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In this lecture, I am going to put the computational circuit design techniques into a real example of 
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a traffic light controller.
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This figure shows the intersection of a main highway (the horizontal road) with a secondary access
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road (the vertical road). 
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Four sensors along lanes U, L, D, and R detects vehicles. The sensor output is 0 when no vehicle
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is presented and 1 if at least a car is on the corresponding lane. 
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There are two traffic lights on this junction, one for North-South traffic and another for East-West 
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traffic.
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The logic value “1” set the traffic light to green and the logic value “0” set the traffic
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light to red. The traffic lights should meet the following rules:
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1. The EW traffic light is green, and NS traffic light is red if both lanes R and L are occupied
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2. The EW traffic light is green, and NS traffic light is red if one of the lanes 
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R and L is occupied and both lanes U and D are also occupied.
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3. The NS traffic light is green, and EW traffic light is red if both lanes U and D 
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are occupied, and cars are not detected on both lanes L and R.
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4.
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The NS traffic light is green, and EW traffic light is red if one of the lanes U 
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and D is occupied while both lanes L and R are vacant.
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5. The EW traffic light is green, and NS traffic light is red if all lanes are vacant. 
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The final logic circuit has four inputs corresponding to the four sensors and two outputs for traffic 
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lights.
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This table shows all combinations of input values and the corresponding rules that they apply to set 
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the traffic lights.
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For example, if the sensors do not detect any cars then based on Rule 5, the EW traffic 
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light should be green, and the NS traffic light should be red.
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Or if there is traffic on the U lane and other lanes are vacant then based on Rule 4, the EW 
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traffic light should be red, and the NS traffic light should be green.
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Now let’s design the controller. For this purpose, we propose a logic circuit representing each rule 
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and then we will combine them to create the final traffic-light controller.
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The first rule says EW is 1 
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if both R and L sensors are one. We can use an AND gate to implement this rule. 
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The second rule says EW is 1 if one of the R and L sensors is 1, and both U and D sensors
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are not 1. An XOR can model the first part of the rule, 
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and a NAND gate can represent the second part. And then an AND gate can combine these two parts. 
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The third rule says NS light is 1 if both U and D sensors are 1 and both L and R sensors 
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are 0. A NAND can model the first part of the rule, and an AND gate represents the second
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part.
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Then an AND gate can combine them to make the rule. 
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Similarly, this logic circuit with three gate models the fourth rule. 
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Two XNOR gates combined by an AND gate can implement the fifth rule. 
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We have two groups of rules, Rules 1, 2 and 5 set the EW to 1 and Rules 3, and 4
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set the NS to 1.
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Now we should combine these rules to design the controller.
First, let’s combine the rules inside a group.
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Using OR gates, we can combine rules inside a group. 
Now we have the controller that generates EW 
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and NS outputs.
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Now let’s write a C code to implement this controller. For this purpose, we write a C function for 
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Each function receives the values of four sensors and returns a value controlling the EW
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or NS traffic light.
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These are the codes for Rules 1, 2 and 3.
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And these are the codes for Rules 4 and 5.
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Now an HLS top function can call and combine these rules.
The function has six arguments; two of them 
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are pointers to be connected to the traffic lights. 
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First, we call rules.
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Then we combine them with OR operators.
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Finally, we should add interfaces. This picture shows the HLS report after synthesis. The propagation 
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delay of the circuit is about 2.934 ns.
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The hardware is a combinational circuit that utilises 34 LUTs. And ports are a set of single 
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wires without any specific protocol as we expected.
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Now that we have designed the traffic light controller, we should see its behaviour in action. The 
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next lecture follows the implementation of this design on the Basys3 
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board.  
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This is our takeaway message.  Dividing a complex problem and algorithm into a set of subfunctions is 
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the key to manage the design process.
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Now the quiz question. The two traffic lights in an intersection must not be green at the same time.
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Check that the FW signal is the opposite of the NS 
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signal in slide number 10.