Microprocesors, AdvancedWhat You Need to Know About RTOSes

February 1, 2012Jack Ganssle

The RTOS

• The kernel – the multitasker– The kernel handles tasking– It also has resources to pass data between tasks

safely, and to synchronize tasking.

• An RTOS also has resources like communications, a GUI, a file system, and debugging tools.

Polled Loop

Wait for something

Do something

Polled Loop With Interrupts

Wait for something

Do something

Wait for interruptsignal

Handle Interrupt

Interrupt event

Signal main loop

Enter the RTOS

Handle Interrupt

Interrupt event

Signal RTOS

Handle screen Handle TCP/IP Process data

Kernel

When Do You Need an RTOS?

• Multiple independent activities• Communications• File system• GUIs

An Infusion Pump

What About Linux?

• Memory needs• Real-time response• Reliability, validation and security• Licensing issues• Porting Linux is hard!

Multitasking and Tasks

void App_TaskADC (void *p_arg){ while (1) { ADC_Read(); OSTimeDly(10); }}

Multitasking and Tasks

void App_TaskLowFluid (void *p_arg){ code to handle low fluid; OSTaskDel(. . .);}

Task Creation

void main (void){ OSInit(); /* Kernel initialization */

OSTaskCreate(...); /* Task creation */ OSTaskCreate(...);

OSStart(); /* Start multitasking */}

TCBs and Stacks

Stack pointer Registers

Other task data

Rest of stackTask Priority

Task state

Task Control Block Stack

Task States

Scheduling• Preemption– The suspension of one task and start of another

• Context switching– The process of switching from one task to another

• Round robin– The giving of equal time to all same-priority tasks

Round-Robin Scheduling

Priorities

Scheduling Points

– When a task decides to wait for time to expire– When a task sends a message or a signal to

another task– When an ISR sends a message or a signal to a task– When a kernel object is deleted– When the priority of a task is changed– Other places

I/O Perils

Task 1

Task 2

“Hi There”

“Howdy”

Terminal

“HHio wTdhyere”

The Mutex

Acquire_mutex;Access_device;Release_mutex;

Semaphores

AtoD_Read (uint16 *result){

start ADC conversionwait for semaphoreRead A/Dreturn result

}

ADC_ISR (void){ release semaphore

clear interrupt}

Message Queues

Rx ISR

Serial Stream

Rx Task

Message queue

Roll Your Own?

• Consider the cost of buying vs. cost to write• Cost to validate• Messaging and synchronization• Modularity

Rate Monotonic Scheduling

If tasks are periodic, and do not share resources or sync with each other, then RMA says:

if: E1/T1 + E2/T2 + E3/T3 … <= n(2 1/n - 1)

all hard real time deadlines will be met.

n Time

1 1.0002 0.8283 0.7804 0.7575 0.743

10000 0.693

// Task A – if angle> 90 swap modesvoid a (void){ int raw; float angle; float scale=2*pi; raw=inport(encoder); angle=(float)raw * scale; if(angle>90)swap_mode;}

0013DA 9A 00 00 CALL ddload0013DF 83 C4 04 ADD SP,40013E2 9A 00 00 CALL dpush0013E7 FF 76 FC PUSH WORD PTR [BP-4]0013EA 9A 00 00 CALL dicvt0013EF 83 C4 02 ADD SP,20013F2 9A 00 00 CALL dpush0013F7 9A 00 00 CALL drmul0013FC 83 C4 10 ADD SP,160013FF 9A 00 00 CALL dpush

k=(l1>l2);

000E8A 8B 56 CA MOV DX,[BP-54]000E8D 8B 46 C8 MOV AX,[BP-56]000E90 3B 56 D6 CMP DX,[BP-42]000E93 7F 07 JG SHORT L42000E95 7C 0A JL SHORT L40000E97 3B 46 D4 CMP AX,[BP-44]000E9A 76 05 JBE SHORT L40000E9C B8 01 00 L42 MOV AX,1000E9F EB 02 JMP L41 000EA1 2B C0 L40 SUB AX,AX000EA3 89 46 F6 L41 MOV BP-10],AX000EA6 FF 46 FE INC [BP-2]

Talk to the vendors

We’re mad as hell and we’re not gonna take it anymore!

Software Selection

• CPU/Language/Tool compatibility• Pricing model• Source or object?• Size and performance• Debugging tools• Services, drivers• Other software components

• Reputation/Tech Support• Reliability

Questions?