DT-DMA Mechanism
7-17Emulation Features
Figure 7−5. Process for Handling a DT-DMA Request
polite or rude?
Request
DBGM = 0?
nonpreemptive or
Mode
Access performed
Wait for hole
Rude
Access denied
Force a hole
preemptive?
DT-DMA mechanism
Polite
No
Yes
Preemptive
Nonpreemptive
requests access
Some key concepts of the DT-DMA mechanism are:
- Even if DBGM = 0, when the mechanism is in nonpreemptive mode, it
must wait for a hole. This minimizes the intrusiveness of the debug access
on a system.
- Real-time-mode accesses are typically polite (although there may be rea-
sons, such as error recovery, to perform rude accesses in real-time
mode). If the DBGM bit is permanently set to 1 due to a coding bug but
you need to regain debug control, use rude accesses, which ignore the
state of DBGM.
- In stop mode, DBGM is ignored, and the DT-DMA mode is set to preemp-
tive. This ensures that you can gain visibility to and control of your system
if an otherwise unrecoverable error occurs (for example, if ST1 is changed
to an undesired value due to stack corruption).
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