A combination lock is installed on the entrance door of a room to ensure that only authorized

persons can enter. This lock is microprocessor controlled and its circuit is shown below. The red LED

used in the circuit is used to give the user a wait signal to log in$.$ The green LED is used to indicate

that the entered password is correct. The solenoid used in the lock is connected to the PC$2$ port. A

four$-$digit password is used. The password is entered by pressing four keys in sequence. Assume that

the password information is stored in $4$ consecutive addresses starting from the PASSWORD address.

The debounce time of the keys is $15ms.$ Assume that the DEBOUNCE subroutine, which provides a

delay of this duration, is ready.

a$)$ Design the address decoder circuit suitable for the memory and I$/$O Map design you choose. Use

the minimum number of switching elements for the decoder circuit. The $8155$ onboard memory will

be used to store the necessary variables. The stack region will also be in the same RAM.

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