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MICROCONTROLLERS MADE EASY

2.5 INTERRUPTS

Polling. Polling is what you have to do if your microcontroller does not have interrupts or if

what you want to do is not time critical. It is a software technique whereby the controller con-

tinually asks a peripheral if it needs servicing. The peripheral sets a flag when it has data

ready for transferring to the controller, which the controller notices on its next poll. Several pe-

ripherals can be polled in succession, with the controller jumping to different software routines,

depending on which flags have been set.

Figure 14. Polling versus Interrupt

Interrupts. Rather than have the microcontroller continually polling - that is, asking periph-

erals (timers / UARTS / A/Ds / external components) whether they have any data available

(and finding most of the time they do not), a more efficient method is to have the peripherals

tell the controller when they have data ready. The controller can be carrying out its normal

function, only responding to peripherals when there is data to respond to. On receipt of an in-

terrupt, the controller suspends its current operation, identifies the interrupting peripheral, then

jumps to the appropriate interrupt service routine. The advantage of interrupts, compared with

polling, is the speed of response to external events and reduced software overhead (of con-

tinually asking peripherals if they have any data ready).

Most microcontrollers have at least one external interrupt, which can be edge selectable

(rising or falling) or level triggered. Both systems have advantages. Edge is not time sensitive,

but it is susceptible to glitches. Level must be held high (or low) for a specific duration (which

can be a pain but is not susceptible to glitches).

VR02101M

TASK 1

POLLINGLOOP

TASK 3

TASK 2

Event occurs

POLLINGLOOP

POLLINGLOOP

PROCESS

EVENT

TASK 1

TASK 3

TASK 2

Event occurs

INTERRUPT

PROCESS

EVENT

Save State

Restore State

POLLING

INTERRUPT

TASK 4

TASK 4