Every time you turn on your computer it is doing multitasking. I still find that amazing - to think of all that the PC is capable of doing and all with one instruction at a time!
It simply means switching attention from one task to another to make it appear that many tasks are happening all at the same time.
To make it work in a PC you need to balance the speed of the processor with the tasks you ask it to do and in some cases extra external hardware is used to take the load off the processor e.g. maths co-processor (built into Pentium processor), graphics cards etc.
This is exactly the same for a PIC microcontroller but since it is not going at GHz speeds you have to carefully decide what tasks you need to do and how to make it work distributing a fair share of processing time to each task (or peripheral).
An RTOS (Real Time Operating System) is the ultimate multitasking system and it does make writing code easier using flag signals for inter-process communication and providing prioritized task scheduling.
The penalty for using them is they take up code space and RAM resource (and they have a learning curve as there are a lot of details for RTOS control and task switching).
On the web you can find RTOS code for PIC micros but this is probably more appropriate for the larger 18F series micros as RTOS's tend to require larger RAM and program resources (Ram is used to save all the current task data between task switching) so you wont fit many tasks into a mid-range PIC micro.
You can make a simple multiple task scheduler using interrupts and careful coding and it won't need a huge amount of memory or resources.
The advantage of this method is that you have full control over the code generated and can therefore fit it into tiny devices e.g. a 12F675 with 1k of program memory.
The disadvantage is that you have to be completely on top of how your code is working. With an RTOS time slices are assigned to each task giving them a fair share of processing time. With this method you need to decide how tasks are assigned processor time as you design the code.
All you do is place all your code in an loop and use variables within the loop to test the conditions you need e.g polling an input. Each test section is more or less equivalent to a task in an RTOS.
As with any multitasking system the heart of a task scheduler is the main timer that can assign time slices to each process. For this method a timer interrupt operates every n milliseconds and calls subordinate functions that require the highest priority. Everything else is a low priority task that gets interrupted by the highest priority task.
The multitasking example I use here is taken from the 12F675 tutorial section - this uses a Timer 0 interrupt to execute the highest priority task(s) - in this case generating the servo motor output signal every 18ms. If this signal is delayed then the servo motor position will change so this must be updated regularly and must have the highest priority.
The subordinate tasks are to send and receive serial data coded in software to/from separate pins. Reception is done by continuously polling an input pin - obviously a better way would be to use another interrupt to detect the pin change - but there was not one available and it illustrates the principle well.
In this case the serial RS232 signals are not as important as the accurate servo motor signal as they are operating at 2400 Baud and can be interrupted (as the interrupt is slow compared to the transmission time of the RS232).
When you use this multitasking method you have to balance the processing time of the interrupt routine so that enough time is spent doing the RS232 communication and all this really means is make the ISR (Interrupt Service Routine) as short as possible. Also in this case the interrupt routine occurs very infrequently i.e. every 18ms leaving plenty of time for all the other tasks.
Note: This multitasking method is useful for fairly small programs but I have worked in companies that used this method for an entire large project - you end up with zillions of variables and state machine operations transferring operation to different sections within the main loop (about 20 pages long!). The whole thing becomes an unmanageable mess and if you have more than a few simple tasks it is better to switch to an RTOS as you can think of each task separately (even though the underlying code might be similar). The RTOS lets you think of solving the task rather than overcoming hardware operation (or code operation!).
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