Introduction

There often exist scenarios where flexible traffic signal sequences are required for the coordination of traffic through the intersection of a busy street and a lightly used side street. In such situations, the sequences can be controlled using different timers and a traffic detection signal from the side street. These requirements can be met through conventional methods e.g. using building blocks from discrete electronic components or microcontrollers. However, the concept of integrated circuits (IC) provides an attractive alternative considering its design flexibility, low cost, development time, and convenience.

Many regions and countries are progressing to more complicated grids that can accommodate a greater number of variables to control traffic lights. However, many traffic lights still utilize fixed time control, such as electro-mechanical signal controllers. The purpose of this application note is to show how one can use a GreenPAK's Asynchronous State Machine (ASM) to develop a simplified traffic signal controller to replace a fixed-time controller. This traffic signal regulates traffic passing through the intersection of a busy main street and a lightly used side street. The controller would control the sequence of two traffic signals, which are installed at the main and side street. A sensor signal, detecting the presence of side street traffic, is fed to the controller that, in conjunction with two timers, would control the sequence of the traffic signals. A finite state machine (FSM) scheme is developed that ensures the requirements of the traffic signals sequence are met. The controller logic is implemented using a GreenPAK™ SLG46537 programmable mixed-signal ASIC.

Requirements

Figure 1: Timing Requirements of The Traffic Control System

Consider a traffic scenario with the timing requirements of traffic signals from the main and side street, as shown in Figure 1. The system has six states, and will move from one state to the other depending upon certain predefined conditions. These conditions are based on three timers; a long timer TL =25 s, a short timer TS = 4 s and a transitory timer T= 1 s. Additionally, the digital input from side traffic detection sensor is required. A thorough description of each of the six system states and the state transition control signals is given below:

In the first state, the main signal is green while the side signal is red. The system will stay in this state until the long timer (TL = 25 s) expires or as long as there is no vehicle on the side street. If a vehicle is present on the side street after the expiration of the long timer, the system will undergo a state change moving to the second state.

In the second state, the main signal turns yellow while the side signal remains red for the duration of the short timer (TS = 4 s). After 4 seconds the system moves into the third state.

In the third state, the main signal changes to red and the side signal remains red for the duration of transitory timer (Tt =1 s). After 1 second, the system moves to the fourth state.

During the fourth state the main signal is red while the side signal turns to green. The system will stay in this state until the expiry of long timer (TL = 25 s) and there are some vehicles present on the side street. As soon as the long timer expires, or there is no vehicle on the side street, the system will transition into the fifth state.

During the fifth state the main signal is red while the side signal is yellow for the duration of the short timer (TS = 4 s). After 4 seconds the system will move into the sixth state.

In the sixth and the last state of the system, both the main and side signals are red for the period of the transitory timer (Tt =1 s). After that, the system goes back to the first state and starts over again.

The third and sixth states provide a buffer state...

Read more »