Write a Verilog description for the state-machine diagram for the batch mixing system derived in Example 4-10.

Question:

Example 4-10:

A mixing system for large batches of liquids is designed to add up to three ingredients to a large circular mixing tank, mix the ingredients, and then empty the mixed liquid from the tank. There are three inlets for ingredients, each with an on– off valve. There are three movable fluid sensors in the tank that can be set to turn off the respective valves at the level required for the first ingredient alone, for the first and second ingredients, and for all three ingredients. A switch is used to select either a two or three ingredient operation. There is a button for starting the operation and a second button for stopping the operation at any time. There is a timer for timing the mixing cycle. The length of the mixing cycle is specified by a manually operated dial that provides a starting value to a timer. The timer counts downward to zero to time the mixing. After mixing, the output valve is opened to remove the mixed liquid from the tank.

A sequential circuit is to be designed to control the batch mixing operation. The inputs and outputs for the circuit are given in Table 4-8. Before starting the operation of the mixing system, the operator places the fluid sensor L1, L2, and L3 in the proper locations. Next, the operator selects either two or three ingredients with switch NI and sets dial D to the mixing time. Then, the operator pushes the START to begin the mixing operation which proceeds automatically unless the STOP button is pushed. Valve V1 is opened and remains open until L1 indicates ingredient level 1 has been reached. Valve 1 closes and valve 2 opens and remains open until L2 indicates level 1 plus 2 has been reached. Valve 2 closes, and, if switch NI = 1, valve 3 opens and remains open until L3 indicates level 1, 2 plus 3 has been reached. If NI = 0, the value on dial D is then read into the timer, the mixing begins, and the timer starts counting down. In the case where NI = 1, these actions all occur when L3 indicates that the level for all three ingredients has been reached. When the timer reaches 0 as indicated by the signal TZ, the mixing stops. Next, the Output valve is opened and remains open until sensor L0 indicates the tank is empty. If STOP is pushed at any time, addition of ingredients stops, mixing stops, and the output valve closes.

Table 4-8: