HS_USB2.0 host
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Doc ID 018672 Rev 1
22.6.8 USBCMD
register
The USBCMD is a RW register which indicates the command to be executed by the serial
bus EHCI host controller.
[07:04]
IST
4’h1
Isochronous scheduling threshold.
This field indicates, relative to the current position of
the executing EHCI host controller, where software
can reliably update the isochronous schedule. When
bit [7] of this field is 1‘b0 (default), the value of the
least significant 3 bits indicates the number of micro-
frames a EHCI host controller can hold a set of
isochronous data structures (one as default or more)
before flushing the state. When bit [7] is set to 1‘b1,
then host software assumes the EHCI host controller
may cache an isochronous data structure for an
entire frame.
[03]
Reserved
-
Read: undefined.
[02]
ASPC
1‘h0
Asynchronous schedule park capability.
If this bit is set, then the EHCI host controller supports
the park feature for high-speed (HS) queue heads in
the asynchronous schedule. The park feature can be
disabled or enabled as well as set to a specific level
by using the asynchronous schedule park mode
enable and asynchronous schedule park mode count
fields in the USBCMD register.
[01]
PFLF
1‘h0
Programmable frame list flag.
This bit states the frame list length, according to
encoding:
1‘b0 = System software must use a frame list length
of 1024 elements with this EHCI host controller. In
this case, the frame list size (FLS) in the USBCMD
register is a read only field and it should be set to
2‘b00.
1‘b1 = System software can specify and use a smaller
frame list, configured by the frame list size (FLS) field
in the USBCMD register.
The frame list must always be aligned on a 4K page
boundary, in order to ensure that the frame list is
always physically contiguous.
[00]
64BAC
1‘h0
64 bits addressing capability.
This bit documents the addressing range capability of
this implementation, according to encoding:
1‘b0 = Data structures using 32 bit address memory
pointers.
1‘b1 = Data structures using 64 bit address memory
pointers.
Table 353.
HCCPARAMS register bit assignments (continued)
Bit
Name
Reset value Description