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Architecture
616
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Enhanced Direct Memory Access (EDMA3) Controller
17.2.11.1.2 Command Fragmentation
The TC read and write controllers in conjunction with the source and destination register sets are
responsible for issuing optimally-sized reads and writes to the slave endpoints. The transfer controller
read/write transaction as specified by the transfer request packet is internally broken down into smaller
bursts; this determines the default burst size (DBS) for the transfer controller. See
for
the DBS value of each EDMA3TC.
The EDMA3TC attempts to issue the largest possible command size as limited by the DBS value or the
ACNT/BCNT value of the TR. EDMA3TC obeys the following rules:
•
The read/write controllers always issue commands less than or equal to the DBS value.
•
The first command of a 1D transfer is always issued so that subsequent commands align to the DBS
value.
shows the command fragmentation for a DBS of 32 bytes. In summary, if the ACNT value is
larger than the DBS value, then the EDMA3TC breaks the ACNT array into DBS-sized commands to the
source/destination addresses. Each BCNT number of arrays are then serviced in succession.
Example 17-4. Command Fragmentation (DBS = 32)
The pseudo code:
1. ACNT = 8, BCNT = 8, SRCBIDX = 8, DSTBIDX = 10, SRCADDR = 64, DSTADDR = 191
Read Controller: This is optimized from a 2D-transfer to a 1D-transfer such that the read side is equivalent
to ACNT = 64, BCNT = 1.
Cmd0 = 32 byte, Cmd0 = 32 byte
Write Controller: Since DSTBIDX != ACNT, it is not optimized.
Cmd0 = 8 byte, Cmd1 = 8 byte, Cmd2 = 8 byte, Cmd3 = 8 byte, Cmd4 = 8 byte, Cmd5 = 8 byte, Cmd6 = 8
byte, Cmd7 = 8 byte.
2. ACNT = 64, BCNT = 1, SRCADDR = 31, DSTADDR = 513
Read Controller: Read address is not aligned.
Cmd0 = 1 byte, (now the SRCADDR is aligned to 32 for the next command)
Cmd1 = 32 bytes
Cmd2 = 31 bytes
Write Controller: The write address is also not aligned.
Cmd0 = 31 bytes, (now the DSTADDR is aligned to 32 for the next command)
Cmd1 = 32 bytes
Cmd2 = 1 byte
17.2.11.1.3 TR Pipelining and Data Ordering
The transfer controller(s) can issue back-to-back transfer requests (TR). The number of outstanding TRs
for a TC is limited by the number of destination FIFO register entries that is controlled by the
DSTREGDEPTH parameter (fixed in design for a given transfer controller). TR pipelining refers to the
ability of the TC read controller to issue read commands for a subsequent TR, while the TC write
controller is still performing writes for the previous TR. Consider the case of 2 TRs (TR0 followed by TR1),
because of TR pipelining, the TC read controller can start issuing the read commands for TR1 as soon as
the last read command for TR0 has been issued, meanwhile the write commands and write data for TR0
are tracked by the destination FIFO registers. In summary, the TC read controller is able to process
n
TRs
ahead of the write controller, where
n
is the number of destination FIFO register entries (typically 4).
TR pipelining is useful for maintaining throughput on back-to-back small TRs. It eliminates the read
overhead because reads start in the background of a previous TR writes.