C541U_99 Datasheet, PDF (90/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

On-Chip Peripheral Components
C541U
Another way to initiate buffer switching is setting bit DONE by software. This feature, which is shown
in figure 6-28 for USB read access, can be used to transfer a variable number of bytes. The
maximum number of bytes to be transferred is still determined by MaxLen, which is not changed
when bit DONE is set. The actual packet length (Len1 or Len2) is the number of bytes which have
been written to the buffer before bit DONE is set.
Number of
Data Bytes
MaxLen
Len1
Len2
DONE = 1
2
DONE = 1
3
CBF
=0
MaxLen
Len1
Len2
Page 0
1
Swap
Buffer
UBF = 1
Page 1
Page 1
Page 0
Time
Swap
Buffer
UBF = 1
Page 0
Page 1
UBF
=0
SOF (n)
set
USB read accesses
Frame n
UBF = 0
Frame n+1
SOF (n+1)
set
CPU write accesses
Time
SOF (n+2)
set
MCT03408
Figure 6-28
Dual Buffer Mode USB Read Access : Buffer Switching by Setting Bit DONE
Semiconductor Group
6-45
1999-04-01