Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 871

31.12.1 USB Receive Buffer Descriptor (RxBD) for Host and Function
The CP reports information about each buffer of received data using RxBDs. The CP closes the current
buffer, generates a maskable interrupt, and starts receiving data in the next buffer when the current buffer
is full. Additionally, it closes the buffer on the following conditions:
• End of packet was detected.
• Overrun error occurred.
• Bit stuff violation detected
As shown in Figure 31-19, the first word of the RxBD contains status and control bits. These bits are
prepared by the user before reception and are set by the CP after the buffer has been closed. The second
word contains the data length—in bytes—that was received. The third and fourth words contain a pointer
that always points to the beginning of the received data buffer.
The RxBD is identical for both the host mode and the function mode.
0 1
OFFSET + 0 E —
OFFSET + 2
OFFSET + 4
OFFSET + 6
1
Entries in boldface must be initialized by the user.
2
All fields should be written by the CPU core before enabling the USB
Table 31-14 describes USB receive buffer descriptor fields.
Offset Bit Name
0x00 0 E
1 —
2 W
Freescale Semiconductor
2 3 4 5
W I L F
RX DATA BUFFER POINTER
Figure 31-19. USB Receive Buffer Descriptor (RxBD)
Table 31-14. USB RxBD Fields
Empty
0 The data buffer associated with this RxBD has been filled with received data, or data
reception has been aborted due to an error condition. The CPU core is free to examine
or write to any fields of this RxBD. The CP will not use this BD again while the E-bit
remains zero.
1 The data buffer associated with this BD is empty, or reception is currently in progress.
This RxBD and its associated receive buffer are owned by the CP. Once the E-bit is
set, the CPU core should not write any fields of this RxBD.
Reserved, should be cleared.
Wrap (Final BD in Table)
0 This is not the last BD in the RxBD table.
1 This is the last BD in the RxBD table. After this buffer has been used, the CP will receive
incoming data into the first BD in the table (the BD pointed to by RBASE). The number
of RxBDs in this table is programmable and is determined only by the W-bit and the
overall space constraints of the dual-port RAM.
MPC885 PowerQUICC Family Reference Manual, Rev. 2
6 7 8 9 10
— PID —
DATA LENGTH
Description
Universal Serial Bus (USB)
11 12 13 14 15
NO AB CR OV —
1,2
31-23