Most courses treat multitasking as an advanced topic. I hope your perspective will change as you come to see that it is the most fundamental part of efficient programming and deserves your attention as early as possible. Once you understand it, you can have your microcontroller look like it is doing many things all at the same time—it will never seem to be unavailable and will always be checking for something to do.
Beyond single-program thinking
If you have used only traditional techniques of programming, you will find, as your applications grow more time-critical, that a single program approach becomes awkward. When you have several external hardware devices that need to be served at the same time, your challenge is to make sure each device is satisfied.
Perhaps all your software has only had one thing happens at a time…in sequence (that is why computers were called sequential machines). If a speech chip had to say something, you programmed the processor to put out the appropriate code, pulse the write line, and then wait until the phrase was done before going on to check for some new input from the keypad. Nothing could be recognized while the processor was waiting for the ‘done’ from the speech chip. To move on in your programming skills you must develop a new way of thinking about program flow…you can’t let the flow get stuck if something isn’t ready yet.
Realtime
Although the term means different things to different people, I apply realtime to any system that respond to inputs and supply outputs fast enough to meet user or external hardware requirements. For example, a keyboard entry system is realtime if it gives you some feedback quickly enough for you to feel confident that the system “heard” you. If a “beep” is fed back to you within 100mSec, you feel confident that the system recognized the input “right away.” So, a keyscan routine that repeated every 100mSec would probably meet the requirement of fast enough. Likewise, your eyes and brain cannot assimilate new digital display information more quickly than perhaps 5 times a second….numeric values that are rapidly changing might better be updated only a few times a second. On the other hand, a stepper motor ought to be sent a new step pulse at a regular time with millisecond precision, so this is a much more critical time requirement. Most serial ports have a single-character buffer, so at 9600 baud every incoming character must be picked up within about 1mSec (10 bits @9600 bits/Sec=960 char/Sec »1mSec). Outgoing (asynchronous) characters can go whenever the processor is not busy, assuming there is no other time constraint on message transfer. The same considerations would apply to collecting data via an A-D converter. Depending on how rapidly the incoming voltage is changing, the reading might need immediate retrieval or might be held for quite some time. How rapidly should a flow valve be adjusted for a process? How quickly should a motor be supplied a new speed setting? All of these are questions relating to what is fast enough.