Block Guide — S12BDM V4

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Section 4 Functional Description

The BDM receives and executes commands from a host via a single wire serial interface. There are two

types of BDM commands, namely, hardware commands and firmware commands.

Hardware commands are used to read and write target system memory locations and to enter active

background debug mode, see 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 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 4.3 BDM Hardware Commands. Firmware commands can only be executed when the

system is in active background debug mode (BDM).

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 EEPROM and FLASH EEPROM are erased. This

being the case, the UNSEC 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

EEPROM or FLASH do 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

EEPROM and FLASH. After execution of the secure firmware, regardless of the results of the erase tests,

the CPU registers, INITEE and PPAGE, will no longer be in their reset state.

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.