The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 7
Listing 7-1. Displays a message when external interrupt 1 is detected.
10 ONEX1 100
20 DO
30 WHILE 1=1
40 END
100 PRINT “Interrupt detected”
110 RETI
For a pushbutton-triggered interrupt, connect a switch like Figure 7-1’s to the 8052-BASIC’s
INT1 input at pin 13. Pin 13 will then be normally high. When you press and release the
switch, pin 13 will briefly go low.
Listing 7-1 is a simple program that waits for interrupts and jumps to an interrupt-handling
routine when it detects one, signified by pin 13 going low. Line 10 enables external interrupt
1 and specifies line 100 as the location to branch to when the 8052-BASIC detects an
interrupt request at pin 13. Lines 20-30 are an endless loop that waits for an interrupt. Lines
100 and 110 are the interrupt-service routine. In this example the routine doesn’t do very
much; it just displays an on-screen message that the interrupt was detected, then returns to
the main program loop.
Edge-detecting Interrupts
BASIC-52’s TCON operator allows you to write to the 8052’s special-function register
TCON, which enables you to set up interrupt 1 as edge-detecting or level-detecting. The
default after bootup or reset is edge-detecting, where interrupts are triggered by a falling
edge at pin 13. If you want a rising edge to trigger an interrupt, you’ll have to add an inverter
at pin 13.
Edge-triggering is handy for detecting switch presses, because the interrupt routine executes
only once, when the switch is first pressed, no matter how long you hold down the switch.
Switch debouncing. Even with edge-triggered interrupts, however, switch bounce can cause
multiple interrupts to occur with a single switch press. Switch bounce occurs because manual
presses of mechanical switches tend to be sloppy. When you press a switch, the contacts
normally bounce open and closed several times before they close positively, and bounce
again as you lift your finger and the contacts open.
The computer has to be able to tell the difference between a bounce and a genuine switch
press. Otherwise, each time you press a switch, and again when you release it, the computer
will detect several rapid switch presses. One way to handle switch bounce is to ignore
keypresses that are less than a certain length, usually around 10-20 milliseconds, with the
112 The Microcontroller Idea Book