Current
Endpoint
BDT
Buffer in Memory
BDT Page
Start of Buffer
•
000
ODD
TX
BDT_PAGE Registers
END_POINT
••
••
•
System Memory
End of Buffer
Figure 42-3. Buffer descriptor table
42.3.2 RX vs. TX as a USB target device or USB host
The USBFS 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 USBFS
core-centric nomenclature is used to describe the direction of the data transfer between
the USBFS core and USB:
• "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 how the data direction corresponds to the USB token type in
host and target device applications.
Table 42-1. Data direction for USB host or USB target
RX
TX
Device
OUT or SETUP
IN
Host
IN
OUT or SETUP
Chapter 42 Universal Serial Bus Full Speed OTG Controller (USBFSOTG)
K22F Sub-Family Reference Manual , Rev. 3, 7/2014
Freescale Semiconductor, Inc.
993
