Bootloader Functionality The bootloader can be activated by two means: Hardware conditions or regular boot process. The Hardware conditions (EA = 1, PSEN = 0) during the Reset falling edge force the on-chip bootloader execution. As PSEN is a an output port in normal operating mode after reset, user application should take care to release PSEN after falling edge of reset signal. The hardware conditions are sampled at reset signal falling edge, thus they can be released at any time when reset input is low. To ensure correct microcontroller startup, the PSEN pin should not be tied to ground during power-on.