Multi-Tasking and Real-Time Operating Systems
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1. Define which timer to use and minor_cycle for RTOS use rtos timer 0 minor_cycle 10ms II Declare TASK 1 called every 250ms task rate 250ms max 10ms void task_BO output_toggle PIN_BO Toggle RBO II Declare TASK 2 called every 500ms II task rate 500ms max 10ms void task _B1 output_toggle PIN_B1 Toggle RB1 Declare TASK 3 called every second Figure 10 8 Program listing of the project Multi Tasking and Real Time Operating Systems 527 II task rate 1s max 10ms void task_B2 output_toggle PIN_B2 Toggle RB2 Declare TASK 4 called every 2 seconds task rate 2s max 10ms void task_B3 output_toggle PIN_B3 Toggle RB3 Start of MAIN program void main set_tris_b 0 Configure PORTB as outputs rtos_run Start RTOS Figure 10 8 Cont d a 4MHz clock Then the RTOS timer is declared as Timer 0 and minor_cycle time is declared as 10ms using the preprocessor command use rtos The program consists of four similar tasks e task_BO flashes the LED connected to RBO at a rate of 250ms Thus the LED is ON for 250ms then OFF for 250ms and so on CCS statement output _ toggle is used to change the state of the LED every time the task is called In the CCS compiler PIN BO refers to port pin RBO of the microcontroller e task_BI flashes the LED connected to RB1 at a rate of 500ms as described2. set_tris_d 0 PORTD all outputs output_d 0 Clear PORTD set_tris_a OxFF PORTA all inputs setup_adc_ports ALL_ANALOG A D ports setup_adc ADC_CLOCK_DIV_32 A D clock set_adc_channel 0 Select channel 0 ANO delay_us 10 sem 1 Semaphore is 1 rtos_run Start RTOS Figure 10 16 Cont d Multi Tasking and Real Time Operating Systems 539 program RTOS4 C is given in Figure 10 16 The operation of the new program is as follows The semaphore variable sem is set to 1 at the beginning of the program Task Get_voltage decrements the semaphore calls rtos_wait variable so that task To_RS232 is blocked semaphore variable sem 0 and cannot send data to the PC When a new A D sample is ready the semaphore variable is incremented calls rtos_signal and task To_RS232 can continue Task To_RS232 then sends the measured voltage to the PC and increments the semaphore variable to indicate that it had access to the data Task Get_voltage can then get a new sample This process is repeated forever
3. e Preemptive scheduling Multi Tasking and Real Time Operating Systems 519 Cooperative scheduling is perhaps the simplest scheduling algorithm available Each task runs until it is complete and gives up the CPU voluntarily Cooperative scheduling cannot satisfy real time system needs since it cannot support the prioritization of tasks according to importance Also a single task may use the CPU too long leaving too little time for other tasks And the scheduler has no control of the various tasks execution time A state machine construct is a simple form of a cooperative scheduling technique In round robin scheduling each task is assigned an equal share of CPU time see Figure 10 4 A counter tracks the time slice for each task When one task s time slice completes the counter is cleared and the task is placed at the end of the cycle Newly added tasks are placed at the end of the cycle with their counters cleared to 0 This like cooperative scheduling is not very useful in a real time system since very often some tasks take only a few milliseconds while others require hundreds of milliseconds or more TASK 1 TASK 2 TASK3 TASK1 TASK 2 Figure 10 4 Round robin scheduling Preemptive scheduling is considered a real time scheduling algorithm It is priority based and each task is given a priority see Figure 10 5 The task with the highest priority gets the CPU time Real time
4. 10 9 Block diagram of the project Multi Tasking and Real Time Operating Systems 529 Push button Switch 4 Figure 10 10 Circuit diagram of the project The program listing of the project RTOS2 C is given in Figure 10 11 The main part of the program is in the later portion and it configures PORTB pins as outputs Also bit 0 of PORTD is configured as input and other pins of PORTD are configured as outputs Timer 0 is used as the RTOS timer and the minor_cycle is set to 1s The program consists of three tasks Task Live runs every 200ms and flashes the LED connected to port pin RD7 This LED indicates that the system is working Task Generator runs every millisecond and increments a byte variable called count continuously When the push button switch is pressed pin 0 of PORTD RDO goes to logic 0 When this happens the current value of count is sent to task Display using RTOS function call rtos_msg_send display count where 530 Chapter 10 TITTLE SIMPLE RTOS EXAMPLE RANDOM NUMBER GENERATOR This is a simple RTOS example 8 LEDs are connected to PORTB of a PIC18F452 microcontroller Also a push button switch is connected to port RCO of PORTC and an LED is connected to port RC7 of the microcontroller The push button switch is normally at logic 1 The program consists of 3 tasks called Generator Display and Live Task Generator runs in a loop and increments a
5. a second e Task 4 called task_B3 flashes the LED connected to port RB3 once every two seconds Figure 10 7 shows the circuit diagram of the project A 4MHz crystal is used as the clock PORTB pins RBO RB3 are connected to the LEDs through current limiting resistors Figure 10 7 Circuit diagram of the project The software is based on the CCS C compiler and the program listing RTOS1 C is given in Figure 10 8 The main program is at the end of the program and inside the main program PORTB pins are declared as outputs and RTOS is started by calling function rtos_run The file that contains CCS RTOS declarations should be included at the beginning of the program The preprocessor command use delay tells the compiler that we are using 526 Chapter 10 ITLL LTT II SIMPLE RTOS EXAMPLE This is a simple RTOS example 4 LEDs are connected to lower half of PORTB of a PIC18F452 microcontroller The program consists of 4 tasks Task task_BO flashes the LED connected to port RBO every 250ms Task task_B1 flashes the LED connected to port RB1 every 500ms Task task_B2 flashes the LED connected to port RB2 every second Task task_B3 flashes the LED connected to port RB3 every 2 seconds The microcontroller is operated from a 4MHz crystal Programmer Dogan Ibrahim Date September 2007 File RTOS1 C II TTT include C NEWNES PROGRAMS rtos h use delay clock 4000000
6. but tasks in a multi tasking application do not have any arguments and do not return any values Before a task is declared a task preprocessor command is needed to specify the task options The format of this preprocessor command is task rate n max m queue p where rate specifies how often the task should be called The number specified must be followed by s ms us or ns max specifies how much processor time a task will use in one execution of the task The time specifed here must be equal to or less than the time specified by minor_cycle queue is optional and if present specifies the number of bytes to be reserved for the task to receive messages from other tasks The default value is 0 In the following example a task called my_ticks is every 20ms and is expected to use no more than 100ms of processor time This task is specified with no queue option task rate 20ms max 100ms void my ticks PROJECT 10 1 LEDs In the following simple RTOS based project four LEDs are connected to the lower half of PORTB of a PIC18F452 type microcontroller The software consists of four tasks where each task flashes an LED at a different rate e Task 1 called task_BO flashes the LED connected to port RBO at a rate of 250ms e Task 2 called task_B1 flashes the LED connected to port RB1 at a rate of 500ms Multi Tasking and Real Time Operating Systems 525 e Task 3 called task_B2 flashes the LED connected to port RB2 once
7. manual The CCS language provides the following RTOS functions in addition to the normal C functions rtos_run initiates the operation of RTOS All task control operations are implemented after calling this function rtos_terminate terminates the operation of RTOS Control returns to the original program without RTOS In fact this function is like a return from rtos_run rtos_enable receives the name of a task as an argument The function enables the task so function rtos_run can call the task when its time is due Multi Tasking and Real Time Operating Systems 523 rtos_disable receives the name of a task as an argument The function disables the task so it can no longer be called by rtos_run unless it is re enabled by calling rtos_enable rtos_ yield when called from within a task returns control to the dispatcher All tasks should call this function to release the processor so other tasks can utilize the processor time rtos_msg_send receives a task name and a byte as arguments The function sends the byte to the specified task where it is placed in the task s message queue rtos_msg_read reads the byte located in the task s message queue rtos_msg_poll returns true if there is data in the task s message queue This function should be called before reading a byte from the task s message queue rtos_signal receives a semaphore name and increments that semaphore rtos_wait receives a sem
8. runs every 200ms and it flashes the LED conencted to port pin RD7 of the microcontroller to indicate that the program is running and is ready to measure voltages task Get_voltage reads analog voltage from port ANO and then converts the voltage into millivolts and stores in a variable called Volts II Task To_RS232 gets the measured voltage converts it into a character array and then sends to the PC over the RS232 serial line The serial line is configured to operate at 2400 Baud higher Baud rates can also be used if desired II Programmer Dogan Ibrahim Date September 2007 I File RTOS3 C MUTT include lt 18F8520 h gt device adc 10 use delay clock 10000000 use rs232 baud 2400 xmit PIN_C6 rcv PIN_C7 unsigned int16 adc_value unsigned int32 Volts Define which timer to use and minor_cycle for RTOS II use rtos timer 0 minor_cycle 100ms Declare TASK Live called every 200ms Figure 10 14 Program listing of the project Multi Tasking and Real Time Operating Systems 535 This task flashes the LED on port RD7 II task rate 200ms max 1ms void Live output_toggle PIN_D7 Declare TASK Get_voltage called every 10ms task rate 2s max 100ms void Get_voltage adc_value read_adc Volts unsigned int82 adc_value 5000 Volts Volts 1024 Declare TASK To_RS232 called every millisec
9. systems generally support priority levels ranging from 0 to 255 where 0 is the highest priority and 255 is the lowest Priority TASK 3 Figure 10 5 Preemptive scheduling 520 Chapter 10 In some real time systems where more than one task can be at the same priority level preemptive scheduling is mixed with round robin scheduling In such cases tasks at higher priority levels run before lower priority ones and tasks at the same priority level run by round robin scheduling If a task is preempted by a higher priority task its run time counter is saved and then restored when it regains control of the CPU In some systems a strict real time priority class is defined where tasks above this class may run to completion or run until a resource is not available even if there are other tasks at the same priority level In a real time system a task can be in any one of the following states see Figure 10 6 e Ready to run e Running e Blocked When a task is first created it is usually ready to run and is entered in the task list From this state subject to the scheduling algorithm the task can become a running task According to the conditions of preemptive scheduling the task will run if it is the highest priority task in the system and is not waiting for a resource A running task becomes a blocked task if it needs a resource that is not available For example a task may need data from an A D converter and is blocked unti
10. these services are e Semaphores e Event flags 522 Chapter 10 e Mailboxes e Pipes e Message queues Semaphores are used to synchronize access to shared resources such as common data areas Event flags are used to synchronize the intertask activities Mailboxes pipes and message queues are used to send messages among tasks 10 5 CCS PIC C Compiler RTOS The CCS PIC C compiler is one of the popular C compilers for the PIC16 and PIC18 series of microcontrollers In addition to their PIC compilers Customer Computer Services offers PIC in circuit emulators simulators microcontroller programmers and various development kits The syntax of the CCS C language is slightly different from that of the mikroC language but readers who are familiar with mikroC should find CCS C easy to use CCS C supports a rudimentary multi tasking cooperative RTOS for the PIC18 series of microcontrollers that uses their PCW and PCWH compilers This RTOS allows a PIC microcontroller to run tasks without using interrupts When a task is scheduled to run control of the processor is given to that task When the task is complete or does not need the processor any more control returns to a dispatch function which gives control of the processor to the next scheduled task Because the RTOS does not use interrupts and is not preemptive the user must make sure that a task does not run forever Further details about the RTOS are available in the compiler s user
11. Multi Tasking and Real Time Operating Systems Nearly all microcontroller based systems perform more than one activity For example a temperature monitoring system is made up of three tasks that normally repeat after a short delay namely e Task 1 Reads the temperature e Task 2 Formats the temperature e Task 3 Displays the temperature More complex systems may have many complex tasks In a multi tasking system numerous tasks require CPU time and since there is only one CPU some form of organization and coordination is needed so each task has the CPU time it needs In practice each task takes a very brief amount of time so it seems as if all the tasks are executing in parallel and simultaneously Almost all microcontroller based systems work in real time A real time system is a time responsive system that can respond to its environment in the shortest possible time Real time does not necessarily mean the microcontroller should operate at high speed What is important in a real time system is a fast response time although high speed can help For example a real time microcontroller based system with various external switches is expected to respond immediately when a switch is activated or some other event occurs A real time operating system RTOS is a piece of code usually called the kernel that controls task allocation when the microcontroller is operating in a multi tasking 516 Chapter 10 environment RTOS decides for inst
12. ance which task to run next how to coordinate the task priorities and how to pass data and messages among tasks This chapter explores the basic principles of multi tasking embedded systems and gives examples of an RTOS used in simple projects Multi tasking code and RTOS are complex and wide topics and this chapter describes the concepts pertaining to these tools only briefly Interested readers should refer to the many books and papers available on operating systems multi tasking systems and RTOS There are several commercially available RTOS systems for PIC microcontrollers At the time of writing mikroC language did not provide a built in RTOS Two popular high level RTOS systems for PIC microcontrollers are Salvo www pumpkin com which can be used from a Hi Tech PIC C compiler and the CCS Customer Computer Services built in RTOS system In this chapter the example RTOS projects are based on the CCS www ccsinfo com compiler one of the popular PIC C compilers developed for the PIC16 and PIC18 series of microcontrollers 10 1 State Machines State machines are simple constructs used to perform several activities usually in a sequence Many real life systems fall into this category For example the operation of a washing machine or a dishwasher is easily described with a state machine construct Perhaps the simplest method of implementing a state machine construct in C is to use a switch case statement For example our temperat
13. aphore The program given in Figure 10 14 is working and displays the measured voltage on the PC screen This program can be improved slightly by using a semaphore to synchronize the display of the measured voltage with the A D samples The modified Multi Tasking and Real Time Operating Systems 537 TLL TLL SIMPLE RTOS EXAMPLE VOLTMETER WITH RS232 OUTPUT This is a simple RTOS example Analog voltage to be measured between OV and 5V is connected to analog input ANO of a PIC18F8520 type microcontroller The microcontroller is operated from a 10MHz crystal In addition an LED is connected to port in RD7 of the microcontroller RS232 serial output of the mirocontroller RC6 is connected to a MAX232 type RS232 voltage level converter chip The output of this chip can be connected to the serial input of a PC e g COM1 so that the measured voltage can be seen on the PC screen The program consists of 3 tasks called live Get_voltage and To_RS232 Task Live runs every 200ms and it flashes the LED connected to port RD7 of the microcontroller to indicate that the program is running and is ready to measure voltages task Get_voltage reads analog voltage from port ANO and then converts the voltage into millivolts and stores in a variable called Volts Task To_RS232 gets the measured voltage and then sends to the PC over the RS232 serial line The se
14. aphore name and waits for the resource associated with the semaphore to become available The semaphore count is then decremented so the task can claim the resource rtos_await receives an expression as an argument and the task waits until the expression evaluates to true rtos_overrun receives a task name as an argument and the function returns true if that task has overrun its allocated time rtos_stats returns the specified statistics about a specified task The statistics can be the minimum and maximum task run times and the total task run time The task name and the type of statistics are specified as arguments to the function 10 5 1 Preparing for RTOS In addition to the preceding functions the use rtos preprocessor command must be specified at the beginning of the program before calling any of the RTOS functions The format of this preprocessor command is use rtos timer n minor cycle m where timer is between 0 and 4 and specifies the processor timer that will be used by the RTOS and minor_cycle is the longest time any task will run The number entered here must be followed by s ms us or ns 524 Chapter 10 In addition a statistics option can be specified after the minor_cycle option in which case the compiler will keep track of the minimum and maximum processor times the task uses at each call and the task s total time used 10 5 2 Declaring a Task A task is declared just like any other C function
15. counter from 0 to 255 This task also checks the state of the push button switch When the push button switch is pressed the task sends the value of the count to the Display task using messaging passing mechanism The Display task receives the value of count and displays it on the PORTB LEDs Task Live flashes the LED connected to port RC7 at a rate of 250ms This task is used to indicate that the system is working and is ready for the user to press the push button The microcontroller is operated from a 4MHz crystal II Programmer Dogan Ibrahim Date September 2007 File RTOS2 C TIT include C NEWNES PROGRAMS rtos h use delay clock 4000000 int count Define which timer to use and minor_cycle for RTOS use rtos timer 0 minor_cycle 1ms Declare TASK Live called every 200ms This task flashes the LED on port RC7 task rate 200ms max 1ms void Live output_toggle PIN_D7 II Figure 10 11 Program listing of the project Multi Tasking and Real Time Operating Systems 531 Declare TASK Display called every 10ms task rate 10ms max 1ms queue 1 void Display if rtos_msg_poll gt 0 Is there a message output_b rtos_msg_read Send to PORTB j Declare TASK Generator called every millisecond task rate 1ms max 1ms void Generator count Increment count if inpu
16. e task_B2 flashes the LED connected to RB2 every second as described e Finally task_B3 flashes the LED connected to RB3 every two seconds as described 528 Chapter 10 The program given in Figure 10 8 is a multi tasking program where the LEDs flash independently of each other and concurrently PROJECT 10 2 Random Number Generator In this slightly more complex RTOS project a random number between 0 and 255 is generated Eight LEDs are connected to PORTB of a PIC18F452 microcontroller In addition a push button switch is connected to bit 0 of PORTD RDO and an LED is connected to bit 7 of PORTD RD7 Three tasks are used in this project Live Generator and Display e Task Live runs every 200ms and flashes the LED on port pin RD7 to indicate that the system is working e Task Generator increments a variable from 0 to 255 continuously and checks the status of the push button switch When the push button switch is pressed the value of the current count is sent to task Display using a messaging queue e Task Display reads the number from the message queue and sends the received byte to the LEDs connected to PORTB Thus the LEDs display a random pattern every time the push button is pressed Figure 10 9 shows the project s block diagram The circuit diagram is given in Figure 10 10 The microcontroller is operated from a 4MHz crystal LIVE flashes every 200ms PIC 18F452 Push button switch Figure
17. l it is Resource available but not highest priority Highest priority Not the highest priority Unblocked and highest priority Blocked Resource not available Figure 10 6 Task states Multi Tasking and Real Time Operating Systems 521 available Once the resource can be accessed the blocked task becomes a running task if it is the highest priority task in the system otherwise it moves to the ready state Only a running task can be blocked A ready task cannot be blocked When a task moves from one state to another the processor saves the running task s context in memory loads the new task s context from memory and then executes the new instructions as required The kernel usually provides an interface to manipulate task operations Typical task operations are e Creating a task e Deleting a task e Changing the priority of a task e Changing the state of a task 10 3 RTOS Services RTOS services are utilities provided by the kernel that help developers create real time tasks efficiently For example a task can use time services to obtain the current date and time Some of these services are e Interrupt handling services e Time services e Device management services e Memory management services e Input output services 10 4 Synchronization and Messaging Tools Synchronization and messaging tools are kernel constructs that help developers create real time applications Some of
18. nfigured the A D clock is set and the A D channel 0 is selected AN0 The RTOS is then started by calling function rtos_run The program consists of three tasks e Task Live runs every 200ms and flashes an LED connected to port pin RD7 of the microcontroller to indicate that the project is working e Task Get_voltage reads the analog voltage from channel 0 pin RAO or ANO of the microcontroller The value is then converted into millivolts by multiplying by 5000 and dividing by 1024 in a 10 bit A D there are 1024 quantization levels and when working with a reference voltage of 5V each quantization level corresponds to 5000 1024mV The voltage is stored in a global variable called Volts 534 Chapter 10 ITLL i SIMPLE RTOS EXAMPLE VOLTMETER WITH RS232 OUTPUT This is a simple RTOS example Analog voltage to be measured between OV and 5V is connected to analog input ANO of a PIC18F8520 type microcontroller The microcontroller is operated from a 10MHz crystal In addition an LED is connected to port in RD7 of the microcontroller II RS232 serial output of the mirocontroller RC6 is connected to a MAX232 type RS232 voltage level converter chip The output of this chip can be connected to the serial input of a PC e g COM1 so that the measured voltage can be seen on the PC screen The program consists of 3 tasks called live Get_voltage and To_RS232 Task Live
19. o_RS232 reads the formatted voltage and sends it over the RS232 line to a PC every second Figure 10 12 shows the block diagram of the project The circuit diagram is given in Figure 10 13 A PIC18F8520 type microcontroller with a 1OMHz crystal is used in this project though any PIC18F series microcontroller can be used The voltage to be measured is connected to analog port ANO of the microcontroller The RS232 TX output of the microcontroller RC6 is connected to a MAX232 type RS232 level converter chip and then to the serial input of a PC e g COM1 using a 9 pin D type connector Port pin RD7 is connected to an LED to indicate whether the project is working Live LED PIC 18F8520 Voltage to be measured Figure 10 12 Block diagram of the project The program listing RTOS3 C of the project is given in Figure 10 14 At the beginning of the program the A D is defined as 10 bits the clock is defined as 1OMHz and the RS232 speed is defined as 2400 baud The RTOS timer and the minor_cycle are then defined using the use rtos preprocessor command Multi Tasking and Real Time Operating Systems 533 Voltage To be Measured a RC6 i PIC 18F8520 OSC1 OSC2 i 10MHz a Figure 10 13 Circuit diagram of the project In the main part of the program PORTD is configured as output and all PORTD pins are cleared Then PORTA is configured as input RAO is the analog input the microcontroller s analog inputs are co
20. ond task rate 2s max 100ms void To_RS232 printf Measured Voltage LumV n r Volts Start of MAIN program void main set_tris_d 0 output_d 0 set_tris_a OxFF setup_adc_ports ALL_ANALOG setup_adc ADC_CLOCK_DIV_32 set_adc_channel 0 delay_us 10 rtos_run Figure 10 14 Cont d II Toggle RD7 LED Read A D value Noltage in mV send to RS232 PORTD all outputs Clear PORTD PORTA all inputs A D ports A D clock Select channel 0 ANO Start RTOS 536 Chapter 10 e Task To_RS232 reads the measured voltage from common variable Volts and sends it to the RS232 port using the C printf statement The result is sent in the following format Measured voltage nnnn mV The HyperTerminal program is run on the PC to get an output from the program A typical screen output is shown in Figure 10 15 E Hyperterminal Gle Edit View Call Transfer Help Ose 23 04 f 4946mV 4936mV 4912mV 4931mV 4931mV 4809mV 4926mV 4931mV 4951mV 4926mV 4907mV 4897mV 4892mV 2353mV mV OmV 4995mV 4995mV 4995mV 4912mV 4936mV 4838mV 4887mV Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage Voltage NNNUNN Auto detect 2400 8 N 1 Figure 10 15 Typical output from the program Using a Sem
21. rial line is configured to operate at 2400 Baud higher Baud rates can also be used if desired In this modified program a semaphore is used to synchronize the display of the measured value with the A D samples II Programmer Dogan Ibrahim Date September 2007 File RTOS4 C II TLL include lt 18F8520 h gt device adc 10 use delay clock 10000000 use 1s232 baud 2400 xmit PIN_C6 rcv PIN_C7 unsigned int16 adc_value unsigned int32 Volts int8 sem Define which timer to use and minor_cycle for RTOS use rtos timer 0 minor_cycle 100ms Figure 10 16 Modified program listing eae ontinue Declare TASK Live called every 200ms This task flashes the LED on port RD7 task rate 200ms max 1ms void Live output_toggle PIN_D7 Toggle RD7 LED Declare TASK Get_voltage called every 10ms task rate 2s max 100ms void Get_voltage rtos_wait sem decrement semaphore adc_value read_adc Read A D value Volts unsigned int32 adc_value 5000 Volts Volts 1024 Noltage in mV rtos_signal sem increment semaphore Declare TASK To_RS232 called every millisecond task rate 2s max 100ms void To_RS232 rtos_wait sem Decrement semaphore printf Measured Voltage LumV n r Volts Send to RS232 rtos_signal sem Increment semaphore Start of MAIN program void main
22. t PIN_DO 0 Switch pressed rtos_msg_send Display count send a message I Start of MAIN program void main set_tris_b 0 Configure PORTB as outputs set_tris_d 1 RDO input RD7 output rtos_run Start RTOS Figure 10 11 Cont d Display is the name of the task where the message is sent and count is the byte sent Task Display runs every 10ms This task checks whether there is a message in the queue If so the message is extracted using RTOS function call rtos_msg_read and the read byte is sent to the LEDs connected to PORTB Thus the LEDs display the binary value of count as the switch is pressed The message queue should be checked by using function rtos_msg_poll as trying to read the queue without any bytes in the queue may freeze the program 532 Chapter 10 PROJECT 10 3 Voltmeter with RS232 Serial Output In this RTOS project which is more complex than the preceding ones the voltage is read using an A D converter and then sent over the serial port to a PC The project consists of three tasks Live Get_voltage and To_RS232 e Task Live runs every 200ms and flashes an LED connected to port RD7 of the microcontroller to indicate that the system is working e Task Get_voltage reads channel 0 of the A D converter where the voltage to be measured is connected The read value is formatted and then stored in a variable This task runs every two seconds e Task T
23. ure monitoring system has three tasks named Task 1 Task 2 and Task 3 as shown in Figure 10 1 The state machine implementation of the three tasks using switch case statements is shown in Figure 10 2 The starting state is 1 and each task increments the state number by one to select the next state to be executed The last state selects state 1 and there is a delay at the end of the switch case statement The state machine construct is executed continuously inside an endless for loop Figure 10 1 State machine implementation Multi Tasking and Real Time Operating Systems 517 for state 1 switch state CASE 1 CASE 2 CASE 3 Delay_ms n implement TASK 1 state break implement TASK 2 state break implement TASK 3 state 1 break Figure 10 2 State machine implementation in C In many applications the states need not be executed in sequence Rather the next state is selected by the present state either directly or based on some condition This is shown in Figure 10 3 for state 1 switch state CASE 1 CASE 2 CASE 3 Delay_ms n implement TASK 1 state 2 break implement TASK 2 state 3 break implement TASK 3 state 1 break Figure 10 3 Selecting the next state from the current state 518 Chapter 10 State machines although easy to implement are primitive and have limited application They can only be
24. used in systems which are not truly responsive where the task activities are well defined and the tasks are not prioritized Moreover some tasks may be more important than others We may want some tasks to run whenever they become eligible For example in a manufacturing plant a task that sets off an alarm when the temperature is too hot must be run This kind of implementation of tasks requires a sophisticated system like RTOS 10 2 The Real Time Operating System RTOS Real time operating systems are built around a multi tasking kernel which controls the allocation of time slices to tasks A time slice is the period of time a given task has for execution before it is stopped and replaced by another task This process also known as context switching repeats continuously When context switching occurs the executing task is stopped the processor registers are saved in memory the processor registers of the next available task are loaded into the CPU and the new task begins execution An RTOS also provides task to task message passing synchronization of tasks and allocation of shared resources to tasks The basic parts of an RTOS are e Scheduler e RTOS services e Synchronization and messaging tools 10 2 1 The Scheduler A scheduler is at the heart of every RTOS as it provides the algorithms to select the tasks for execution Three of the more common scheduling algorithms are e Cooperative scheduling e Round robin scheduling
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