TARGET
SYSTEM
BUS STATE
ANALYZER
TARGET
MCU
1129A
Figure 5-8 Bus State Analyzer Configuration
5.10.2.1 Enabling BDM
Accidentally entering BDM in a non-development environment can lock up the CPU32
when the serial command interface is not available. For this reason, BDM is enabled
during reset via the breakpoint (BKPT) signal.
5
BDM operation is enabled when BKPT is asserted (low), at the rising edge of RESET.
BDM remains enabled until the next system reset. A high BKPT signal on the trailing
edge of RESET disables BDM. BKPT is latched again on each rising transition of RE-
SET. BKPT is synchronized internally, and must be held low for at least two clock cy-
cles prior to negation of RESET.
BDM enable logic must be designed with special care. If hold time on BKPT extends
into the first bus cycle following reset, the bus cycle could inadvertently be tagged with
a breakpoint. Refer to the SIM Reference Manual (SIMRM/AD) for timing information.
5.10.2.2 BDM Sources
When BDM is enabled, any of several sources can cause the transition from normal
mode to BDM. These sources include external breakpoint hardware, the BGND in-
struction, a double bus fault, and internal peripheral breakpoints. If BDM is not enabled
when an exception condition occurs, the exception is processed normally. Table 5-3
summarizes the processing of each source for both enabled and disabled cases. As
shown in Table 5-3, the BKPT instruction never causes a transition into BDM.
Table 5-3 BDM Source Summary
Source
BDM Enabled
Background
Background
Background
BDM Disabled
Breakpoint Exception
Halted
BKPT
Double Bus Fault
BGND Instruction
BKPT Instruction
Illegal Instruction
Opcode Substitution/
Illegal Instruction
Opcode Substitution/
Illegal Instruction
MC68331
USER’S MANUAL
CENTRAL PROCESSING UNIT
MOTOROLA
5-19
MOTOROLA [ MOTOROLA ]
