C and the 8051: Hardware, Modular Programming & Multitasking

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Thomas W. Schultz, Purdue University, Indiana. If You're an Educator Additional order info. Most microcontrollers just don't have the facilities for that, and things an Intel CPU does with a couple specific instructions would require you to write miles of code. Better forget classic multitasking for microcontrollers unless you really have nothing better to do with your time.

Most microcontrollers have different interrupt sources, often including timers. So, the main loop runs one task continuously, and when the timer counts to zero, interrupt is issued. The main loop is stopped and execution jumps to an address known as 'interrupt vector'.


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There, a different procedure is launched, performing a different one-off task. Once that finishes possibly resetting the timer if need be , you return from the interrupt and main loop is resumed. Microcontrollers often have a few timers, and you can assign one task per timer, not to mention tasks on other, external interrupts say, keyboard input - key pressed, or data arriving over RS While this approach is very limited, it really suffices for great most cases; specifically yours: set up the timer to cycle 1s, on interrupt calculate the new hour, change display, then leave the interrupt.

In main loop wait for date to reach birthday, and when it does start playing the music and blinking the LEDs. This is how it was done in the early days. You need to write your 'tasks' as subroutines, each with a finite state machine or a single pass of a loop inside, and the "OS" is a simple loop of jumps to consecutive tasks, in sequence. After each jump the MPU starts executing given task, and will continue until the task returns control, after first saving up its state, to recover it when it's started again.

Programming embedded microcontrollers in the C language

Each pass of the task job should be very short. Any delay loops must be replaced with wait states in the finite state engine if the condition is not satisfied, return. If it is, change the state. Writing that way is more difficult, but the solution is more robust. In your case you might have four tasks:.

Clock returns control if no new second arrived. If it did, it recalculates the number of seconds, minutes, hours, date, and then returns. Display updates the displayed values. If you multiplex over the digits on the 8-segment display, each pass will update one digit, next pass - next one etc.

Playing sound will wait yield while it's not birthday. If it's birthday, pick the sample value from memory, output it to speaker, yield. Optionally yield if you were called earlier than you were supposed to output next sound. Very short loops - say, 10 iterations of 5 lines - are still allowed, but anything longer should be transformed into a state of the finite state engine which the process is.

Each task is a procedure that would normally execute infinitely, doing just its own thing. Upon getting started by the interrupt, the OS task must save all current volatile state of the last task - registers, the interrupt return address from which the task should be resumed , current stack pointer, keeping that in a record of that task. Then, using the scheduler algorithm, it picks another process from the list, which should start now; restores all of its state, then overwrites own return-from-interrupt address with the address of where that process left off, when preempted previously.

Upon ending the interrupt normal operation of the preempted process is resumed, until another interrupt which switches control to OS again. As you can see, there's a lot of overhead, with saving and restoring the complete state of the program instead of just what the task needs at the moment, but the program doesn't need to be written as a finite state machine - normal sequential style suffices.

While SF provides an excellent overview of multitasking there is also some additional hardware most microcontrollers have that let them do things simultaneously. Illusion of simultaneous execution - Technically your professor is correct and updating simultaneously cannot be done.

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C and the 8051, Vol.I : Hardware, Modular Programming and Multitasking / Edition 2

However, processors are very fast. For many tasks they can execute sequentially, like updating each 7 segment display one at a time, but it does it so fast that human perception cannot tell that each display was updated sequentially. The same applies to sound. Most audible sound is in the kilohertz range while processors run in the megahertz range. The processor has plenty of time to play part of a sound, do something else, then return to playing a sound without your ear being able to detect the difference. Interrupts - SF covered the execution of interrupts well so I'll gloss over the mechanics and talk more about hardware.

Most micro controllers have small hardware modules that operate simultaneously with instruction execution. When a given module completes its task it notifies the processor and the processor jumps to the interrupt code for the module. This mechanism allows you to do things like transmit a byte over uart which is relatively slow while executing instructions. PWM - PWM Pulse Width Modulation is a hardware module that essentially generates a square wave, two at a time, but the squares don't have to be even I am simplifying here.

A SystemC Model for RTOS Kernel

One could be longer than the other, or they could be the same size. You configure in hardware the size of the squares and then the PWM generates them continuously. This module can be used to drive motors or even generate sound, where the speed of the motor or the frequency of sound depends on the ratio of the two squares. To play music, a processor would only need to change the ratio when it is time for the note to change perhaps based on a timer interrupt and it can execute other instructions in the meantime.

Something like an ADC might continuously write a converted value to a specific register in memory. A DMA controller can be configured to read continuously from one address the ADC output while writing sequentially to a range of memory like the buffer to receive multiple ADC conversions before averaging.

All of this happens in hardware while the main processor executes instructions. Meanwhile, most micro-controllers have additional hardware that accomplishes specific tasks simultaneously with program execution.

C and the | Programming embedded microcontrollers in the C language

The answers by Zack and SF. But sometimes a working example is valuable. While I could glibly suggest browsing the source kit to the Linux kernel which is both open source and provides multitasking even on single-core machines , that is not the best place to start for an understanding of how to actually implement a scheduler.

A much better place to start is with the source kit to one of the hundreds if not thousands of real time operating systems. Many of these are open source, and most can run even on extremely small processors, including the With those names as a starting point along with keywords like "RTOS" Google will reward you with names of many others. This is a commercial product that started life as an educational exercise for readers of Embedded Systems Design magazine.


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  6. The magazine articles and their attached source code became the subject of one of the better books on the subject. The OS is open source, but does carry license restrictions on commercial use. Since it was originally written as an educational tool, the source code is well documented. The text book as of the 2nd edition on my shelf here somewhere, at least is well written as well, and goes into a lot of theoretical background on each of the features it supports.

    I successfully used it in several product development projects, and would considering it again for a project that needs multitasking but does not need to carry the weight of a full OS like Linux. It is written entirely in a subset of standard C meeting almost all requirements of the the MISRA-C guidelines which helped make it possible for it to it to receive a variety of safety critical certifications.

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