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AN2104
Using Background Debug Mode for the M68HC12 Family
By Timothy J. Airaudi
Applications Engineering, Microcontroller Division
Austin, Texas
Introduction
This application note describes the basic operation of the background
debug mode (BDM) and some of its applications, as it relates to
Motorola’s M68HC12 Family of microcontrollers (MCU). Examples of in-
circuit programming of internal FLASH memory and in-circuit debugging,
using P&E Microcomputer Systems’ BDM interface cable and its
software, are also contained in this document.
The BDM’s main purpose is to allow debugging of the actual
microcontroller being used in the user’s target application. This takes the
place of hardware such as an in-circuit emulator, which uses external
components to attempt to duplicate operation of the MCU from outside
of the target application.
Instead of having this external hardware, and a variety of potential
problems, the debug logic is built into the MCU’s on-chip integration
module. This differs from other systems that have the debugging logic
located in the central processor unit (CPU). Not having the debugging
logic in the CPU allows for reading and writing of memory locations,
while the CPU is executing user code, with no degradation in real-time
operation. This is an example of the BDM being enabled but not active.
© Freescale Semiconductor, Inc., 2004. All rights reserved.
© Motorola, Inc., 2001
For More Information On This Product,
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1 page  
Freescale Semiconductor, Inc.
Application Note
Theory of Operation
To execute firmware commands, the user must have the BDM enabled
and active. See Table 2. When the BDM is active, it has control of the
CPU, which executes code out of the BDM ROM.
Table 2. BDM Firmware Commands
Command Opcode (Hex)
Data
Description
READ_NEXT
62
16-bit data out
X = X + 2; Read next word
pointed to by X
READ_PC
63 16-bit data out Read program counter
READ_D
64 16-bit data out Read D accumulator
READ_X
65 16-bit data out Read X index register
READ_Y
66 16-bit data out Read Y index register
READ_SP
67 16-bit data out Read stack pointer
WRITE_NEXT
42
16-bit data in
X = X + 2; Write next word
pointed to by X
WRITE_PC
43 16-bit data in Write program counter
WRITE_D
44 16-bit data in Write D accumulator
WRITE_X
45 16-bit data in Write X index register
WRITE_Y
46 16-bit data in Write Y index register
WRITE_SP
47 16-bit data in Write stack pointer
GO
08
None
Go to user program
TRACE1
10
None
Execute one user instruction
then return to BDM
TAGGO
18
None
Enable tagging and go to
user program
AN2104
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Application Note
Background Debug Mode Application Examples
• When in normal operating mode, another normal operating mode
can be accessed, but this can be done only once.
• To change to the normal operating mode, when the part is in
special operating mode, a 1 (one) must be written twice to the
SMODN bit in the mode register.
• When the part comes up in special single-chip mode, the BDM is
enabled and active.
• When the part comes up in special single-chip mode, it accesses
the BDM ROM, not the normal memory mapped locations at
$FF00–$FFFF.
• To perform hardware commands, the BDM does not need to be
active (see Table 1).
• To perform firmware commands, the BDM must be enabled and
active (see Table 2).
• The BDM does not operate in stop mode.
Background Debug Mode Application Examples
Two BDM application examples are given here in a step-by-step format.
In-Circuit
Programming
of Internal FLASH
This application example of the BDM explains how to perform in-circuit
programming of the internal FLASH memory of an MC68HC912B32
using P&E Microcomputer Systems’ Cable12 POD and software (see
Figure 1). The target board for this example is the M68EVB912B32
evaluation board.
AN2104
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