Motorola PowerQUICC II MPC8280 Series Reference Manual page 884

USB Buffer Descriptor Ring
27.6.1 USB Receive Buffer Descriptor (Rx BD) for Host and
Function
The CP reports information about each buffer of received data using Rx BDs. 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 detected
• Overrun error occurred
• Bit stuff violation detected
As shown in Figure 27-18, the first word of the Rx BD 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 (when using the packet-level interface) and
the function mode. There are no RxBDs in host mode when using the transaction-level
interface.
0 1
OFFSET + 0 E
—
OFFSET + 2
OFFSET + 4
OFFSET + 6
Figure 27-18. USB Receive Buffer Descriptor (Rx BD)
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 27-16 describes USB receive buffer descriptor fields.
Offset Bit Name
0x00 0 E
1 —
27-26
Freescale Semiconductor, Inc.
2 3 4 5 6
I L
W F
RX DATA BUFFER POINTER
Table 27-16. USB Rx BD Fields
Empty
0 The data buffer associated with this Rx BD 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 Rx BD. 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 Rx BD 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 Rx BD.
Reserved, should be cleared
MPC8280 PowerQUICC II Family Reference Manual
For More Information On This Product,
Go to: www.freescale.com
7 8 9 10
— PID —
DATA LENGTH
Description
11 12 13 14 15
NO AB CR OV —
1, 2
MOTOROLA