Universal Serial Bus (USB)
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
31-21
31.11.7 USB Status Register (USBS)
The USB status register, described in
Figure 31-17
and
Table 31-13
, is a read-only register that allows the
user to monitor the real-time status condition on the USB lines.
Table 31-13
describes USBS field.
31.12 USB Buffer Descriptor Ring
The data associated with the USB channel is stored in buffers that are referenced by BDs organized in BD
rings located in the dual-port RAM (refer to
Figure 31-18
). These rings have the same basic configuration
as those used by the SCCs and SMCs.
There are four separate transmit BD rings and four separate receive BD rings, one for each endpoint. The
BD ring allows the user to define buffers for transmission and buffers for reception. Each BD ring forms
a circular queue. The CP confirms reception and transmission or indicates error conditions using the BDs
to inform the processor that the buffers have been serviced.
The buffers may reside in either external memory or internal memory.
0
1
2
3
4
5
6
7
Field
—
IDLE
Reset
0000_0000
R/W
R
Addr
0xA17
Figure 31-17. USB Status Register (USBS)
Table 31-13. USBS Fields
Bit
Name
Description
0–6 —
Reserved
7
IDLE
Idle status. IDLE is set when an idle condition is detected on the USB lines, it is cleared
when the bus is not idle.
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