exercise:

The purpose of this exercise is to create Finite State Machine (FSM) which can detect a

certain eight digit binary sequence in a continuous serial binary input stream. When the

correct sequence is detected, the single output signal should be a logical true value, in all

other cases it should be a logical false. This exercise is designed to allow a gradual

transition from strictly combinational circuits to a simple sequential circuit. Note that this

lab assignment is a group assignment with each group having 2 students.

Problem Statement: The "Gottcha Anti-Theft" Machine

There are many anti-theft devices on the market that attempt to foil a would-be robber

from starting your car and driving off with it. One popular item has a keypad like a touchtone

telephone. In order to start your car, you must key in a secret four digit decimal

code, such as '3719'. It only "remembers" the most recent four digits you have keyed in.

Thus the sequence '3723719' will let you start your car.

For simplicity here we use a code based on a sequence of eight bits, and use two push

buttons to enter a sequence serially. Each press of a button enters the corresponding digit.

Figure 1. The Gottcha machine in action.

As you key in the bits in sequence, the device outputs E = 0 until the most recent eight

bits agree with a built-in secret code byte. Then E switches to 1.

The shut-off gottcha: If 16 bits are toggled in without the secret code being observed,

the system shuts off and won't accept any more bits. Add a reset button to the system that

presumably only the owner could control: when RESET is pressed the system is again

enabled and can accept bits.

Design and test (on LogicWorks) the miracle Gottcha machine. Do this in two different

ways (thus designing and simulating two different circuits):

1) The Factory Preset Model: The specific secret byte preset at the factory MUST be

one of your team members secret keys. The Secret key is: 11000111

2) The User-Programmable Model: This machine allows the car owner to enter a secret

byte into a register. The user selects a byte using two hex keyboards, and presses an

ENTERCODE pushbutton to store the code word. Now when the machine is used "in the

field", the car owner enters a sequence using the same pushbutton arrangement as for the

previous machine. When the sequence so entered matches the code word stored in the

register, E goes HIGH. As with the Factory Preset Model, if more than 16 bits are entered

without the proper sequence being observed, the system shuts off (until RESET is

pressed).

secret bjte enable for the starter ImOtor fnger 0