Datasheet

5-46 MCF5407 User’s Manual
Real-Time Debug Support
5.6.1 Theory of Operation
Breakpoint hardware can be configured to respond to triggers in several ways. The response
desired is programmed into TDR. As shown in Table 5-25, when a breakpoint is triggered,
an indication (CSR[BSTAT]) is provided on the PSTDDATA output port when it is not
displaying captured processor status, operands, or branch addresses. See Section 5.3.2,
“Processor Stopped or Breakpoint State Change (PST = 0xE).”
The breakpoint status is also posted in CSR. Note that CSR[BSTAT] is cleared by a CSR
read when either 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.
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 is executed. 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] = 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] = 10, the 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. 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
Table 5-25. 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