MC68HC908LB8_05 Datasheet, PDF (203/234 Pages) Freescale Semiconductor, Inc – M68HC08 Microcontrollers

Chapter 19
Development Support
19.1 Introduction
This section describes the break module, the monitor read-only memory (MON), and the monitor mode
entry methods.
19.2 Break Module (BRK)
The break module can generate a break interrupt that stops normal program flow at a defined address to
enter a background program.
Features of the break module include:
• Accessible input/output (I/O) registers during the break Interrupt
• Central processor unit (CPU) generated break interrupts
• Software-generated break interrupts
• Computer operating properly (COP) disabling during break interrupts
19.2.1 Functional Description
When the internal address bus matches the value written in the break address registers, the break module
issues a breakpoint signal (BKPT) to the system integration module (SIM). The SIM then causes the CPU
to load the instruction register with a software interrupt instruction (SWI) after completion of the current
CPU instruction. The program counter vectors to $FFFC and $FFFD ($FEFC and $FEFD in monitor
mode).
The following events can cause a break interrupt to occur:
• A CPU generated address (the address in the program counter) matches the contents of the break
address registers.
• Software writes a 1 to the BRKA bit in the break status and control register.
When a CPU generated address matches the contents of the break address registers, the break interrupt
begins after the CPU completes its current instruction. A return-from-interrupt instruction (RTI) in the
break routine ends the break interrupt and returns the microcontroller unit (MCU) to normal operation.
Figure 19-1 shows the structure of the break module.
Figure 19-2 provides a summary of the I/O registers.
MC68HC908LB8 Data Sheet, Rev. 1
Freescale Semiconductor
203