PALOS:Power Aware Light-weight Operating System

(Pseudo-realtime, Application-specific, Light-weight Operating System)

Sung ParkElectrical Engineering DepartmentsUniversity of California, Los Angeles

September 18, 2002

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Features of RTOS (hard)

• Structure - OS components and user defined Tasks

• Multi-tasking• Provides hard time guarantee

• Schedulability test guarantees task deadline

• Priority Support• Pre-emption of tasks

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Drawbacks of RTOS

• Interrupt latency of context switching may introduce inefficiency

• Pre-emption requires extra attention to shared memory

• RTOS is difficult to debug• Commercial RTOS

• Portability can be difficult• License Fee• Learning Curve

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Do we need RTOS in Wireless Sensor Network?

• Application Specific – Need for RTOS• Sensor network monitoring plane’s position and

orientation – a missed deadline could lead to plane crash

• Sensor network monitoring nuclear plant – a missed deadline could lead to nuclear melt down

• Other cases• If a missed deadline results in minor degradation

of performance or data qaulity, we can live without an RTOS

• As a research organization, we are more concerned about algorithms and development flexibility -> need to address possible migration to RTOS

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PALOS: Pseudo-Realtime (soft)

• Structure: OS functions and user defined Tasks

• Multi-tasking (Event-Driven)• Each tasks is a routine which processes

events that are stored in event queues

• Supports Inter-task Communication• By queuing events to other tasks, the data

between tasks can be exchanged

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PALOS: Pseudo-Realtime (soft)

• Instead of hard-realtime, palos provides soft-realtime gaurantee (best-effort)

• Tasks cannot be pre-emptied (by other tasks)• Tasks run one after the other• No shared memory protection required

• Has mechanisms to provide priority, stopping and resuming of tasks

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PALOS architecture

PALOSCore

Drivers (Hardware Abstraction Layer)

ManagerT

AS

K 1

TA

SK

2

TA

SK

5

TA

SK

N

TA

SK

3

TASK 4

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PALOS features

• Stripped Core• Code Size: 956 Bytes• Mem Size: 548 Bytes

• ATmega128 • FLASH size(Code): 128Kbytes• RAM Size (Memory): 4Kbytes.

• Typical( 3 drivers, 3 user tasks)• Code Size: 8 KBytes• Mem Size: 1.3 KBytes

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Compared to tinyOS

• Notion of well-defined tasks• Inter-task communication through

the use of separate event queues • More elaborate scheduling scheme

where multiple tasks can be periodically or aperiodically scheduled

• Easier to debug (minimum use of macros)

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PALOS Core

• Processor independent algorithms• Provides means of managing event

queues and exchanging events among tasks

• Provides means of task execution control(slowing, stopping, and resuming)

• Supports a scheduler: periodic, and aperiodic functions can be scheduled

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Tasks

• A task belongs to the palos main control loop

• Each task has an entry in palos task table (along with eventQs)

PALOSTaskTable

TASK 1

TASK 2

TASK 3

TASK N

Task Routine

Event Q

Task Routine

Event Q

Task Routine

Event Q

Task Routine

Event Q

MainControl

Loop

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Inter-task Communication

• Events are exchanged using the service provided by PALOS core

Task 2

Task 3 Event Q

Task 1

Task 3

PALOSCore

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Scheduling with Software Timer

• Periodic or aperiodic events can be scheduled using Delta Q and Timer Interrupt

• When event expires appropriate event handler is called

Task 2

Delta Q

Task 1

TimerInterrupt

Expired Event Q

TimerTask

Handler 1

Handler 2

Handler 3

PALOSCore

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Event Driven Task

• Typical task routine

while (eventQ != isEmpty){

dequeue event;

process event;

}

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Task Execution Control

• Execution Control using task counter• A task counter is associated with each

task• Counters are initialized to pre-defined

values• Counters are decremented 1) every main

control loop iteration (relative timing) 2) by timer interrupts (exact timing)

• When counter reaches zero, the task routine is called. The counter is reset to reload value.

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Task Execution Control

• Normal Task: Set the counter to 0 and reload value to 0

• Task slow down: Set the counter to large positive value

• Task Suspension: Set the counter to pre-set value (e.x. –1)

• Task Resumption: Reset the counter to positive value

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PALOS v0.1 implementation – Task Table Structure

/* Generic event queue structure */

typedef struct {

SHORT (*initHandler)(void);

SHORT (*taskHandler)(void);

SHORT execCounter; /* Counter to be used for task speed control */

/* when counter reaches zero the task is executed */

SHORT reloadCounter; /* execCouter is reset the reload counter value after it */

/* reaches zero */

SHORT maxEvent; /* stores max number of events that can be processed per */

/* iteration. can be used to give priority */

BOOL isExactTiming; /* indicates whether the counter is decremented */

/* following exact timing */

USHORT header; /* header ptr */

USHORT trailer; /* trailer ptr */

USHORT eventStrSize; /* member structure size */

USHORT maxQsize; /* max number queue size */

USHORT curQsize; /* current queue size */

CHAR isValid; /* indicates whether this is valid entry */

void *event;

} taskStr;

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PALOS v0.1 implementation – Main Control Loop

// main loop while (1){ // run each task in order for (i=0; i< globalTaskID; i++){ isExact = qArray[i].isExactTiming; tmpCntr=qArray[i].execCounter; if ( tmpCntr != TASK_DISABLED) { /* task is not disabled */

if ( tmpCntr ) { /* counter hasn't expired */ if (!isExact) qArray[i].execCounter--;}else { /* exec counter expired */ if (isExact) PALOSSCHED_TIMER_INTR_DISABLE; qArray[i].execCounter = qArray[i].reloadCounter; if (isExact) PALOSSCHED_TIMER_INTR_ENABLE; /* run the task routine */ (*qArray[i].taskHandler)();}

} } }

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PALOS Core functions

SHORT palosEvent_register(SHORT (*initFunc)(void), SHORT (*taskFunc)(void), LONG xCounter, LONG rCounter, USHORT maxEv,

BOOL exactTiming,USHORT eventStrSize, USHORT maxQsize, void *ev);

SHORT palosEvent_putEvent( USHORT taskID, void *ev, CHAR isAtomic);

SHORT palosEvent_getEvent( USHORT taskID, void *ev, CHAR isAtomic);

SHORT palosEvent_start(USHORT taskID, LONG excCntr, LONG reldCntr);

SHORT palosEvent_stop(USHORT taskID);

SHORT palosEvent_maxEvent(USHORT taskID, USHORT maxEv);

SHORT palosEvent_exactTiming(USHORT taskID, BOOL exactTiming);

SHORT palosSched_schedule( USHORT tid, ULONG param, hndlrWrapper *tmrHandler, ULONG ticks, CHAR isPeriodic);

SHORT palosSched_cancel( USHORT tid, hndlrWrapper *tmrHandler );

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PALOS architecture – Drivers and Managers

PALOSCore

Drivers (Hardware Abstraction Layer)

ManagerT

AS

K 1

TA

SK

2

TA

SK

5

TA

SK

N

TA

SK

3

TASK 4

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Drivers (HAL)

• Abstracts hardware (processor-specific and platform-specific) from the task level

• The layering supports portability• Processors: ATmega103, ATmega128L,

TMS320, STrongThumb• Platforms: iBadge, MICA, MK2

• Examples: • Processor-specific: UART, SPI, Timers.. • Platform-specific: RFM, LEDs, Sensors

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Managers (Optional)

• Extra abstraction layer that handles a protocol or handshaking with external modules• iBadge: Bluetooth Manager

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UART0 Driver

void UART0_Init();

void UART0_Enable();

void UART0_Disable();

UCHAR UART0_NewData();

UCHAR UART0_GetByte();

USHORT UART0_Get2Bytes();

UCHAR UART0_GetNBytes( UCHAR * ptr_ch, UCHAR nN );

USHORT UART0_Check2Bytes();

UCHAR UART0_GetError();

UCHAR UART0_FreeSpace();

BOOL UART0_WriteByte( UCHAR ch );

BOOL UART0_Write2Bytes( USHORT sh );

BOOL UART0_WriteNBytes( UCHAR * ptr_ch, UCHAR nN );

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Task Implementation with PALOS

• Need to define event structure• Implement initialization function• Implement main task function• Implement initTask()

• Performs system initialization• Registers different task to PALOS core

• Implement initSched()• Initial scheduling of events

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Task Implementation with PALOS

void main(void){ SHORT i; USHORT tmpCntr; BOOL isExact; // event handler initialization palosEvent_init(); // The user's task is registered and // scheduled by this function

initTask(); // initialize each function for (i=0; i< globalTaskID; i++){ (*qArray[i].initHandler)(); } // User needs to define this function to // schedule events

initSched();// main loop while (1){ // run each task in order for (i=0; i< globalTaskID; i++){ isExact = qArray[i].isExactTiming; tmpCntr=qArray[i].execCounter;

if ( tmpCntr != TASK_DISABLED) { /* task is not disabled */if ( tmpCntr ) { /* counter hasn't expired */ if (!isExact) qArray[i].execCounter--;}else { /* exec counter expired */ if (isExact) PALOSSCHED_TIMER_INTR_DISABLE; qArray[i].execCounter = qArray[i].reloadCounter; if (isExact) PALOSSCHED_TIMER_INTR_ENABLE; /* run the task routine */ (*qArray[i].taskHandler)();}

} } } /* should never get here */ return;}

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Example Application for MICA node

• StringIn Task: gets string from stdin• StringOut Task: outputs string to stdout• Menu Task: runs the menu state machine to

control the frequency of LED flashing frequency

PALOSCore

Timer1Driver

StringOutTask

StringInTask

MenuTask

UART0 DriverLED

Driver

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stringOutTask.h

/* stringOut task event structure */

typedef struct {

UCHAR *str; /* pointer to string */

UCHAR size; /* size of the string */

USHORT eventID;

ULONG eventParam;

hndlrWrapper stringOut_TXdone; /* handler called when tx is done */

} stringOut_Event;

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stringOutTask.c

SHORT stringOut_init() { stringOut_hoqValid=false;

// stringOut initialization

UART0_Init();

return 0;

}

SHORT stringOut_task() { UCHAR availSize;

while ((stringOut_hoqValid == true) ||

(palosEvent_getEvent(stringOutID, &stringOut_hoq, TASK_NON_ATOMIC) !=PALOSEVENT_QEMPTY)){

availSize = UART0_FreeSpace(); if ( stringOut_hoq.size <= availSize ) { UART0_WriteNBytes(stringOut_hoq.str,

stringOut_hoq.size); HANDLER_CALL(stringOut_hoq.stringOut_TXdone,

stringOut_hoq.eventID, stringOut_hoq.eventParam);

stringOut_hoqValid=false; } else { UART0_WriteNBytes(stringOut_hoq.str, availSize); stringOut_hoq.str += availSize; stringOut_hoq.size -= availSize; stringOut_hoqValid=true; break; } } return PALOSEVENT_TASK_DONE;}

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initTask.c : initTask()void initTask() {SYS_Init();stringOutID=palosEvent_register(stringOut_init, stringOut_task,

STRINGOUT_DEF_CNTR, STRINGOUT_DEF_RCNTR, STRINGOUT_DEF_MAXEVENT, false, sizeof(stringOut_Event), STRINGOUT_Q_SIZE, (void *)stringOutEvent);

stringInID=palosEvent_register(stringIn_init, stringIn_task, STRINGIN_DEF_CNTR, STRINGIN_DEF_RCNTR, STRINGIN_DEF_MAXEVENT, false, sizeof(stringIn_Event), STRINGIN_Q_SIZE, (void *)stringInEvent);

menuID=palosEvent_register(menu_init, menu_task, MENU_DEF_CNTR, MENU_DEF_RCNTR, MENU_DEF_MAXEVENT, false, sizeof(menu_Event), MENU_Q_SIZE, (void *)menuEvent);

palosSchedID=palosEvent_register(palosSched_init, palosSched_task, PALOSSCHED_DEF_CNTR, PALOSSCHED_DEF_RCNTR, PALOSSCHED_DEF_MAXEVENT, false, sizeof(palosSched_Event), PALOSSCHED_EVENTQ_SIZE, (void *)tEvent);

}

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initTask.c : initSched()

void initSched() { //schedule an event

stringOut_msg(initMsg, MENU_START,0, HANDLER_WRAP(menu_handler));}

stringOutTask.c : void stringOut_msg(CHAR *str, USHORT id, ULONG param, hndlrWrapper *hnd){ stringOut_Event outgoingMsgEvent;

outgoingMsgEvent.str=str;

outgoingMsgEvent.size=strLength(str);

outgoingMsgEvent.eventID=id;

outgoingMsgEvent.eventParam=param;

HANDLER_COPY(&(outgoingMsgEvent.stringOut_TXdone), hnd);

palosEvent_putEvent(stringOutID, &outgoingMsgEvent, TASK_NON_ATOMIC);}

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MICA demo

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menuTask.c

void menu_ledsched(USHORT id, ULONG param){ CHAR tmpID;

tmpID=(CHAR)id; switch(tmpID){ case LEDCHOICE_RED: redToggle(); break; case LEDCHOICE_GREEN: greenToggle(); break; case LEDCHOICE_YELLOW: yellowToggle(); break; default: /* shouldn't get here */ redOn(); yellowOn(); greenOn(); break; }}

case MENU_FREQ_CHECK: periodVal=strToShort(incomingMsg); if ( periodVal > 0 && periodVal <=10000) { palosSched_cancel(ledChoice,HANDLER_WRAP(menu_ledsched)); palosSched_schedule(ledChoice, 0, HANDLER_WRAP(menu_ledsched),

periodVal, PALOSSCHED_PERIODIC); stringOut_msg(msg5, MENU_RESTART_TX, 0,

HANDLER_WRAP( menu_handler)); }

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PALOS Development Environment

Directory Structure

PALOS v0.1

palosCoreDriversampleProject

-MICA

palosMain.c, .h

palosEvent.c, .h

palosSched.c, .h

menuTask.c, .h

stringInTask.c, .h

stringOutTask.c, .h

ATmega128L TMS320

Processor

iBadge

MICA

MK2 iBadge

StrongThumb

MK2

uart0.c ,.h

timer1.c ,.h

timer1.c ,.h

Processor Processor

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PALOS Development Environment

• Compiler: CVAVR (possible port to AVR-GCC)

• Current release v.01• https://sourceforge.net/project/showfiles.php?grou

p_id=61125

• If you want to do development• Needs to obtain account from

http://sourceforge.net• Email me your account id and what part of the

code you will be working on• Need to use CVS client to access CVS repository

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Installing CVS client under Windows

• Windows CVS client setup(wincvs)• Download and install wincvs from

http://prdownloads.sourceforge.net/cvsgui/WinCvs13b8.zip?download

• Download and install ssh from http://prdownloads.sourceforge.net/sfsetup/ssh-1.2.14-win32bin.zip?download

• Download and install sourcefourge setup utility “sfssetup” from http://prdownloads.sourceforge.net/sfsetup/sfsetup-1.3.zip?download

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Setting up wincvsPALOS CVS Repository Setup: Admin->Preferences

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Setting up wincvsChecking Out Palos0.1 module: Create->Check Out Module

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Software Engineering Practices

• Make use of CVS for development coordination (also backup and revision control)

• If you write something, make it modular(drivers, task, library)

• Spend extra time to refine the interface so that other people can easily use it

• Write a simple example code that can test your contributed module

• More you contribute, more useful it becomes Everybody counts!

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Contributions Needed

• Communication Module: • RFM driver, Encoding and Decoding Task, MAC

layer etc..

• Drivers • SPI, timer0, timer2, watchdog timer, ADC, etc..

• Tasks• Sensing, Networking related tasks

• PALOS core• Random number generator, and other library

functions

• Porting to AVR-GCC

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References

• Michael Melkonian, “Get by Without an RTOS”, Embedded Systems Programming Mag, vol 3. No. 10 Sept., 2000

• Jack W. Crenshaw, “Mea Culpa (Is RTOS needed?)”, http://www.embedded.com/story/OEG20020222S0023

• Karl Fogel, “Open Source Development with CVS”, http://cvsbook.red-bean.com/

• CVS FAQ, http://www.cs.utah.edu/dept/old/texinfo/cvs/FAQ.txt