Multi-Tasking and Real-Time Operating Systems - HCMUT
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1. if rtos_msg_poll gt 0 ls there a message output_b rtos_msg_read Send to PORTB II Declare TASK Generator called every millisecond task rate 1ms max 1ms void Generator count Increment count if input PIN_DO 0 Switch pressed rtos_msg_send Display count send a message 2 19 2012 31 TROS2 C 4 4 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 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_RS23z2 Task Live runs every 200ms and flashes an LED connected to port RD7 of the microcontroller to indicate that the system is working 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 ina variable This task runs every two seconds Task To_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 P2. inti6 min minimum tick time used inti6 max maximum tick time used inti6 hns us ticks hns 10 rtos_stats RTOS Application Ideas User 1 O Communication Protocols 23 2 19 2012 10 5 1 Preparing for RTOS e 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 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 e A task is declared just like any other C function 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 e The format of this preprocessor command is task rate n max m queue p 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
3. 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 m ____RTO83 C 1 4 34 RTOS3 C 2 4 RTOS3 C 3 4 2 19 2012 35 2 19 2012 RTOS3 C 4 4 Declare TASK To_RS232 called every millisecond task rate 2s max 100ms void To_RS232 printf Measured Voltage LumV n r Volts send to RS232 I I Start of MAIN program I void main set_tris_d Q 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 O ANO delay_us 10 rtos_run Start RTOS E il HyperTerminal Fie Edit View Cal Transfer Help Da 00 g 4946mV 4936mV 4912mV 4991 mV 4931mV 4809mV 4926mV Measured Voltage Measured Voltage Measured Voltage Measured Voltage d Voltage Vol tage Voltage Voltage Voltage Vol tage Voltage Voltage Voltage Voltage Voltage Voltage Yal tage Voltage Voltage Yol tage Voltage Voltage Vol tage E a a a a a 488 7mV Connected 02 06 18 Auto detect 2400 amp N 1 Figure 10 15 Typical output from the program 36 2 19 2012 Using a Semaphore e The program given in Figure 10 14 is working and displays the measured voltage on
4. 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 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 17 2 19 2012 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 semaphore name and waits for the resou
5. systems Cons e Not efficient in overall usage of CPU processing e Does not provide optimal response time 2 19 2012 2 19 2012 Multitasking e Round Robin Systems Several processes execute sequentially to completion Often in conjunction with a cyclic executive Each task is assigned a fixed time slice Fixed rate clock initiates an interrupt at a rate corresponding to the time slice e Task executes until it completes or its execution time expires e Context saved if task does not complete Multitasking e Round Robin Systems Time Slicing of 3 Tasks 5 Y O T a lt Time 2 19 2012 Multitasking e Pre emptive Priority Systems Higher priority task can preempt a lower priority task if it interrupts the lower priority task Priorities assigned to each interrupt are based upon the urgency of the task associated with the interrupt Priorities can be fixed or dynamic Multitasking e Round Robin Systems Preemptive Scheduling of 3 Tasks Medium Priority Time 2 19 2012 Multitasking e Preemptive Priority Systems An Example Aircraft Navigation System e High priority task Task that gathers accelerometer data every 5 msec e Medium priority task Task that collects gyro data and compensates this data and the accelerometer data every 40 msec e Low priority task Display update Built in Test BIT Multitasking e Multitasking is not perfect High priority tasks ho
6. 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 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 TaskTo_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 13 RTOS C 1 4 FETE CECT LTD TELE EL CELLED EEL ETT LEE LETT GG ETT TELE EET TET LET TET EE i SIMPLE RTOS EXAMPLE VOLTMETER WITH RS232 OUTPUT i This is a simple RTOS example Analog voltage to be measured between OV and 5 is connected to analog input ANO of a PIC18F8520 type H microcontroller The microcontroller is operated from a 10MHz crystal In addition an LED is connected to port in RDF of the microcontroller i H RS232 serial output of the mirocontrollar RC6 is connected to a MAX232 type R5232 voltage level converter chip The output of this chip can be i connected to the serial input cf a PC e
7. 2 19 2012 HCM IU Subject ERTS Instructor Ho Trung My Multi Tasking and Real Time Operating Systems Ref Dogan Ibrahim Outline 10 1 State Machines 10 2 The Real Time Operating System RTOS 10 2 1 The Scheduler 10 3 RTOS Services 10 4 Synchronization and Messaging Tools 10 5 CCS PIC C Compiler RTOS 10 5 1 Preparing for RTOS 10 5 2 Declaring a Task PROJECT 10 1 LEDs PROJECT 10 2 Random Number Generator PROJECT 10 3 Voltmeter with RS232 Serial Output 2 19 2012 Multitasking e 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 Task 1 Reads the temperature Task 2 Formats the temperature 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 Multitasking e What is Multitasking Separate tasks share one processor or processors Each task executes within its own context e Owns processor e Sees its own variables e May be interrupted Tasks may interact to execute as a whole program 2 19 2012 M
8. IC18F 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 2 19 2012 32 Figure 10 12 Block diagram of the project Figure 10 13 Circuit diagram of the project 2 19 2012 33 2 19 2012 e Inthe 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 microcontrollers analog inputs are configured the A D clock is set and the A D channel 0 is selected ANO The RTOS is then started by calling function rtos_run e The program consists of three tasks Task Live runs every 200ms and flashes an LED connected to port pin RD7 of the microcontroller to indicate that the project is working 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 b5V each quantization level corresponds to 5000 1024mV The voltage is stored in a global variable called Volts
9. RTB 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 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 Programmer Dogan Ibrahim Date September 2007 RTOS2 C 30 TROS2 C 2 4 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 TROS2 C 3 4 Declare TASK Display called every 10ms II task rate 10ms max 1ms queue 1 void Display
10. e called every 1Gms r task rate 2s max 100ms void Get_voltage rtos_wail sam H decrement semaphore adc_value reacd_ade H Raad A D value Volts unsigned int32 adc_value 5000 Volts Volts 1024 H Voltage in mV rtos_siqnal sem increment semaphore 2 19 2012 38 RTOS C 4 4 II Declare TASK To_RS232 called every millisecond II task rate 2s max 100ms void To_RS232 rtos_wait sem printf Measured Voltage LumV n r Volts rtos_signal sem Start of MAIN program void main set_tris_d 0 output_d 0 set_tris_a OxFF setup_adc_ports ALL_ANALOG setup_ade ADC_CLOCK_DIV_32 set_adc_channel 0 delay_us 10 sem 1 rtos_run Decrement semaphore Send to RS232 Increment semaphore PORTD all outputs Clear PORTD PORTA all inputs A D ports A D clock Select channel 0 ANO Semaphore ts 1 Start RTOS 2 19 2012 39
11. econds I task rate 2s max 10ms void task_B3 output_toggle PIN_B3 Toggle RB3 II Start of MAIN program void main set_tris_b 0 Configure PORTB as outputs rtos_run Start RTOS PROJECT 10 2 Random Number Generator e 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 O of PORTD RDO and an LED is connected to bit 7 of PORTD RD7 e Three tasks are used in this project Live Generator and Display Task Live runs every 200ms and flashes the LED on port pin RD7 to indicate that the system is working 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 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 e 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 28 Figure 10 9 Block diagram of the project Figure 10 10 Circuit diagram of the project 2 19 2012 29 2 19 2012 The pro
12. ep RTOS Task Control rtos enable task rtos_disable task Dynamic task control Enable Disable the specified task Task is the function name All tasks are enabled at start 20 2 19 2012 RTOS Messaging rtos msg_send task char sends char to task avail rtos_msg_poll TRUE if a char is waiting for this task byte rtos msg_read Read next char destined for this task RAE EA Slide 53 RTOS Yielding rtos_yield Stops processing current task Returns to this point on next cycle rtos_await expression rtos_yield if expression not TRUE 2 2 19 2012 task rate 100ms max 5ms void Taskinput void if KeyReady rtos_msg_send TaskSystem KeyGet task rate 25ms queue 1 void TaskSystem void SystemPrepare rtos_await rtos_msg_poll SystemDo rtos_msg_read rtos_yield SystemVerify RTOS Semaphores Semaphore Determine shared resource availability A user defined global variable Set to non zero if used Set to zero if free rtos_wait semaphore rtos_yield until semaphore free Once free sets semaphore as used rtos_signal semaphore Pi SB TG ww 22 2 19 2012 RTOS Timing Statistics overrun rtos_overrun task TRUE if task took longer than max rtos stats task rtos_ stats Get timing statistics for specified task typedef struct int32 total total ticks used by task
13. escribed 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 temperature 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 a k a a A lia S a Sf e SY Figure 10 1 State machine implementation state 1 for switch state CASE 1 implement TASK 1 state break iimplement TASK 2 state break implement TASK 3 state 1 break delay_ms n Figure 10 2 State machine implementation in C 10 2 19 2012 e 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 State machines although easy to implement are primitive and have limited application They can only be used in systems which are not truly responsive where the task activities are well defined and the tasks are not prioritized Moreover some tasks ma
14. 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 24 2 19 2012 e Inthe following example a task called my_ticks is every 20ms and is expected to use no more than 100ms of processor time e This task is specified with no queue option task rate 20ms max 100ms void my_ticks PROJECT 10 1 LEDs e Inthe 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 Task 1 called task_BO flashes the LED connected to port RBO at a rate of 250ms Task 2 called task_B1 flashes the LED connected to port RB1 at a rate of 500ms Task 3 called task_B2 flashes the LED connected to port RB2 once a second 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 25 Figure 10 7 Circuit diagram of the project e The software is based on the CCS C compiler and the program listing RTOS1 C is given in Figure 10 8 The main progra
15. g COM1 so that the measured F voltage can be seen on the PC screen fi i The program consists of 3 tasks called live Get_voltage and To_RS 32 i Task Livs runs every 200ms and it flashes the LED connected to port RD H of the microcontroller to indicate that the program is running and ts ready to i measure voltages i task Get_voltage reads analog voltage from port ANO and then converts the voltage into millivolts and stores in a variable called Volts i 37 RTOS C 2 4 f Task To_RS232 gets the measured voltage and then sends to the PC over the RS232 serial line The serial line is configured to operate at 2400 Baud f higher Baud rates can also be used if desired if Jf In this modified program a semaphore is used to synchronize the display of the measured value with the A D samples It Programmer Dogan Ibrahim f Date September 2007 I I File RTOS4 C It LETTE EE EEE EE 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 int8 sem If l Define which timer to use and minor_cycle for RTOS if use rtos timer 0 minor_cycle 100ms RTOS C 3 4 if Declare TASK Live called every 200ms i This task flashes the LED on port RD fi task rate 200ms max 1ms void Live output_toggle PIN_D7 H Toggle ROY LED if i Declare TASK Get_voltag
16. g resources and starve low priority tasks Low priority tasks share a resource with high priority tasks and block high priority task How does a RTOS deal with some of these issues Rate Monotonic Systems higher execution frequency higher priority Priority Inheritance 2 19 2012 RTOS e 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 environment RTOS decides for instance which task to run next how to coordinate the task priorities and how to pass data and messages among tasks Multitasking Priority Inversion Priority Inheritance Task A and Task C share a resource Task A is High Priority Task C is Low Priority Task A 1s blocked when Task C runs effectively assigning A to C s priority hence Priority Inversion Task A will be blocked for longer if a Ta
17. gram 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 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 59 TROS2 C 1 4 CE UE LLL SIMPLE RTOS EXAMPLE RANDOM NUMBER GENERATOR This is a simple RTOS example 8 LEDs are connected to PO
18. heduling 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 until it is 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 Task operations e The kernel usually provides an interface to manipulate task operations Typical task operations are Creating a task Deleting a task Changing the priority of a task Changing the state of a task 15 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 Interrupt handling services Time services Device management services Memory management services Input output services 10 4 Synchronization and Messaging T
19. ithms to select the tasks for execution Three of the more common scheduling algorithms are Cooperative scheduling Round robin scheduling Preemptive scheduling e 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 12 2 19 2012 Round robin scheduling 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 Figure 10 4 Round robin scheduling Preemptive scheduling Preemptive scheduling is considered a real time scheduling algorithm It is priorityoased and each tas
20. k is given a priority see Figure 10 5 The task with the highest priority gets the CPU time Real time systems generally support priority levels ranging from 0 to 255 where 0 is the highest priority and 255 is the lowest Priority TASK 3 TASK 2 TASK 2 TASK 1 TASK 1 Figure 10 5 Preemptive scheduling 13 2 19 2012 Mixed scheduling 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 Task states e Inareal time system a task can be in any one of the following states Ready to run Running Blocked Resource available Highest but not highest priority priority Not the highest priority Unblocked and highest priority Blocked Resource not available Figure 10 6 Task states 14 2 19 2012 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 sc
21. m 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 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 e The program consists of four similar tasks task_B0O 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 task_B1 flashes the LED connected to RB1 at a rate of 500ms as described task_B2 flashes the LED connected to RB2 every second as described Finally task_B3 flashes the LED connected to RB3 every two seconds as described The program given in Figure 10 8 is a multi tasking program where the LEDs flash independently of each other and concurrently 2 19 2012 26 RTOS1 c 1 8 RTOS1 c 2 3 yy have been corrupted Restart your computer and then 2 19 2012 2 2 19 2012 RTOS1 c 3 3 task rate 1s max 10ms void task_B2 output_toggle PIN_B2 Toggle RB2 Declare TASK 4 called every 2 s
22. ools synchronization and messaging tools are kernel constructs that help developers create real time applications Some of these services are Semaphores Event flags Mailboxes Pipes 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 2 19 2012 16 2 19 2012 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 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 compilers user manual RTOS in CCS C
23. rce 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 35 arguments to the function SEM RTOS Setup use rtos timer X minor_cycle cycle_time imer can be any timer available Minor_Cycle is rate of fastest task Example use rtos timer 1 minor_cycle 50ms 18 2 19 2012 RTOS Tasks task rate xxxx max yyyy queue z Following function is RTOS task Will be called at specitied rate Max is slowest execution time used for budgeting Queue defines RX message size RAE T Slide 49 RTOS Start and Stop rtos_run Starts the RTOS Will not return until rtos_terminate rtos_terminate Stops the RTOS 19 2 19 2012 use rtos timer 1 task rate 100ms max 5ms void Taskinput void get user input task rate 25ms void TaskSystem void x do somestuff void main void while TRUE rtos_run sle
24. sk B of medium priority comes along to keep Task C from finishing A good RTOS would sense this condition and temporarily promote task C to the High Priority of Task A Priority Inheritance 2 19 2012 Multitasking 7 the resource Priority Inversion The problem Po ee a high priority low Kei 5A aiso keeps A waiting high GAK priority time K C inherits A s priority so it can finish and then A can have the resource 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 There are several commercially available RTOS systems for PIC microcontrollers 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 2 19 2012 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 d
25. ultitasking Example Clock interrupt Navigation System Software Navigation Filter Navigation Sensor Tasks Sensors Sensor data pools Shared Memory be ccc mcs memes eco e Navigation Sensor Task retrieves data from sensor Each task can only see its data Clock driven scheduler drives tasks at various frequencies to write sensor data to shared memory e Navigation filter retrieves data at scheduled intervals Post to mailbox Task D Data Transfer _____ Multitasking Context switching When the CPU switches from running one task to running another it 1s said to have switched contexts Save the MINIMUM needed to restore the interrupted process Back to the book example what might be needed name page paragraph word In a Computer System the MINIMUM is often contents of registers contents of the program counter contents of coprocessor registers if applicable memory page registers memory mapped I O special variables During context switching interrupts are often disabled Real Time Systems require minimal times for context switches Multitasking e How do many tasks share the same CPU Cyclic Executive Systems Round Robin Systems Pre emptive Priority Systems Multitasking e Cyclic Executive Calls to statically ordered threads Thread calls OO timer timer Pros e Easy to implement used extensively in complex safety critical
26. y 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 state 1 for switch state CASE 1 implement TASK 1 state 2 break i mplement TASK 2 state 3 break implement TASK 3 state 1 break delay_ms n Figure 10 3 Selecting the next state from the current state 11 2 19 2012 10 2 The Real Time Operating System RTOS e 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 e The basic parts of an RTOS are Scheduler RTOS services Synchronization and messaging tools 10 2 1 The Scheduler e A scheduler is at the heart of every RTOS as it provides the algor
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