C541U_99 Datasheet, PDF (91/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

On-Chip Peripheral Components
C541U
If bit SOFDE is set, buffer switching is done automatically after SOF (start of frame) has been
detected by the USB. Figure 6-29 describes this functionality for USB read access for this case.
The buffer which contains the latest data from the CPU is tagged valid for USB access (UBF=1) at
1 and the buffers are swapped if the USB buffer is empty. After the USB read access has occured
at 2 , this buffer assigned to USB is empty again (UBF=0) and can be swapped again as soon as
the CPU has filled its buffer (at 3 ). The number of bytes in the buffer is less or equal MaxLen. The
MaxLen threshold is always active, but an occurrence of SOF (if SOFDE=1) or setting bit DONE by
software are used to tag the CPU buffer full before reaching MaxLen.
Number of
Data Bytes
MaxLen
Len1
Len2
CBF
=0
MaxLen
Len1
Len2
UBF
=0
SOF (n)
set
USB read accesses
Page 0
1
Swap
Buffer
Page 1
3
Time
Swap
Buffer
Page 1
Frame n
Page 0
2
UBF = 0
SOF (n+1)
set
CPU write accesses
Frame n+1
Time
SOF (n+2)
set
MCT03409
Figure 6-29
Dual Buffer Mode USB Read Access: Buffer Switching on SOF with SOFDE=1
Semiconductor Group
6-46
1999-04-01