Table of Contents
Advertisement
DMA Configuration Parameters
8.2.4 VLEN Setting
The DMA channel is capable of using the first byte or word (for word, bits 12:0 are used) in source data as
the transfer length. This allows variable-length transfers. When using variable-length transfer, various
options regarding how to count number of bytes to transfer are given. In any case, the transfer-count
(LEN) setting is used as a maximum transfer count. If the transfer length specified by the first byte or word
is greater than LEN, then LEN bytes or words are transferred. When using variable-length transfers, then
LEN should be set to the largest allowed transfer length plus one.
Note that the M8 bit
Options which can be set with VLEN are the following:
1. Transfer number of bytes or words commanded by first byte/word + 1 (transfers the length byte/word,
and then as many bytes/words as dictated by the length byte/word)
2. Transfer number of bytes or words commanded by first byte/word
3. Transfer number of bytes or words commanded by first byte/word + 2 (transfers the length byte/word,
and then as many bytes/words as dictated by the length byte/word + 1)
4. Transfer number of bytes or words commanded by first byte/word + 3 (transfers the length byte/word,
and then as many bytes/words as dictated by the length byte/word + 2)
Figure 8-2
shows the VLEN options.
Byte/Word n
Byte/Word n – 1
•
•
•
Byte/Word 3
Byte/Word 2
Byte/Word 1
LENGTH = n
VLEN = 001
8.2.5 Trigger Event
Each DMA channel can be set up to sense on a single trigger. This field determines which trigger the
DMA channel senses.
8.2.6 Source and Destination Increment
When the DMA channel is armed or rearmed, the source and destination addresses are transferred to
internal address pointers. The possibilities for address increment are:
•
Increment by zero. The address pointer remains fixed after each transfer.
•
Increment by one. The address pointer increments one count after each transfer.
•
Increment by two. The address pointer increments two counts after each transfer.
•
Decrement by one. The address pointer decrements one count after each transfer.
where a count equals 1 byte in byte mode and 2 bytes in word mode.
96
DMA Controller
(Section
8.2.11) is only used when byte-size transfers are chosen.
Byte/Word n – 1
•
•
•
Byte/Word 3
Byte/Word 2
Byte/Word 1
LENGTH = n
VLEN = 010
Figure 8-2. Variable Length (VLEN) Transfer Options
Copyright © 2009–2014, Texas Instruments Incorporated
Byte/Word n + 1
Byte/Word n
Byte/Word n – 1
•
•
•
Byte/Word 3
Byte/Word 2
Byte/Word 1
LENGTH = n
VLEN = 011
SWRU191F – April 2009 – Revised April 2014
Submit Documentation Feedback
www.ti.com
Byte/Word n + 2
Byte/Word n + 1
Byte/Word n
Byte/Word n – 1
•
•
•
Byte/Word 3
Byte/Word 2
Byte/Word 1
LENGTH = n
VLEN = 100
M0103-02
Hide quick links:
Advertisement
Table of Contents
