Practical Design Using Counter and Compare Instructions

You are to design a traffic light control system. It controls North$-$South and East$-$West lights at an

intersection according to the following steps.

When the program starts running, all lights $($including walk$)$ turn $ON$ for $\frac{1}{2}$ second $($light bulb test$).$

Following step $0,$ the system starts operating normally by repeating steps $1$ to $7$ automatically:

Red in both directions for $1$ second.

Occasional extra step: See Walk light below.

North$-$South goes green for $6$ seconds and East$-$West stays red.

North$-$South goes yellow for $1$ second and East$-$West stays red.

Red in both directions for $1$ second.

East West goes green for $4$ seconds and North$-$South stays red.

East$-$West goes yellow for $1$ second and North$-$South stays red.

IMPORTANT: A master reset switch is required on the system.

The sequencer reset must force the system back to step $0($light bulb test$).$

Walk light

To be able to get the full $3\mathrm{.}5$ marks for this lab a Walk light must be implemented as described below.

You may choose to omit the Walk feature but

your lab demo will only be worth a maximum of $\frac{2\mathrm{.}0}{3\mathrm{.}5}$

The Walk pushbutton is momentary pressed when pedestrians want to cross the street in the North$-$South

direction. When walk is active, $3$ seconds are added to the complete light cycle as described below:

Pushing the walk button has no effect during step $3$ when the North$-$South light is already green.

If the button is momentarily pushed during any other step $($except step $3)$ then:

a$)$ To allow pedestrians to start walking before the cars get the green light, a Walk light turns

oN for $3$ seconds at step $2.$ This means that the North$-$South remains red longer before the

green light is turned on at step $3.$ Note that East$-$West remains red for the whole duration of

the North$-$South cycle.

b$)$ The walk light remains oN during the whole duration of step $3$ while North$-$South is green.

c$)$ The Walk light turns OFF at the beginning of step $4($yellow light$)$ and will remain OFF until

the Walk pushbutton is used again.