Background debug mode interface
The Background Debug Mode interface (BDM) is a single-wire signal and associated on-chip hardware that gives in-circuit debugging functionality in microcontrollers. It appears in many Freescale Semiconductor products.
The interface allows a Host to manage and query a Target. Specialized hardware is required in the target device. No special hardware is required in the host; a simple bi-directional I/O pin is sufficient.
I/O signals
The signals used by BDM to communicate data to and from the target are initiated by the host processor. The host negates the transmission line, and then either
- asserts the line sooner, to output a 1,
- asserts the line later, to output a 0,
- tri-states its output, allowing the target to drive the line. The host can sense a 1 or 0 as an input value.
At the start of the next bit time, the host negates the transmission line, and the process repeats. Each bit is communicated in this manner.
BDM commands
The BDM host can issue commands with parameters to the target. Some commands allow reading or writing of blocks of the target's memory, individual registers in the CPU, or registers not available to the target.
Examples include:
READ_BYTE |
Read a byte from memory |
WRITE_BYTE |
Write a byte to memory |
GO |
Start/resume executing at current program counter location |
WRITE_PC |
Write a value to the program counter |
READ_PC |
Read the value of the program counter |
WRITE_X |
Write a value to the index register |
READ_X |
Read the value of the index register |
READ_SP |
Read the value of the stack pointer |
Bibliography
Motorola (Freescale Semiconductor Inc.). CPU12 Reference Manual (CPU12RM/AD).