Motorola MPC823e Reference Manual page 816

Communication Processor Module
Note: MRBLR is not intended to be dynamically changed while the USB channel is
operating. However, if it is modified in a single bus cycle with one 16-bit move
(not two 8-bit bus cycles back-to-back), then a dynamic change in the receive
buffer length can be successfully achieved. This occurs when the
communication processor module transfers control to the next RX buffer
descriptor in the table. Thus, a change to MRBLR will not have an immediate
effect. To guarantee the exact RX buffer descriptor on which the change will
occur, you must change MRBLR only while the USB receiver is disabled.
• RBPTR—The receive buffer descriptor pointer points to the next buffer descriptor that
the receiver will transfer data to when it is in an idle state or to the current buffer
descriptor while processing a frame. RBPTR must be initialized by the software after
reset. When the end of the buffer descriptor table is reached, the communication
processor module initializes this pointer to the value programmed in the RBASE entry.
Although you never need to write RBPTR in most applications (except initialization),
you can modify it when the receiver is disabled or when you are sure that no receive
buffer is currently in use.
• TBPTR—The transmit buffer descriptor pointer for each USB endpoint points to the
next buffer descriptor that the transmitter will transfer data from when it is in an idle state
or to the current buffer descriptor during frame transmission. TBPTR must be initialized
by the software after reset. When the end of buffer descriptor table is reached, the
communication processor module initializes this pointer to the value programmed in the
TBASE entry. Although you never need to write TBPTR in most applications (except
initialization), you can modify it when the transmitter is disabled or when you are sure
that no transmit buffer is currently in use.
• TSTATE—The transmit internal state entry must be initilaized to zero before enabling
the USB controller.
• Other General Parameters—These are the additional parameters listed in Table 16-34
and Table 16-35. You do not need to access these parameters for normal operation and
they are only listed because they provide helpful information for debugging purposes.
The RX and TX internal data pointers (RPTR and TPTR) are updated by the SDMA
channels to indicate the next address in the buffer to be accessed.
TX internal byte count (TBCNT) is a down-count value that is initialized with the TX
buffer descriptor's DATA LENGTH field and decremented with every byte read by the
SDMA channels. RX internal byte count (RBCNT) is a down-count value that is
initialized with the MRBLR value and decremented with every byte written by the SDMA
channels. RSTATE, TSTATE, RTEMP, TTEMP, and the reserved areas are only used
by the RISC microcontroller.
16-362 MPC823e REFERENCE MANUAL MOTOROLA