C541U_99 Datasheet, PDF (86/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

On-Chip Peripheral Components
C541U
The start-of-frame-done enable feature (SOFDE=1) is useful for USB memory read accesses when
the number of data bytes to be transferred from CPU to USB is not predictable (see figure 6-24).
The CPU can write data as desired to USB memory until a SOF occures (every 1 ms). The
automatic setting of bit SOF causes bit EOD to be set (at 1 ). This indicates the CPU that no CPU
action on this buffer is required until a USB read operation has been finished (bit SOD set at 2 ).
Setting of SOD indicates an empty USB memory to the CPU which can start again writing data into
USB memory.
Number of
Data Bytes
in USB Buffer
MaxLen
1
2
SOD
SOD
set
set
USB read accesses
SOF (n)
and EOD
set
Frame n
CPU write accesses
Time
Frame n+1
SOF (n+1)
and EOD
set
MCT03404
Figure 6-24
Single Buffer Mode : USB Read Access with Start-of-Frame-Done Enabled
Semiconductor Group
6-41
1999-04-01