1. Overview – Multitasking Basics
Multitasking and RTOS
Overview – Multitasking Basics
August 26, 2013 Charles J. Lord, PE President, Consultant, Trainer Blue Ridge Advanced Design and Automation

2. This Week’s Agenda
8/26 Multitasking Basics 8/27 State Machines I 8/28 State Machines II / Round Robin 8/29 Finishing it Up 8/30 An Example RTOS

3. This Week’s Agenda
8/26 Multitasking Basics 8/27 State Machines I 8/28 State Machines II / Round Robin 8/29 Finishing it Up 8/30 An Example RTOS

4. Multitasking, RTOS, and Real Life
We hear a lot about the proliferation of RTOS on myriad processors, even some 8-bit machines, but when do we need a real-time operating system (RTOS)?
Just how critical is timing and interoperation?
Cost of chip size, power consumption
“small ARM” vs 8/16 bit
“Free” RTOS and many paid ones, plus alternatives (kernals, etc)

5. Distributed Processing
8-bit
8-bit
Control / Data (typically i2c)
8-bit

6. Multitasking Approach
32-bit (typically)

7. Multitasking Dividing duties requires capability to:
Switch tasks cleanly without corruption
Determine priorities when conflicts occur
Provide clean, independent storage for each task
Provide communications between tasks when needed
Accomplishing the above usually requires robust timing control

8. Simple Example – RTOS?
Picture a simple digital alarm clock with hours and minutes display
Primary task: update the counters every minute to display the current time
Other tasks: set time, set alarm, determine if alarm should sound, enable snooze, other possible operator interaction.
RTOS not necessary – a few simple interrupts
State machine?

9. A little more realistic example
We will look at an industrial washer (similar to a dishwasher) – simplified
The I/O is simple: start button, a door switch, a two digit minutes countdown display, two LEDs, a water valve solenoid, a motor on/off, and a motor direction (wash / drain), and a ‘water level full’ input.
Please note for simplicity that we are ignoring many real-world features like error sense and recovery!

10. Our Washer CPU Motor Run Motor Direction Water On Door Lock
Water Level Full
Wash
Rinse

11. Our States Idle – unit is not operating Filling to wash
Wash cycle (10 minutes)
Drain from wash
Fill to rinse
Rinse cycle (10 minutes)
Drain from rinse

12. Our States IDLE: Motor, water, all LEDs and display off
Door Locked and start button pressed, go to FILL-TO-WASH
FILL-TO-WASH:
Water on, motor off, counter “FF”
If door locked = FALSE, go to IDLE
If water level full, go to WASH

13. Our States WASH: Water off, motor on, motor direction = wash
Start countdown timer from 10 minutes on entry
Display time
Turn on Wash LED
If door unlocked, go to IDLE
On time = 00, go to DRAIN-FROM-WASH

14. Our States DRAIN-FROM-WASH:
Motor on, Motor direction = drain, water off
Counter display = “dr”, LEDs off
Set timer for one minute
If door unlocked, go to IDLE
If timer = 00, go to FILL-TO-RINSE

15. Our States FILL-TO-RINSE: Water on, motor off, counter “FF”
If door locked = FALSE, go to IDLE
If water level full, go to RINSE

16. Our States RINSE: Water off, motor on, motor direction = wash
Start countdown timer from 10 minutes on entry
Display time
Turn on Wash LED
If door unlocked, go to IDLE
On time = 00, go to DRAIN-FROM-RINSE

17. Our States DRAIN-FROM-RINSE:
Motor on, Motor direction = drain, water off
Counter display = “dr”, LEDs off
Set timer for one minute
If door unlocked, go to IDLE
If timer = 00, go to IDLE

18. What’s Next?
As we have described the project in “states”, we will first build the project as a simple state machine over the next two days.
We will look at how to initialize each state, and may add other states as needed.
Later this week, we will look at another approach – the round robin – and re-define our states as “tasks”
On Friday we will look at an RTOS implementation

19. This Week’s Agenda
8/26 Multitasking Basics 8/27 State Machines I 8/28 State Machines II / Round Robin 8/29 Finishing it Up 8/30 An Example RTOS

20. Please stick around as I answer your questions!
Please give me a moment to scroll back through the chat window to find your questions
I will stay on chat as long as it takes to answer!
I am available to answer simple questions or to consult (or offer in-house training for your company)