MC68HC12 Datasheet, PDF (401/478 Pages) Motorola, Inc – The MC68HC912DT128A microcontroller unit (MCU) is a 16-bit device composed of standard on-chip peripherals including a 16-bit central processing unit

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Background Debug Mode
Figure 20-3 shows the host receiving a logic zero from the target
MC68HC912DT128A MCU. Since the host is asynchronous to the target
MCU, there is a 0-to-1 cycle delay from the host-generated falling edge
on BKGD to the start of the bit time as perceived by the target MCU. The
host initiates the bit time but the target MC68HC912DT128A finishes it.
Since the target wants the host to receive a logic zero, it drives the
BKGD pin low for 13 BDMCLK cycles, then briefly drives it high to speed
up the rising edge. The host samples the bit level about ten cycles after
starting the bit time.
20.4.3 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 BDMCLK cycles, the CPU12 is
momentarily frozen so the control logic can steal a cycle.Commands
implemented in BDM control logic are listed in Table 20-2.
MC68HC912DT128A — Rev 4.0
MOTOROLA
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Technical Data
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