C541U_99 Datasheet, PDF (87/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

On-Chip Peripheral Components
C541U
6.4.2.3 Dual Buffer Mode
In dual buffer mode, both USB memory pages (page 0 and page 1) are used for data transfers. The
logical assignment of the memory pages to CPU or USB is automatically switched. The following
two figures show the buffer handling concept in dual buffer mode for the USB read access and USB
write access.
CPU Buffer Handling
USB Buffer Handling
CPU page is empty : CBF = 0
CPU write access enabled
USB page is full : UBF = 1
USB read access enabled
CPU writes 1 Byte
SOD = 1
No
CPU buffer
full?
Yes
CPU buffer is full : CBF = 1
CPU write access disabled
USB read
No
request?
Yes
USB reads buffer
USB buffer is empty: UBF = 0
USB read access disabled
EOD = 1
No
USB page
empty?
CPU page
full?
Yes
Yes
Pages are swapped (CBF = 1 and UBF = 0)
Figure 6-25
USB Read Access in Dual Buffer Mode - Buffer Handling
No
MCB03405
Semiconductor Group
6-42
1999-04-01