Hi, friend
I think you mean the active background debug mode, you can find the words in the datasheet of your device, below is the way to enter into the active background debug mode for one S08 device, hope it's helpful.
Active background mode is entered in any of five ways:
• When the BKGD/MS pin is low at the rising edge of reset
• When a BACKGROUND command is received through the BKGD pin
• When a BGND instruction is executed
• When encountering a BDC breakpoint
• When encountering a DBG breakpoint