Exercise - Using Interrupts

<sidebar>Sidebar: What Is an Interrupt?</sidebar>

This exercise shows how to use an interrupt to sense when a switch is closed, (an external interrupt.)

Polling vs interrupt

Interrupt are a way of grabbing the microcontroller's attention immediately. It is often connected to a large number of peripheral input devices - switches, sensors, memory, timers etc. There are two broad ways in which one of these devices can be serviced by the microcontroller:

• polling - each device is 'asked' in turn if it has data to transfer to the controller;
• interrupts - to allow the device to interrupt the task being carried out by the controller.

They are very widely used, both in microcontrollers and microprocessors. For example, the keyboard and mouse in your computer probably use interrupts to talk to the CPU.

Interrupts can also be used to save energy. In many battery-powered applications, the microcontroller is 'put to sleep' when inactive, and so requires little energy. An interrupt is used to 'awaken' the controller, and bring it back into operation, when needed.

The first exercise aims to show the difference between polling and using interrupts. The second section looks .......

Exercise 1

This exercise sets up a system with two switches.
Switch 1 is polled by the program, at regular intervals.
Switch 2 initiates an interrupt.

The flowchart sequence will be:

Check if switch 1 is pressed.

If it isn't, make the red LED flash slowly.

If it is, make the yellow LED flash slowly.

Go back to the beginning and repeat the process.

When switch 2 is pressed, make both LEDs flash quickly ten times and then go back to the main program.

New flowchart

• Start a new Flowcode flowchart, using the default microcontroller.

• Make sure that the System Panel is visible. If necessary, click on View and then select 'System Panel' a check-box will appear next to the option when enabled.

Set up the input

• Drag and drop a Loop icon between the BEGIN and END icons.

• Inside the loop, drag and drop an Input icon from the Icons toolbar.

Set up the switch

• Drag and drop a Decision icon after the 'Input' icon.

Next, add icons to control what happens when switch is not pressed, (and so the program follows the 'No' branch:

Set up the lamp=

Next we concentrate on the 'Yes' branch of the 'Decision' icon.

• Drag and drop an Output icon in the 'Yes' branch.

This icon is going to function as a trigger to turn the lamp on.

Next we want the lamp to stay on for ten seconds and then turn off.

• Drag and drop a Delay icon after the 'Output' icon.

• This is the icon which will allow us to delay the circuit for a specified period of time.
• The delay will last for 10 seconds, in this time the circuit will not be able to carry out any other function until the delay is complete.

Finally we need to turn the lamp off, after the ten second delay.

To do this we will need another 'Output' icon.

• Drag and drop a second 'Output' icon after the 'Delay' icon.

Your flowchart should now be set up to resemble the image to the right.

You should now save the flowchart as "Lamp1.fcf", and close Flowcode.