Application Note

AN2104

8

•

Next the BDM firmware saves the contents of the user’s D register

in another temporary register and then saves the user’s CCR

register in the CCRSAV register.

•

The BDM firmware then checks the ENBDM bit in the STATUS

register to see if it will be allowed to go into the active BDM. If it is,

the BDM firmware enters a software loop and waits for a valid

firmware command in which to execute. The user’s program

counter (PC), stack pointer (SP), and X and Y registers are not

changed by the BDM firmware, so the user doesn’t need to save

or stack these values.

During exit from the BDM, the user’s register values are restored and a

value is stored in the BDM STATUS register. Then a jump command is

executed to resume execution of the user’s program.

M68HC12 Operating Modes

The two basic modes of operation (see Table 4) for the M68HC12

Family are:

•

Normal modes — Provide protection for control registers from

being accidentally modified

•

Special modes — Allow access to these control registers for

system development and special testing

If any of the normal operating modes are entered (BKGD high), the BDM

is available, but must be enabled and/or made active.

If the special single-chip mode is selected (BKGD, MODA, and MODB

all low), the BDM comes up enabled and active.

Table 4 also shows that the states of the BKGD, MODA, and MODB pins

determine a specific mode where the port A and port B pins are

configured for different functions.

Freescale Semiconductor, Inc.

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