Page 161 - ARM Based Microcontroller Projects Using MBED

8.4 POLLING AND INTERRUPTS 147

FIG. 8.12 Program using polling.

operations and at the same time, it is required to respond when an external button is pressed.
It is clear from this figure that the CPU is not very responsive to the button event and it can
only check the state of the button when it completes executing the other code.
Polling has the disadvantage that the CPU cannot respond to external events quickly dur-
ing a polling routine. Also, if there is an error in the other code and the code happens to stay in
an indefinite loop, then the CPU will never respond to the external events.

8.4.2 Interrupts
Interrupts are very important parts of all microcontrollers as they allow the microcontroller
to respond to external events very quickly. As shown in Fig. 8.13, when an interrupt occurs
due to an external or an internal event (e.g., the timer), the CPU stops executing the current
instruction and jumps to execute the code named the interrupt service routine (ISR). After

FIG. 8.13 The interrupt process.