
Break Module (BRK)
MC68HC908QL4  MC68HC908QL3  MC68HC908QL2 Data Sheet, Rev. 4
Freescale Semiconductor
189
Figure 16-2. Break Module Block Diagram
When the internal address bus matches the value written in the break address registers or when software 
writes a 1 to the BRKA bit in the break status and control register, the CPU starts a break interrupt by:
Loading the instruction register with the SWI instruction
Loading the program counter with $FFFC and $FFFD ($FEFC and $FEFD in monitor mode)
The break interrupt timing is:
When a break address is placed at the address of the instruction opcode, the instruction is not 
executed until after completion of the break interrupt routine.
When a break address is placed at an address of an instruction operand, the instruction is executed 
before the break interrupt.
When software writes a 1 to the BRKA bit, the break interrupt occurs just before the next instruction 
is executed.
By updating a break address and clearing the BRKA bit in a break interrupt routine, a break interrupt can 
be generated continuously.
CAUTION
A break address should be placed at the address of the instruction opcode. 
When software does not change the break address and clears the BRKA 
bit in the first break interrupt routine, the next break interrupt will not be 
generated after exiting the interrupt routine even when the internal address 
bus matches the value written in the break address registers.
16.2.1.1  Flag Protection During Break Interrupts
The system integration module (SIM) controls whether or not module status bits can be cleared during 
the break state. The BCFE bit in the break flag control register (BFCR) enables software to clear status 
bits during the break state. See 
13.8.2 Break Flag Control Register
 and the 
Break Interrupts
 subsection 
for each module.
16.2.1.2  TIM During Break Interrupts
A break interrupt stops the timer counter.
16.2.1.3  COP During Break Interrupts
The COP is disabled during a break interrupt with monitor mode when BDCOP bit is set in break auxiliary 
register (BRKAR).
ADDRESS BUS[15:8]
ADDRESS BUS[7:0]
8-BIT COMPARATOR
8-BIT COMPARATOR
CONTROL
BREAK ADDRESS REGISTER LOW
BREAK ADDRESS REGISTER HIGH
ADDRESS BUS[15:0]
BKPT
(TO SIM)