Auto brakes are armed in the RTO position for take off. (RTO = Rejected Take Off) The system isn't active until reaching a certain airspeed. I believe that is 80 knots in the 320. Below 80 knots, the autobrakes will not activate. Above 80 knots, one or two things have to happen. I don't remember exactly what, but pulling the power levers to idle will do it. Loss of an engine is NOT one of the things that will activate autobrakes for very good reason.
On the A320(319) the auto brakes and spoilers arm at 72 knots. And the settings on Airbus are "Low", "Medium" and "Max". On takeoff the auto brakes are selected "Max".
A318/A319/A320/A321 FLIGHT CREW OPERATING MANUAL
AIRCRAFT SYSTEMS LANDING GEAR
BRAKES AND ANTISKID - DESCRIPTION
AUTO BRAKE - GENERAL Applicable to: ALL
GENERAL The purposes of this system are : to reduce the braking distance in case of an aborted takeoff to establish and maintain a selected deceleration rate during landing, thereby improving passenger comfort and reducing crew workload.
SYSTEM ARMING The system arms when the crew presses the LO, MED, or MAX pushbutton switch if: Green pressure is available. The anti-skid system has electric power. There is no failure in the braking system. At least one ADIRS is functioning. Note: 1.Auto brake may be armed with the parking brake on. 2.MAX autobrake mode cannot be armed in flight.
SYSTEM ACTIVATION Automatic braking is activated: at the command for ground spoilers extension (Refer to DSC-27-10-20-C Speed Brakes and Ground Spoilers - Speed Brake Control), for LO and MED mode. at the command for ground spoilers extension and the aircraft speed is above 40 kt, for MAX mode. Therefore, if the aircraft makes an acceleration stop and begins to decelerate when its speed is under 72 kt, the automatic braking will not activate because the ground spoilers will not extend. For autobrake to activate, at least two SEC's must be operative.
SYSTEM DEACTIVATION The system deactivates : When it is disarmed (Refer to DSC-32-30-10-A Auto Brake - System Disarming). When the ground spoilers retract. In this case it remains armed.
SYSTEM DISARMING The system disarms when: Flight crew presses the pushbutton switch or, One or more arming conditions is lost or, Flight crew applies enough deflection to at least one brake pedal when autobrake is active in MAX, MED or LO mode. After take-off/touch and go.