42.7 UHPHS USB Frame Index Register

This register is used by the host controller to index into the periodic frame list. The register updates every 125 μs (once each microframe). Bits [N:3] are used to select a particular entry in the Periodic Frame List during periodic schedule execution. The number of bits used for the index depends on the size of the frame list as set by system software in the Frame List Size field in the UHPHS_USBCMD register).

This register must be written as a DWord. Byte writes produce undefined results. This register cannot be written unless the Host Controller is in the Halted state as indicated by the HCHLT bit (UHPHS_USBSTS register). A write to this register while the Run/Stop bit is set to 1 (UHPHS_USBCMD register) produces undefined results. Writes to this register also affect the SOF value.

Name:	UHPHS_FRINDEX
Offset:	0x1C
Reset:	0x00000000
Property:	Read/Write


Bit	31	30	29	28	27	26	25	24

Access
Reset


Bit	23	22	21	20	19	18	17	16

Access
Reset


Bit	15	14	13	12	11	10	9	8
			FI[13:8]
Access			R/W	R/W	R/W	R/W	R/W	R/W
Reset			0	0	0	0	0	0


Bit	7	6	5	4	3	2	1	0
	FI[7:0]
Access	R/W	R/W	R/W	R/W	R/W	R/W	R/W	R/W
Reset	0	0	0	0	0	0	0	0

Bits 13:0 – FI[13:0] Frame Index

The value in this register increments at the end of each time frame (microframe, for example). Bits [N:3] are used for the Frame List current index. This means that each location of the frame list is accessed eight times (frames or microframes) before moving to the next index. The following illustrates values of N based on the value of FLS (Frame List Size) in the UHPHS_USBCMD register.


UHPHS_USBCMD.FLS	Number Elements	N
0	1024	12
1	512	11
2	256	10
3	Reserved	–

The SOF frame number value for the bus SOF token is derived or alternatively managed from this register. The value of FRINDEX must be 125 μs (1 microframe) ahead of the SOF token value. The SOF value may be implemented as an 11-bit shadow register. For this discussion, this shadow register is 11 bits and is named SOFV. SOFV updates every eight microframes (1 millisecond). An example implementation to achieve this behavior is to increment SOFV each time the FRINDEX[2:0] increments from 0 to 1.

Software must use the value of FRINDEX to derive the current microframe number, both for high-speed isochronous scheduling purposes and to provide the “get microframe number” function required for client drivers. Therefore, the value of FRINDEX and the value of SOFV must be kept consistent if chip is reset or software writes to FRINDEX. Writes to FRINDEX must also write-through FRINDEX[13:3] to SOFV[10:0]. In order to keep the update as simple as possible, software should never write a FRINDEX value where the three least significant bits are 7 or 0.