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the BDT and buffers in system memory. A semaphore, the OWN bit, is cleared to 0 when
the BD entry is owned by the microprocessor. The microprocessor is allowed read and
write access to the BD entry and the buffer in system memory when the OWN bit is 0.
When the OWN bit is set to 1, the BD entry and the buffer in system memory are owned
by USBFS. USBFS now has full read and write access and the microprocessor must not
modify the BD or its corresponding data buffer. The BD also contains indirect address
pointers to where the actual buffer resides in system memory. This indirect address
mechanism is shown in the following diagram.
BDT_PAGE Registers
33.4.2 USB data transfers—Receive (Rx) and Transmit (Tx)
A USBFS core-centric nomenclature is used to describe the direction of the data transfer
between USBFS core and USB. Throughout this chapter:
• "RX" (or "receive") describes transfers that move data from USB to memory.
• "TX" (or "transmit") describes transfers that move data from memory to USB.
The following table shows the data directions for USBFS.
Table 33-1. Data direction for USB host or USB target
OUT or SETUP
Freescale Semiconductor, Inc.
END_POINT
TX
Figure 33-7. Buffer descriptor table
RX
KL27 Sub-Family Reference Manual , Rev. 5, 01/2016
Chapter 33 Universal Serial Bus (USB) FS Subsystem
BDT Page
ODD
000
Buffer in Memory
System Memory
Current
Endpoint
BDT
Start of Buffer
End of Buffer
TX
IN
535
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