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Debug Module
Freescale Semiconductor
34-50
The breakpoint status is also posted in the CSR. CSR[BSTAT] is cleared by a CSR read when a level-2
breakpoint is triggered or a level-1 breakpoint is triggered and a level-2 breakpoint is not enabled. Status
is also cleared by writing to either TDR or XTDR to disable trigger options.
BDM instructions use the appropriate registers to load and configure breakpoints. As the system operates,
a breakpoint trigger generates the response defined in TDR.
PC breakpoints are treated in a precise manner—exception recognition and processing are initiated before
the excepting instruction executes. All other breakpoint events are recognized on the processor’s local bus,
but are made pending to the processor and sampled like other interrupt conditions. As a result, these
interrupts are imprecise.
In systems that tolerate the processor being halted, a BDM-entry can be used. With TDR[TRC] equals 01,
a breakpoint trigger causes the core to halt (PST = 0xF).
If the processor core cannot be halted, the debug interrupt can be used. With this configuration,
TDR[TRC] equals 10, breakpoint trigger becomes a debug interrupt to the processor, which is treated
higher than the nonmaskable level-7 interrupt request. As with all interrupts, it is made pending until the
processor reaches a sample point, which occurs once per instruction. Again, the hardware forces the PC
breakpoint to occur before the targeted instruction executes and is precise. This is possible because the PC
breakpoint is enabled when interrupt sampling occurs. For address and data breakpoints, reporting is
considered imprecise, because several instructions may execute after the triggering address or data is
detected.
As soon as the debug interrupt is recognized, the processor aborts execution and initiates exception
processing. This event is signaled externally by the assertion of a unique PST value (PST = 0xD) for
multiple cycles. The core enters emulator mode when exception processing begins. After the standard
8-byte exception stack is created, the processor fetches a unique exception vector from the vector table.
describes the two unique entries that distinguish PC breakpoints from other trigger events.
Table 34-28. PSTDDATA Nibble/CSR[BSTAT] Breakpoint Response
PSTDDATA
Nibble
CSR[BSTAT]
1
1
Encodings not shown are reserved for future use.
Breakpoint Status
0000
0000
No breakpoints enabled
0010
0001
Waiting for level-1 breakpoint
0100
0010
Level-1 breakpoint triggered
1010
0101
Waiting for level-2 breakpoint
1100
0110
Level-2 breakpoint triggered
Table 34-29. Exception Vector Assignments
Vector Number
Vector Offset (Hex)
Stacked Program Counter
Assignment
12
0x030
Next
Non-PC-breakpoint debug interrupt
13
0x034
Next
PC-breakpoint debug interrupt
Содержание MCF54455
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