sequence the next-state variables and output variables 14.18 Digital lock, I. Draw a state diagram and a state table for a digital lock. The lock has two inputs, a and b, and one output, unlock. The output is asserted only if the a, b, a, a is observed. Each element of the sequence must last for one or more cycles, and there must be one or more cycles of both inputs low between the sequence elements. After unlocking, either input going high causes unlock to go low. 14.19 Digital lock, II. Implement, in VHDL, your digital lock state machine from Exercise 14.18. 14.20 Basic vending machine, I. Exercises 14.20-14.22 will focus on designing an FSM for a simple vending machine. This machine vends a single $0.40 item and accepts only nickels and dimes. The input signals are nickel and dime, and the outputs are vend and change. The two input signals are pulsed high when a nickel ($0.05) or dime ($0.10) is inserted into the machine (only one will be high at a time). When enough money has been added, the vend signal goes high for a single cycle. If $0.45 had been inserted, change also pulses high for one cycle. After vending an item, the state returns to the initial state where no money has been inserted. We will explore a more flexible vending machine in Section 16.3.1. Draw a state diagram and state table for this machine. 14.21 Basic vending machine, II. Using the state diagram from Exercise 14.20 and a binary state assignment, derive the output signals and next-state logic. 14.22 Basic vending machine, III. Implement the vending machine of Exercise 14.20 in VHDL. Show a waveform of the outputs and state when the user inserts consecutively two nickels, six dimes, and three nickels. 14.23 Vending machine with quarters, I. Modify your state table and diagram from Exercise 14.20 to include a quarter ($0.25) input. Assume the vending machine will output one nickel for every cycle that change remains high. 14.24 Vending machine with quarters, II. Write the VHDL that implements the state machine of Exercise 14.23. 14.25 Airplane indicator lights, I. Draw the state diagram and table for an FSM that controls the seat-belt and no-electronics signs for a commercial airliner. The state machine has three inputs: alt10k, alr25k, and smooth. Whenever the airplane passes 10 000 (25 000) feet moving in either direction, alt 10k (al125k) will pulse high for one cycle. If the plane is not climbing, descending, or experiencing turbulence, the smooth signal will be set