Page 160 - ARM Based Microcontroller Projects Using MBED

146 8. INTERMEDIATE LEVEL PROJECTS

{
if(Digits[3] != 0) // If MSD Digit non zero
{
Segments = LEDS[Digits[3]]; // Send to PORT C
Enable3 = Enable; // Enable Digit 3
wait(0.005); // Wait 5ms
Enable3 = Disable; // Disable Digit 3
}

if(Digits[2] != 0 || (Digits[2] == 0 && Digits[3] != 0))
{
Segments = LEDS[Digits[2]]; // Send to PORT C
Enable2 = Enable; // Enable Digit 2
wait(0.005); // Wait 5ms
Enable2 = Disable; // Disable Digit 2
}

if(Digits[1] != 0 || (Digits[3] != 0 || Digits[2] != 0))
{
Segments = LEDS[Digits[1]]; // Send to PORT C
Enable1 = Enable; // Enable Digit 1
wait(0.005); // Wait 5ms
Enable1 = Disable; // Disable Digit 1
}

Segments = LEDS[Digits[0]]; // Send to PORT C
Enable0 = Enable; // Enable Digit 0
wait(0.005); // Wait 5ms
Enable0 = Disable; // Disable Digit 0
}
}

FIG. 8.11, CONT’D

8.4 POLLING AND INTERRUPTS

In many real-time applications, we may want to respond to external events as soon as they
happen. But this seems not to be possible while the CPU is executing some code. What we
basically want is to interrupt the CPU so that it stops what it has been doing and starts to
execute the code in our real-time routine. After completing the real-time routine, the CPU
should return and carry on what it has been doing before being interrupted. The concept that
stops the CPU and starts executing the real-time code is called Interrupt.
Basically, there are two ways that the CPU can respond to external events: polling and
interrupts.

8.4.1 Polling
In polling, the CPU checks the occurrence of an external event in a loop while executing
some other code. Fig. 8.12 shows a flow diagram where the CPU is carrying out some