From datasheet: “The background debug module (BDM) sub-block is a single-wire, background debug system implemented in on-chip hardware for minimal CPU intervention. All interfacing with the BDM is done via the BKGD pin.”
So, for programming you will need MODC/BKGD pin and additionally also RESET pin.
Ground signal is self-evidence and some programming devices require also MCU power supply voltage (3.3V..5V).
The BKGD/MODC pin is used as a pseudo-open-drain pin. Therefore typically BDM interface drive this pin by open-drain output. In case of S12XS family, the BKGD pin has an internal pull-up device.
For onboard BDM connection is typically used 6pin BDM connector (2x3 pin header with pitch 2.54mm/100mils).
Please look at DEMO9S12XSFAME board schematic from attachment.
