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Background Debug Mode
18.4.3 BDM Commands
The BDM command set consists of two types: hardware and firmware.
Hardware commands allow target system memory to be read or written.
Target system memory includes all memory that is accessible by the
CPU12 including EEPROM, on-chip I/O and control registers, and
external memory that is connected to the target HC12 MCU. Hardware
commands are implemented in hardware logic and do not require the
HC12 MCU to be in BDM mode for execution. The control logic watches
the CPU12 buses to find a free bus cycle to execute the command so the
background access does not disturb the running application programs.
If a free cycle is not found within 128 B-clock cycles, the CPU12 is
momentarily frozen so the control logic can steal a cycle. Commands
implemented in BDM control logic are listed in Table 18-2.
MC68HC912DG128 — Rev 3.0
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