Communication Processor Module
16-366 MPC823e REFERENCE MANUAL MOTOROLA
USB
COMMUNICATION
16
PROCESSOR MODULE
EN—Enable USB
This bit enables USB operation. When the EN bit is cleared, the USB is in a reset state and
consumes minimal power.
0 = USB is disabled.
1 = USB is enabled.
16.10.8.2 USB RECEIVE BUFFER DESCRIPTOR. The USB controller reports
information about each buffer of received data using (RX) buffer descriptors. The USB
controller closes the current buffer, generates a maskable interrupt, and starts receiving
data in the next buffer when the current buffer is full. Additionally, it will close the buffer under
the following conditions:
• When an end of packet is detected
• When an overrun error occurs
• When a bit stuff violation is detected
The first word of the RX buffer descriptor contains status and control bits. It is recommended
that you prepare these bits before reception because they are set by the USB controller after
the buffer has been closed. The second word contains the data length (in bytes) that was
received. The third and fourth words contain a pointer that always points to the beginning of
the received data buffer.
Note: You must not modify any other bits of the USMOD register while the EN bit is set.
Note: The buffer descriptor’s status bits are sticky. Applications that operate on those
bits may need to be reset after every relevant CPM transaction.
BIT 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
OFFSET + 0
E RES WI L F RES PID RES NO AB CR OV RES
OFFSET + 2
DATA LENGTH
OFFSET + 4
RX DATA BUFFER POINTER
OFFSET + 6
NOTE: You are only responsible for initializing the items in bold.
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