FreeRTOS - Common Task Design Patterns in Multi-tasking Applications

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Ltd. Class ID: 9C11L Renesas Electronics America Inc.

1 FreeRTOS - Common Task Design Patterns in Multi-tasking Applications Richard Barry, Founder Real Time Engineers Ltd. Class ID: 9C11L Renesas Electronics America Inc Renesas Electronics America Inc. All rights reserved.

2 Real Time Engineers Ltd. FreeRTOS FreeRTOS+ WITTENSTEIN high integrity systems OpenRTOS SafeRTOS Richard Barry Director, Real Time Engineers Ltd Founder, the FreeRTOS project

3 Working knowledge of the C programming language Appreciation of multi-tasking concepts See how an RTOS can simplify software design Learn how to use task and interrupt prioritisation to ensure determinism See how to use RTOS services to improve maintainability See when an RTOS allows more efficient CPU usage Get confidence in starting an RTOS based design Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd - 3

4 Working knowledge of the C programming language Appreciation of multi-tasking concepts In an hour! Scope is limited See how an RTOS can simplify software design Learn how to use task and interrupt prioritisation to ensure determinism See how to use RTOS services to improve maintainability See when an RTOS allows more efficient CPU usage Get confidence in starting an RTOS based design Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd - 4

5 Real Time Multitasking The RT in RTOS FreeRTOS products and market position A Hypothetical Application A closer look at a real time control task Handling continuous, high frequency and periodic control Putting it all together Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd - 5

6 Real Time Multitasking The RT in RTOS FreeRTOS products and market position In an hour! A Hypothetical Application A closer look at a real time control task Handling continuous, high frequency and periodic control Putting it all together Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd - 6

7 Setting the scene

8 All available tasks appear to be executing... Task1 Task2 Task3 t1 t2 Time tn but only one task is ever executing at any time. Task1 Task2 Task3 t1 t2 Time tn Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd - 8

Deterministic Hard real time it absolutely must Soft real time it should T2 preempts T3 T1 preempts T2 Task1 (high, event) Task2 (med, periodic) Task3

9 Deterministic Hard real time it absolutely must Soft real time it should T2 preempts T3 T1 preempts T2 Task1 (high, event) Task2 (med, periodic) Task3 (low, event) Idle task t1 t2 t3 t4 t5 t6 t7 t8 t9 t11 t10 t12 t13 Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd - 9

Suspended Ready Running Event Blocking API function called Blocked Renesas

in 2010 & 2011 combined Polled top in class for the questions which kernel are you currently using?

12 is everywhere... Professionally developed, quality controlled, robust and supported Downloaded more than 170,000 times in 2010 & 2011 combined Polled top in class for the questions which kernel are you currently using? and which kernel are you most likely to use in your next project in the 2011 and 2012 EETimes Embedded Market Surveys Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

14 Yes Moderated open source Proprietary code remains proprietary FreeRTOS API Application Source Driver Source FreeRTOS Source Middleware Source Closed Source Open Source Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

15 FreeRTOS+ IO Add an open(), read(), write(), ioctl() peripheral interface to your application CLI Enable your application to efficiently process command line input Safety & Certification A pre-certified kernel for microcontrollers, with a similar usage model to FreeRTOS Nabto Exciting new technology that re-defines the web device, making connectivity simpler and safer than ever before RTOS Training Expert instructor led RTOS training to maximise productivity delivered online or on site Trace Get 15 graphically interlinked views of the trace, providing an unprecedented level of insight TCP/IP Pre-built libraries for easy integration of TCP/IP and related protocols into costsensitive applications SSL and TLS State of the art networking security for embedded systems Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

Renesas DevCon October 2012 Copyright 2012

17 Architecting a System

Hard Real Time Soft Real Time Motor commutation Brakes Interlocks Emergency call button Floor indicators Power regeneration Door motors Inter-elevator communication Buttons

19 Hard Real Time Soft Real Time Motor commutation Brakes Interlocks Emergency call button Floor indicators Power regeneration Door motors Inter-elevator communication Buttons in lift Safety functions Emergency stop Audible signals/announcements Buttons on floors Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

20 Motor Control 20kHz PWM Commutation sequencing (~5kHz, ISR) Servo function (PID, 250Hz) Communications CANbus link to other motors, doors, buttons Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

23 Interdependency between timing and functionality Integration of COTS components Code re-use Mixed processing requirements Concurrent team development Testability Mix of hard and soft real time Scalable (not susceptible to application change) Tolerant of hardware change Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

24 Apportioning functionality

Link state HMI Set points State Motor Position Outer loop PID control (periodic, ms resolution) Demand Inner loop commutation (µs resolution) PWM values PWM (continuous)

25 Link state HMI Set points State Motor Position Outer loop PID control (periodic, ms resolution) Demand Inner loop commutation (µs resolution) PWM values PWM (continuous) Demand State Motor Position CAN protocol Sync CAN (event driven) Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd Tasks Interrupts Peripherals

26 Deterministic interrupt processing

Link state HMI Set points State PWM/Time interrupt Motor

Demand Inner loop commutation (µs resolution) PWM values PWM

CAN (event driven) Renesas DevCon October 2012 Copyright 2012

27 Link state HMI Set points State PWM/Time interrupt Motor Position Outer loop PID control (periodic, ms resolution) Demand Inner loop commutation (µs resolution) PWM values PWM (continuous) Demand State Motor Position CAN protocol Sync CAN (event driven) Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd Tasks Interrupts Peripherals

28 Priorities 13, 14 and 15 are never disabled Priority 15 Priority 14 Priority 13 Priority 12 Priority 4 Priority 3 Priority 2 Priority 1 configmax_syscall_interrupt_priority configkernel_interrupt_priority Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

29 Techniques for periodic processing

30 Link state HMI Set points State Motor Position Outer loop PID control (periodic, ms resolution) Demand Inner loop commutation (µs resolution) PWM values PWM (continuous) Demand State Motor Position CAN protocol Sync CAN (event driven) Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd Tasks Interrupts Peripherals

31 Delay (wait) until it is time to start the next control cycle Perform control function Output results Note: It is a simple autonomous and sequential piece of code Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

32 /* Tasks always have the same prototype. */ void vouterlooppidtask( void *pvparameters ) { for( ;; ) { WaitNextControlCycle(); PerformControlFunction(); OutputResults(); } } /* A task cannot exit without first deleting itself. */ vtaskdelete( NULL ); Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

33 xtaskcreate( /* A pointer to the task function. */ vouterlooppidtask, /* Textual name. */ PID, /* Dimensions of the task stack. */ configminimal_stack_depth, /* Parameters passed into the task. */ (void *) 0, /* Assign a high priority to the task. */ ( configmax_priorities 1 ), /* A handle for the task. */ NULL ); Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

34 while( ( xtaskgettickcount() - xlastcycletime ) < CONTROL_PERIOD ); Suspended Ready Running Blocked Non blocking (in RTOS terms) code Consumes CPU time while doing nothing Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

35 What would happen if Task2 used a null loop delay? T2 preempts T3 T1 preempts T2 Task1 (high, event) Task2 (med, periodic) Task3 (low, event) Idle task t1 t2 t3 t4 t5 t6 t7 t8 t9 t11 t10 t12 t13 Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

36 Execution pattern with Task2 using a null loop delay T1 preempts T2 Task1 (high, event) Task2 (med, periodic) Task3 (low, event) Idle task t1 t2 t3 t4 t5 t6 t7 t8 t9 t11 t10 t12 t13 Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

37 Suspended Ready Running Tick event vtaskdelay(atime); vtaskdelayuntil(areferencepoint, atime); Blocked Time is specified in ticks Very efficient, only executes when there is work to be done Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

38 Link state HMI Set points State Motor Position Outer loop PID control (periodic, ms resolution) Demand Inner loop commutation (µs resolution) PWM values PWM (continuous) Demand State Motor Position CAN protocol Sync CAN (event driven) Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd Tasks Interrupts Peripherals

39 ISR Task3 gets pre-empted! ISR Task Wait for semaphore (xsemaphoretake()) Task3 PerformControlFunction() OutputResults() t1 t2 t3 t4 Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

Give semaphore (xsemaphoregivefromisr()) Wait for semaphore (xsemaphoretake()) PerformControlFunction()

41 ISR Task3 gets pre-empted! ISR Task Wait for semaphore (xsemaphoretake()) Task3 PerformControlFunction() OutputResults() t1 t2 t3 t4 Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

42 ISR Task3 gets pre-empted! ISR Task Wait for semaphore (xsemaphoretake()) Task3 PerformControlFunction() OutputResults() t1 t2 t3 t4 Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

specified in ticks Very efficient, only executes when there is work to be

43 Suspended Ready Running xsemaphoretake(xctrlsem,portmax_delay); xsemaphoregivefromisr(xctrlsem,&higherprioritytaskwoken); Blocked Time is specified in ticks Very efficient, only executes when there is work to be done Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

45 Link state HMI Set points State Motor Position Outer loop PID control (periodic, ms resolution) Demand Inner loop commutation (µs resolution) PWM values PWM (continuous) Demand State Motor Position CAN protocol Sync CAN (event driven) Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd Tasks Interrupts Peripherals

46 Link state HMI Set points State Motor Position Outer loop PID control (periodic, ms resolution) Demand Tasks can communicate with each other, and with interrupts, using FreeRTOS queues Inner loop commutation (µs resolution) PWM values PWM (continuous) Demand State Motor Position CAN protocol Sync CAN (event driven) Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd Tasks Interrupts Peripherals

47 Message passing using queues

48 Delay (wait) until it is time to start the next control cycle Perform control function Output results Read and process a value from queue (if any data is available) Note: Read only a single item from the queue to ensure control cycle remains deterministic Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

49 An intelligent FIFO buffer Data is copied byte for byte the only truly flexible way Tasks/interrupts can access queues simultaneously Queues can hold a finite number of items The number and size of each queued item is set when the queue is created Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

50 /* The type of message passed to the PID control task. */ typedef enum { ep_value; ei_value; ed_value; esetpointvalue } eparameterid; /* The structure passed on the queue. How pvvalue is interpreted by the PID control task depends on the econtrolparameterid value. */ typedef struct xctrl_parameter_message { eparameterid; void *pvvalue; } xourqueuemessage_t; Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

51 xqueuehandle xqueue; xqueue = xqueuecreate ( /* The length of the queue */ 3, ); /* The size of each item. */ sizeof( xourqueuemessage_t ) Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

52 xqueuesendtoback /* Or xqueuesendtofront() */ ( /* The handle of the queue */ xqueue, /* The item being sent */ &xfirstmessage, ); /* The number of ticks to block if the queue is already full. */ portmax_delay 1 st Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

53 xqueuesendtoback /* Or xqueuesendtofront() */ ( /* The handle of the queue */ xqueue, /* The item being sent */ &xsecondmessage, ); /* The number of ticks to block if the queue is already full. */ portmax_delay 2 nd 1 st Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

54 xourqueuemessage_t xreceived; xqueuereceive ( /* The handle of the queue */ xqueue, /* Variable into which the item will be placed. */ &xreceived, ); /* This time a block time of zero is specified as the control task must not block on the queue.*/ 0 2 nd 2 nd 1 st Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

55 uint32_t ulsetpoint; uint16_t usp, usi, usd; switch( xreceived.eparameterid ) { case esetpointvalue : ulsetpoint = *( ( uint32_t * ) xreceived.pvvalue ); break; case ei_value : usi = *( ( uint16_t * ) xreceived.pvvalue ); break; } /* Etc. */ Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

56 Link state HMI Set points State Motor Position Outer loop PID control (periodic, ms resolution) Demand Inner loop commutation (µs resolution) PWM values PWM (continuous) Demand State Motor Position CAN protocol Sync CAN (event driven) Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd Tasks Interrupts Peripherals

57 void vouterlooppidtask( void *pvparameters ) { xourqueuemessage_t xmessage; } for( ;; ) { xsemaphoretake( xctrlcyclesemaphore, portmax_delay ); PerformControlFunction(); OutputResults(); if( xqueuereceive( xqueue, &xmessage, 0 ) == pdpass ) { ProcessReceivedMessage( xmessage ); } } Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

Demand State Motor Position CAN protocol Sync CAN (event driven) Renesas DevCon October

59 Link state HMI Set points State Motor Position Outer loop PID control (periodic, ms resolution) Demand Inner loop commutation (µs resolution) PWM values PWM (continuous) Demand State Motor Position CAN protocol Sync CAN (event driven) Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd Tasks Interrupts Peripherals

60 Decoupled, functionally cohesive code Integration of COTS components Code re-use Mixed processing requirements Concurrent team development Testability Mix of hard and soft real time Scalable (not susceptible to application change) Tolerant of hardware change Renesas DevCon October 2012 Copyright 2012 Real Time Engineers Ltd

61 There is a lot more!

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FreeRTOS and LPC Microcontrollers. Richard Barry Design West, San Jose, 2013

FreeRTOS and LPC Microcontrollers Richard Barry Design West, San Jose, 2013 Introductions Real Time Engineers Ltd. FreeRTOS FreeRTOS+ WITTENSTEIN high integrity systems OpenRTOS SafeRTOS Richard Barry