit
(USBSTS register). A write to this register while the Run/Stop bit is set (USBCMD register) is
ignored.
13.3.5 FLBASEADD–Frame List Base Address Register (I/O)
I/O Address: Base + (08-0Bh)
Default Value: Undefined
Attribute: Read/Write
This 32-bit register contains the beginning address of the Frame List in the system memory. HCD
loads this register prior to starting the schedule execution by the Host Controller. When written,
only the upper 20 bits are used. The lower 12 bits are written as zero (4-Kbyte alignment). The
contents of this register are combined with the frame number counter to enable the Host Controller
to step through the Frame List in sequence. The two least significant bits are always 00. This
requires Dword alignment for all list entries. This configuration supports 1024 Frame List entries.
13.3.6 SOFMOD–Start of Frame (SOF) Modify Register (I/O)
I/O Address: Base + (0Ch)
Default Value: 40h
Attribute: Read/Write
This 1-byte register is used to modify the value used in the generation of SOF timing on the USB.
Only the 7 least significant bits are used. When a new value is written into the these 7 bits, 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
Bit Description
15:11 Reserved.
10:0 Frame List Current Index/Frame Number. Bits [10:0] provide the frame number in the SOF
Frame. The value in this register increments at the end of each time frame (approximately every 1
ms). In addition, bits [9:0] are used for the Frame List current index and correspond to memory
address signals [11:2].
Bit Description
31:12 Base Address. These bits correspond to memory address signals [31:12], respectively.
11:0 Reserved. Must be written as 0s.