
MC68F375
CENTRAL PROCESSOR UNIT
MOTOROLA
REFERENCE MANUAL
Rev. 25 June 03
3-21
Table 3-4 summarizes the processing of each source for both enabled and disabled
cases. As shown in Table 3-4, the BKPT instruction never causes a transition into
BDM.
3.10.4.1 External BKPT Signal
Once enabled, BDM is initiated whenever assertion of BKPT is acknowledged. If BDM
is disabled, a breakpoint exception (vector 0x0C) is acknowledged. The BKPT input
has the same timing relationship to the data strobe trailing edge as does read cycle
data. There is no breakpoint acknowledge bus cycle when BDM is entered.
3.10.4.2 BGND Instruction
An illegal instruction, 0x4AFA, is reserved for use by development tools. The CPU32
defines 0x4AFA (BGND) to be a BDM entry point when BDM is enabled. If BDM is
disabled, an illegal instruction trap is acknowledged.
3.10.4.3 Double Bus Fault
The CPU32 normally treats a double bus fault, (an exception that occurs while stack-
ing for another exception) as a catastrophic system error and halts. When this
condition occurs during initial system debug (a fault in the reset logic), further debug-
ging is impossible until the problem is corrected. In BDM, the fault can be temporarily
bypassed, so that the origin of the fault can be isolated and eliminated.
3.10.4.4 Peripheral Breakpoints
CPU32 peripheral breakpoints are implemented in the same way as external break-
points — peripherals request breakpoints by asserting the BKPT signal. Consult the
appropriate peripheral user’s manual for additional details on the generation of
peripheral breakpoints.
3.10.5 Entering BDM
When the processor detects a breakpoint or a double bus fault, or decodes a BGND
instruction, it suspends instruction execution and asserts the FREEZE output. This is
the first indication that the processor has entered BDM. Once FREEZE has been
asserted, the CPU enables the serial communication hardware and awaits a
command.
Table 3-4 BDM Source Summary
Source
BDM Enabled
BDM Disabled
BKPT
Background
Breakpoint Exception
Double Bus Fault
Background
Halted
BGND Instruction
Background
Illegal Instruction
BKPT Instruction
Opcode Substitution/
Illegal Instruction
Opcode Substitution/
Illegal Instruction
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