Is there an alternative for sleep() in C?

Question

In traditional embedded programming, we will give a delay function like so:

for(i=0;i<255;i++)
   for(j=0;j<255;j++);

In the micropro

Answer 1

You would not use the code you published to sleep on an embedded system. A decent compiler would entirely remove it, and even if your compiler does not remove it is suboptimal, since running the processor in a tight loop will burn power, which is an issue for embedded system. Even systems not running on battery care about power usage, since lower power usage means cheaper power supplies and cooling.

The way you normally do this is your CPU will implement some sort of IDLE or SLEEP instructions, that will cause it to temporarily stop processing commands. An external interrupt line connected to a timer circuit will wake the processor back up at regular intervals, and which point the CPU checks to see if it has been asleep for long enough, and if not it goes back to sleep.

//Pseudo code
int start = getTime();
int end = start + sleepTime;

while (getTime() < end) {
       asm("SLEEP");
}

The exact details vary from processor to processor. If you are running as a process on an OS the sleep call generally just tells the scheduler to suspend your process, and then the kernel decides whether to schedule another process or to sleep the CPU. Also, the above code will not be adequete for real time systems, which want deadline guarantees, etc. In those cases you will need to get the time in the loop, know the duration of the time interrupt so ou know if you can resleep without blowing the deadline, and potentially reprogram the timer hardware or busy wait.