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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 the USB-FS. The USB-FS 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.
Current
Endpoint
BDT
Buffer in Memory
BDT Page
Start of Buffer
•
000ODDTXBDT_PAGE Registers END_POINT
• • • • •
End of Buffer
System Memory
Figure 42-3. Buffer descriptor table
42.4.2 RX vs. TX as a USB target device or USB host
The USB-FS core uses software control to switch between two modes of operation:
• USB target device
• USB hosts
In either mode, USB host or USB target device, the same data paths and buffer
descriptors are used for the transmission and reception of data. For this reason, a USB-FS
core centric nomenclature is used to describe the direction of the data transfer between
the USB-FS core and the USB:
• RX (or receive) describes transfers that move data from the USB to memory.
• TX (or transmit) describes transfers that move data from memory to the USB.
The following table shows how the data direction corresponds to the USB token type in
host and target device applications.
Chapter 42 Universal Serial Bus OTG Controller (USBOTG)
K20 Sub-Family Reference Manual, Rev. 1.1, Dec 2012
Freescale Semiconductor, Inc.
Preliminary
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