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Architecture
621
SPRUH82C – April 2013 – Revised September 2016
Copyright © 2013–2016, Texas Instruments Incorporated
Enhanced Direct Memory Access (EDMA3) Controller
17.2.14 EDMA3CC and EDMA3TC Performance and System Considerations
17.2.14.1 System Priority Considerations
The main switched central resource (SCR) (see your device-specific data manual) arbitrates bus requests
from all the masters (CPU, master peripherals, and the EDMA3 transfer controllers) to the shared slave
resources (peripherals and memories). The priorities of transfer requests (read and write commands) from
the EDMA3 transfer controllers with respect to each other and the other masters within the system is
configured as explained in
It is recommended that this priority be altered based on system level considerations. For example,
peripherals servicing audio/video/display threads that typically have real-time deadlines should be
programmed as highest priority requestors in the systems, where as, peripherals responsible for doing
bulk/block/paging transfers with no real-time deadlines, should be programmed as a lower system priority.
The default priority for all transfer controllers is the same, 0 or highest priority relative to other masters;
therefore, it is recommended that a TC servicing audio data requests from serial ports should be
configured at a higher priority as compared to TC service memory to memory (paging/bulk) transfer
requests.
17.2.14.2 TC Transfer Optimization Considerations
The transfer controller can internally optimize the way it issues read commands and write commands for a
given transfer under certain conditions. For 2D transfers (that is, BCNT arrays of ACNT bytes), if the
ACNT value is less than or equal to the DBS value, then the transfer controller will try to optimize the TR
into a 1D transfer in order to maximize efficiency. The optimization only takes place if the EDMA3TC
recognizes that the 2D transfer is organized as a single dimension (SAM/DAM = 0, increment mode),
SRC/DST BIDX = ACNT, the ACNT value is a power of 2, and the BCNT value is less than or equal to
1023. If these conditions are met, then instead of issuing ACNT bytes worth read and/or write commands,
the TC will try to optimize the bus usage by issuing commands as if ACNT' = ACNT × BCNT and
BCNT = 1.
summarizes the conditions in which the optimizations are performed.
Table 17-13. Read/Write Command Optimization Rules
ACNT
≤
DBS
ACNT is power of 2
BIDX = ACNT
BCNT
≤
1023
SAM/DAM = 0
(Increment)
Description
Yes
Yes
Yes
Yes
Yes
Optimized
Yes
No
x
x
Yes
Not Optimized
Yes
x
No
x
Yes
Not Optimized
No
x
x
x
Yes
Not Optimized
x
x
x
x
No
Not Optimized
Consider a case in which it is needed to transfer 4096 bytes where the data is arranged linearly in both
the source and destination locations (SAM/DAM = 0, SRC/DST BIDX = ACNT): Scenario A programs the
ACNT = 4, BCNT = 1024 , AB-synchronized transfer; and Scenario B programs the ACNT = 64,
BCNT = 64. Scenario B will yield a much optimized transfer and higher throughput, as the transfer meets
all the optimization rules, which would result in TC internally treating it as a transfer with an ACNT' = 4096
(ACNT × BCNT). The TC will optimally size, default burst size worth read and write commands. In the
case of Scenario B, since one of the optimization rules is not met (BCNT value is greater then 1023), the
TC will end up issuing several ACNT byte (4 byte) size commands to complete the transfers, which will
result in inefficient usage of the read/write buses.