HCS12 datasheet(123/452 Pages) FREESCALE

123 / 452 page

S12CPUV2 Reference Manual, Rev. 4.0

Freescale Semiconductor

123

Description:

BGND operates like a software interrupt, except that no registers are

stacked. First, the current PC value is stored in internal CPU register TMP2.

Next, the BDM ROM and background register block become active. The

BDM ROM contains a substitute vector, mapped to the address of the

software interrupt vector, which points to routines in the BDM ROM that

control background operation. The substitute vector is fetched, and

execution continues from the address that it points to. Finally, the CPU

checks the location that TMP2 points to. If the value stored in that location

is $00 (the BGND opcode), TMP2 is incremented, so that the instruction that

follows the BGND instruction is the first instruction executed when normal

program execution resumes.

For all other types of BDM entry, the CPU performs the same sequence of

operations as for a BGND instruction, but the value stored in TMP2 already

points to the instruction that would have executed next had BDM not

become active. If active BDM is triggered just as a BGND instruction is

about to execute, the BDM firmware does increment TMP2, but the change

does not affect resumption of normal execution.

While BDM is active, the CPU executes debugging commands received via

a special single-wire serial interface. BDM is terminated by the execution of

specific debugging commands. Upon exit from BDM, the background/boot

ROM and registers are disabled, the instruction queue is refilled starting

with the return address pointed to by TMP2, and normal processing

resumes.

BDM is normally disabled to avoid accidental entry. While BDM is disabled,

BGND executes as described, but the firmware causes execution to return

to the user program. Refer to Section 8. Instruction Queue for more

information concerning BDM.

BGND

Enter Background Debug Mode

BGND

CCR Details:

HIN

––––––––

Source Form

Address

Mode

Object Code

Access Detail

HCS12

M68HC12

BGND

INH

VfPPP

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