Semiconductor Group
6-45
1999-04-01
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.
Figure 6-28
Dual Buffer Mode USB Read Access : Buffer Switching by Setting Bit DONE
Frame n
Frame n+1
Time
SOF (n)
set
Number of
Data Bytes
MaxLen
USB read accesses
CPU write accesses
MCT03408
DONE = 1
Len1
SOF (n+2)
set
Time
SOF (n+1)
set
1
3
Page 0
Page 1
Page 1
Page 0
UBF = 0
Swap
Buffer
Swap
Buffer
UBF
= 0
USB Buffer
CPU Buffer
CBF
= 0
Len2
Len1
MaxLen
Len2
Page 0
2
Page 1
DONE = 1
UBF = 1
UBF = 1
Содержание C541U
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