C541U_99 Datasheet, PDF (89/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

On-Chip Peripheral Components
C541U
Figure 6-27 describes an example of a USB read operation in sequential mode with both buffers
empty at the beginning of the USB read operation.
The CPU starts writing data with sequential access (INCE=1) to the buffer assigned to the CPU at
1 . By definition, the buffer is full when MaxLen is reached at 2 . The second buffer assigned to
the USB is empty (UBF=0) and as a result both buffers are logically swapped. Now the buffer
assigned to USB is full (UBF=1) and an USB read access can take place. After the USB read
access, the buffer assigned to the USB is empty again with UBF=0. During the USB read access
the CPU is still allowed to write into its assigned buffer. When reaching MaxLen at 3 , the CPU
buffer is full and both buffers are again logically swapped. The USB further execute its read access.
Number of
Data Bytes
MaxLen
2
Page 0
Page 1
3
Page 0
CBF
=0
1
MaxLen
Swap
Buffer
UBF = 1
Time
Swap
Buffer
UBF = 1
Page 1
UBF
=0
SOF (n)
set
USB read accesses
Frame n
Page 0
Page 1
UBF = 0
Frame n+1
SOF (n+1)
set
CPU write accesses
Time
SOF (n+2)
set
MCT03407
Figure 6-27
Dual Buffer Mode USB Read Access : Buffer Switching when MaxLen is reached
In dual buffer mode, the physical assignment of the USB memory pages (page 0 or page 1) to either
CPU buffer or USB buffer is controlled automatically in the USB module and cannot be selected by
software.
Semiconductor Group
6-44
1999-04-01