29-04-2014, 01:00 PM
Traffic light controller
[attachment=62635]
Introduction
A busy highway is intersected by a little used farm road. Detectors
C sense the presence of cars waiting on the farm road.
With no car on farm road, light remain green in highway direction. If vehicle on farm road, highway lights go from Green to Yellow to Red, allowing the farm road lights to become green.
These stay green only as long as a farm road car is detected but never longer than a set interval.
When these are met, farm lights transition from Green to Yellow to
Red, allowing highway to return to green. Even if farm road vehicles
are waiting, highway gets at least a set interval as green.
Assume you have an interval timer that generates a short time pulse
(TS) and a long time pulse (TL) in response to a set (ST) signal. TS
is to be used for timing yellow lights and TL for green lights.
Traffic light machine
Build controller out of two machines:
sequencer which sets colors of lights, etc.
timer which is used to control durations of lights.
Separate counter isolates logical design from clock period.
Separate counter greatly reduces number of states in sequencer.
The synchronous philosophy
All operation is controlled by the clock.
All timing is relative to clock.
Separates functional, performance optimizations.
Put a lot of work into designing the clock network so you don’t have to worry about it throughout the combinational logic.
[attachment=62635]
Introduction
A busy highway is intersected by a little used farm road. Detectors
C sense the presence of cars waiting on the farm road.
With no car on farm road, light remain green in highway direction. If vehicle on farm road, highway lights go from Green to Yellow to Red, allowing the farm road lights to become green.
These stay green only as long as a farm road car is detected but never longer than a set interval.
When these are met, farm lights transition from Green to Yellow to
Red, allowing highway to return to green. Even if farm road vehicles
are waiting, highway gets at least a set interval as green.
Assume you have an interval timer that generates a short time pulse
(TS) and a long time pulse (TL) in response to a set (ST) signal. TS
is to be used for timing yellow lights and TL for green lights.
Traffic light machine
Build controller out of two machines:
sequencer which sets colors of lights, etc.
timer which is used to control durations of lights.
Separate counter isolates logical design from clock period.
Separate counter greatly reduces number of states in sequencer.
The synchronous philosophy
All operation is controlled by the clock.
All timing is relative to clock.
Separates functional, performance optimizations.
Put a lot of work into designing the clock network so you don’t have to worry about it throughout the combinational logic.