Background Debug Module (S12SBDMV1)
MC9S12G Family Reference Manual Rev.1.27
288 NXP Semiconductors
Hardware commands are used to read and write target system memory locations and to enter active
background debug mode, see Section 7.4.3, “BDM Hardware Commands”. Target system memory
includes all memory that is accessible by the CPU.
Firmware commands are used to read and write CPU resources and to exit from active background debug
mode, see Section 7.4.4, “Standard BDM Firmware Commands”. The CPU resources referred to are the
accumulator (D), X index register (X), Y index register (Y), stack pointer (SP), and program counter (PC).
Hardware commands can be executed at any time and in any mode excluding a few exceptions as
highlighted (see Section 7.4.3, “BDM Hardware Commands”) and in secure mode (see Section 7.4.1,
“Security”). BDM firmware commands can only be executed when the system is not secure and is in active
background debug mode (BDM).
7.4.1 Security
If the user resets into special single chip mode with the system secured, a secured mode BDM firmware
lookup table is brought into the map overlapping a portion of the standard BDM firmware lookup table.
The secure BDM firmware verifies that the on-chip Flash EEPROM are erased. This being the case, the
UNSEC and ENBDM bit will get set. The BDM program jumps to the start of the standard BDM firmware
and the secured mode BDM firmware is turned off and all BDM commands are allowed. If the Flash does
not verify as erased, the BDM firmware sets the ENBDM bit, without asserting UNSEC, and the firmware
enters a loop. This causes the BDM hardware commands to become enabled, but does not enable the
firmware commands. This allows the BDM hardware to be used to erase the Flash.
BDM operation is not possible in any other mode than special single chip mode when the device is secured.
The device can only be unsecured via BDM serial interface in special single chip mode. For more
information regarding security, please see the S12S_9SEC Block Guide.
7.4.2 Enabling and Activating BDM
The system must be in active BDM to execute standard BDM firmware commands. BDM can be activated
only after being enabled. BDM is enabled by setting the ENBDM bit in the BDM status (BDMSTS)
register. The ENBDM bit is set by writing to the BDM status (BDMSTS) register, via the single-wire
interface, using a hardware command such as WRITE_BD_BYTE.
After being enabled, BDM is activated by one of the following
1
:
• Hardware BACKGROUND command
• CPU BGND instruction
• Breakpoint force or tag mechanism
2
When BDM is activated, the CPU finishes executing the current instruction and then begins executing the
firmware in the standard BDM firmware lookup table. When BDM is activated by a breakpoint, the type
of breakpoint used determines if BDM becomes active before or after execution of the next instruction.
1. BDM is enabled and active immediately out of special single-chip reset.
2. This method is provided by the S12S_DBG module.
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