mc68331cpv20b1 Freescale Semiconductor, Inc, mc68331cpv20b1 Datasheet - Page 119

• Manufacturer Part Number: mc68331cpv20b1
• Description: Mc68331 32 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

5.10.2.1 Enabling BDM
5.10.2.2 BDM Sources
MC68331
USER’S MANUAL
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.
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.
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.
BGND Instruction
Double Bus Fault
BKPT Instruction
TARGET
SYSTEM
Source
BKPT
Figure 5-8 Bus State Analyzer Configuration
Freescale Semiconductor, Inc.
For More Information On This Product,
Table 5-3 BDM Source Summary
CENTRAL PROCESSING UNIT
TARGET
Go to: www.freescale.com
MCU
Opcode Substitution/
Illegal Instruction
BDM Enabled
Background
Background
Background
BUS STATE
ANALYZER
Breakpoint Exception
Opcode Substitution/
Illegal Instruction
Illegal Instruction
BDM Disabled
Halted
1129A
5-19
5