Enhanced Direct Memory Access (eDMA)
MCF52277 Reference Manual, Rev. 1
Freescale Semiconductor
17-3
17.4.2
Debug Mode
In debug mode, the eDMA stops transferring data. If debug mode is entered during the transfer of a data
block described by a minor loop in the current active channel’s TCD, the eDMA continues operation until
completion of the minor loop.
17.5
External Signal Description
This section describes the external signals of the eDMA controller.
17.5.1
External Signal Timing
Asserting the external DMA request signal, DREQ
n
, initiates a service request for that channel. It must
remain asserted until the corresponding DACK
n
signal indicates the channel’s data transfer has started.
The DACK
n
output is asserted for one cycle during the address phase of the channel’s first internal read
access.
•
When no further requests are needed, the DREQ
n
signal must negate after the DACK
n
assertion
and on or before the second cycle following the data phase of the last internal bus write (see
).
•
If another service request is needed, DREQ
n
may simply remain asserted.
•
To request continuous service, DREQ
n
may remain continuously asserted.
Figure 17-2. DREQ
n
and DACK
n
Timing
After a service request has been initiated, it cannot be cancelled. Removing a service request after it has
been asserted may result in one of three actions depending on the DMA engine’s status:
•
The request is never recognized because another channel is executing.
Table 17-1. External Signal List
Signal Name
I/O
Description
DREQ0
I
Provides external requests from peripherals needing DMA service. When
asserted, the device is requesting service. This request pin is tied to DMA
channel 0.
DACK0
O
Indicates when the external DMA request has been acknowledged.
Internal
Internal
DACK
n
DREQ
n
Internal
wr1
rd1
rd2
wr2
wr1
rd1
rd2
wr2
Address Phase
Data Phase
Bus Clock
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