The board in the toolhead is a separate MCU
[mcu extra_mcu]
serial: /dev/ttyACM0
restart_method: command
The extruder motor pins should be addressed as:
[extruder]
step_pin: extra_mcu:PA8
dir_pin: extra_mcu:PB8
enable_pin:!extra_mcu: PB11
[tmc2209 extruder]
uart_pin: extra_mcu:PB10
interpolate: True
run_current: 0.8
hold_current: 0.8
uart_address:3
sense_resistor: 0.150