Creating an Embedded Operating System

2

Super Loop Limitations

| The super loop cannot be used to periodically execute a function at fixed intervals unless:zWe know the precise execution time

for the functionz The execution time is fixed and never

varies

3

Sample Situations with Periodic Timing Needs

| A music sample must be converted every 20 ms.| The current speed of the vehicle must be measured at 0.5 second

intervals.| The display must be refreshed 40 times every second.| The calculated new throttle setting must be applied every 0.5

seconds.| A time-frequency transform must be performed 20 times every

second.| The engine vibration data must be sampled 1000 times per second.| The frequency-domain data must be classified 20 times every

second.| The keypad must be scanned every 200 ms.| The master (control) node must communicate with all other nodes

(sensor nodes and sounder nodes) once per second.| The new throttle setting must be calculated every 0.5 seconds.| The sensors must be sampled once per second.

4A Simple Embedded OS

| Use a timer in conjunction with an interrupt to periodically run an interrupt service routine

| The interrupt signal is often referred to as the system tick

5

Modified Super Loop

void main(void){

Timer_2_Init(); // Set up Timer 2EA = 1; // Globally enable interruptswhile(1); // An empty Super Loop

}

6

Interrupt Sources

7Automatic Timer Reloads

| Timers 0 and 1 can be automatically reloaded in 8-bit mode

| Timer 2 is automatically reloaded every time it overflows

8

Timer-driven vs. Event-driven

| In a timer-driven system, the system relies on a fixed system tick to run tasks

| In an event-driven system, the systems depends on external interrupts to run tasks

Which is more reliable? Why?

9

Co-operative vs. pre-emptive Scheduling| A co-operative schedule lets tasks run

to completion before switching to another task

| A pre-emptive scheduler runs multiple tasks in a round-robin fashion

Which is more reliable? Why?

10

Design Considerations

| It is important that the total time of all tasks not exceed the system tick

| It is prudent to not use any other interrupts in conjunction with the timer interrupt in order to avoid conflict between interrupt service routines

11

Measuring Flow

12

Embedded vs. Real-Time OS