PEB20534H-10 SIEMENS [Siemens Semiconductor Group], PEB20534H-10 Datasheet - Page 90

• Manufacturer Part Number: PEB20534H-10
• Description: DMA Supported Serial Communication Controller with 4 Channels DSCC4
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

Semiconductor Group
Figure 20
For all operations the burst size is limited to the available TFIFO space or to 15 DWORDs
maximum (4 DWORDs in de-multiplexed interface mode).
To guarantee maximum burst length the refill threshold should be programmed such that
(TFIFO size - refill watermark) is equal to 15 DWORDs. A lower refill threshold is no
guarentee for limited burst size because due to bus latency the available TFIFO space
might be greater when transfer starts thus allowing full burst. So 15 DWORDs burst can
happen even if the (TFIFO size - refill watermark) < 15 DWORDs.
A refill threshold such that (TFIFO size - refill watermark) is greater than 15 DWORDs
(the maximum burst size) does not make sense.
Important:
The following condition must be met:
refill watermark > forward watermark
Consider the following situation:
A short frame is loaded and the beginning of a second frame such that the transmit FIFO
is full and DMA operation stops.
Transmit FIFO Section Size i:
FIFOCR1.TFSIZEi
size = TFSIZEi * 4
Central Transmit FIFO Section Thresholds
TFSIZEi*4
Transmit data transferred
1
from shared memory
transmit FIFO
SCCi
90
DMA Controller and Central FIFOs
Transmit FIFO Refill Threshold:
FIFOCR2.TFFTHRESHi
watermark = TFRTHRESH*Mi_x+1
Transmit FIFO Forward Threshold:
FIFOCR4.TFFTHRESHi
watermark = TFFTHRESHi
Data Sheet 09.98
PEB 20534