MC68HC812A4 Datasheet, PDF (311/342 Pages) Motorola, Inc – 16-bit device composed of standard on-chip peripheral modules connected by an intermodule bus. Modules include

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Background Debug Mode (BDM)
17.4.2 Enabling BDM Firmware Commands
BDM is available in all operating modes, but must be made active before
firmware commands can be executed. BDM is enabled by setting the
ENBDM bit in the BDM STATUS register via the single wire interface
(using a hardware command; WRITE_BD_BYTE at $FF01). BDM must
then be activated to map BDM registers and ROM to addresses $FF00
to $FFFF and to put the MCU in active background mode.
After the firmware is enabled, BDM can be activated by the hardware
BACKGROUND command, by the BDM tagging mechanism, or by the
CPU BGND instruction. An attempt to activate BDM before firmware has
been enabled causes the MCU to resume normal instruction execution
after a brief delay.
BDM becomes active at the next instruction boundary following
execution of the BDM BACKGROUND command, but tags activate BDM
before a tagged instruction is executed.
In special single-chip mode, background operation is enabled and active
immediately out of reset. This active case replaces the M68HC11 boot
function and allows programming a system with blank memory.
While BDM is active, a set of BDM control registers are mapped to
addresses $FF00 to $FF06. The BDM control logic uses these registers
which can be read anytime by BDM logic, not user programs. Refer to
17.5 BDM Registers for detailed descriptions.
Some on-chip peripherals have a BDM control bit which allows
suspending the peripheral function during BDM. For example, if the timer
control is enabled, the timer counter is stopped while in BDM. Once
normal program flow is continued, the timer counter is re-enabled to
simulate real-time operations.
17.4.3 BDM Commands
All BDM command opcodes are eight bits long and can be followed by
an address and/or data, as indicated by the instruction. These
commands do not require the CPU to be in active BDM mode for
execution.
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MC68HC812A4 — Rev. 3.0
311