USBFS, a simple semaphore mechanism is used to distinguish who is allowed to update
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.
Current
Endpoint
BDT
Buffer in Memory
BDT Page
Start of Buffer
•
000ODDTXBDT_PAGE Registers END_POINT
• • • • •
System Memory
End of Buffer
Figure 43-6. Buffer descriptor table
43.3.2
RX vs. TX as a USB peripheral device or USB host
The USBFS core uses software control to switch between two modes of operation:
• USB peripheral device
• USB hosts
In either mode, USB host or USB peripheral 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.
Programmers interface
K22F Sub-Family Reference Manual, Rev. 4, 08/2016
1076 NXP Semiconductors
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