A print engine comprising at least one print controller and at least one associated authentication device is provided. Each authentication device having a processor, a memory that the processor can access, a memory access unit for controlling accesses to the memory, an input for receiving power for the authentication device from an external power source, and a power detection unit. The processor is configured to control and trim the amount of power supplied to the input to predetermine a threshold at which operation of the authentication device is established. The power detection unit is configured to monitor a voltage level of the power supplied to the input, and in the event the voltage level drops below the predetermined threshold, disabling a power supply to circuitry for use in writing to the memory under the control of the print controller, such that the memory access unit's ability to alter data in the memory is disabled prior to address or data values to be written to the memory becoming unreliable due to failing power.