Programming Embedded Systems in C and C++

130 Chapter 9: Putting It All Together

int

Timer::waitfor()

{

if (state != Active)

{

return (-1);

}

//

// Wait for the timer to expire.

//

pMutex->take();

//

// Restart or idle the timer, depending on its type.

//

if (type == Periodic)

{

state = Active;

timerList.insert(this);

}

else

{

pMutex->release();

state = Idle;

}

return (0);

} /* waitfor() */

When the timer does eventually expire, the interrupt service routine will release

the mutex and the calling task will awake inside waitfor. In the process of wak-

ing, the mutex will already be taken for the next run of the timer. The mutex need

only be released if the timer is of type OneShot and, because of that, not automat-

ically restarted.

Printing “Hello, World!”

The other part of our example application is a task that prints the text string

“Hello, World!” to one of the serial ports at a regular interval. Again, the timer

driver is used to create the periodicity. However, this task also depends on a serial

port driver that we haven’t seen before. The guts of the serial driver will be

described in the final two sections of this chapter, but the task that uses it is

shown here. The only thing you need to know about serial ports to understand

this task is that a SerialPort is a C++ class and that the puts method is used to

print a string of characters from that port.

#include "timer.h"

#include "serial.h"