Freescale Semiconductor, Inc.
Development Support
Breakpoints
entered by a breakpoint only if an internal signal from the BDM indicates
background debug mode is enabled.
• BKDBE will be used as an enable for the second address only
breakpoint.
• Breakpoints are not allowed if the BDM mode is already active.
Active mode means the CPU is executing out of the BDM ROM.
• BDM should not be entered from a breakpoint unless the ENABLE
bit is set in the BDM. This is important because even if the
ENABLE bit in the BDM is negated the CPU actually does execute
the BDM ROM code. It checks the ENABLE and returns if not set.
If the BDM is not serviced by the monitor then the breakpoint
would be re-asserted when the BDM returns to normal CPU flow.
There is no hardware to enforce restriction of breakpoint operation
if the BDM is not enabled.
18.5.2 Breakpoint Registers
Breakpoint operation consists of comparing data in the breakpoint
address registers (BRKAH/BRKAL) to the address bus and comparing
data in the breakpoint data registers (BRKDH/BRKDL) to the data bus.
The breakpoint data registers can also be compared to the address bus.
The scope of comparison can be expanded by ignoring the least
significant byte of address or data matches.
The scope of comparison can be limited to program data only by setting
the BKPM bit in breakpoint control register 0.
To trace program flow, setting the BKPM bit causes address comparison
of program data only. Control bits are also available that allow checking
read/write matches.
MC68HC912DG128 — Rev 3.0
Technical Data
Development Support
For More Information On This Product,
Go to: www.freescale.com
FREESCALE [ Freescale ]
