PC912XDP512F0MPVR datasheet(578/1348 Pages) FREESCALE

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Chapter 15 Background Debug Module (S12XBDMV2)

MC9S12XDP512 Data Sheet, Rev. 2.21

578

Freescale Semiconductor

15.4

Functional Description

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

types of BDM commands: hardware and ﬁrmware commands.

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

background debug mode, see Section 15.4.3, “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 15.4.4, “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 15.4.3, “BDM Hardware Commands”) and in secure mode (see Section 15.4.1,

“Security”). Firmware commands can only be executed when the system is not secure and is in active

background debug mode (BDM).

15.4.1

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 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 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.

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 S12X_9SEC Block Guide.

15.4.2

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)

interface, using a hardware command such as WRITE_BD_BYTE.

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