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External Interrupts (EXTI)
External interrupt is an extended feature of digital I/Os in input mode. External interrupts make a
micro to respond instantly to changes done on it digital input pin(s) by an external event(s)/trigger(s),
skipping other tasks. Such interrupts are useful in a wide variety of applications. In case of low power
energy efficient micros like the MSP430s, interrupts as such can be used to bring a micro out of sleep
mode. In other words, an external interrupt acts like a wakeup call. For example, it is extremely
important to conserve very limited battery energy in a TV remote controller while at the same time it
is also necessary to keep it completely responsive to button presses. Thus, we need to put its host
micro in sleep mode when we are not using it and make it respond to button presses immediately
when any button is pressed. A MSP430 micro in sleep/idle mode consumes literally no energy at all
and that is why they are the best micros for battery-backed low power applications.
Fortunately for us, most MSP430G2xx digital I/Os have external interrupt handling capability – a much
desired feature. Shown above is the interrupt map for MSP430G2553. Note external interrupts are
maskable interrupts and have low priority compares to other interrupts. We must consider this fact
when coding a multi-interrupt-based application.
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