Semiconductor Group
6-38
1999-04-01
On-Chip Peripheral Components
C541U
Figure 6-21 shows more details of an USB write access to USB memory in single buffer mode. After
SOF(n) (start of frame) occured at
, the USB starts writing at
a fixed number of bytes into the
USB memory. A byte counter is incremented after every USB memory write operation. When the
USB memory write operation (Len(n)) is finished correctly, bit SOD (start of data) is set at
,
indicating a full USB memory buffer. Furthermore, the byte counter value is stored in the
corresponding length register, indicating the number of bytes which have been transferred and can
be now read by the CPU. Subsequently, the CPU can read data bytes from USB memory,
generating an EOD (end of data) at
after the last byte has been read. Bit EOD set indicates an
empty USB buffer, which now can be written again by the USB.
Figure 6-21 also shows a second USB write access operation with a different number of bytes
(Len(n+1)), where the CPU read operation from the USB memory is interrupted twice.
Figure 6-21
Single Buffer Mode : Standard USB Write Access
Note: The CPU accesses shown in the following diagrams assume that bit INCE in the corres-
ponding endpoint control register is set.
A frame is the 1 ms time interval defined by the USB host.
Every frame begins with a SOF token (start-of-frame).
1
2
3
4
1
EOD
set
4
Frame n
Frame n+1
EOD
set
Time
SOF (n)
set
Number of
Data Bytes
in USB Buffer
Len (n)
USB write accesses
CPU read accesses
MCT03401
1
2
3
2
3
4
SOD
set
set
SOD
Len (n+1)
SOF (n+2)
set
SOF (n+1)
set
Содержание C541U
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