1. CSCI1600: Embedded and Real Time Software
Lecture 17: Advanced Programming Concepts
Steven Reiss, Fall 2016
Get lots of feedback from the class. Make them think.
Should have 2-4 different implementations of the task loop on slides to show alternatives. (along with corresponding task prefix)
Code for timer transition?

2. Programming Is about knowing the tricks of the trade
Libraries
Patterns
Techniques
This week I’d like to show you some of these
For real time and embedded programming
Lecture 17: Advanced Programming
6/30/2019

3. Task Loop Tasks are modeled as automata
Code consists of executing each task periodically
What is the main loop
void loop() {
task1();
task2(); …
taskn(); }
What are the problems
Lecture 17: Advanced Programming
6/30/2019

4. Task Loop Coding Issues
Some tasks are periodic
Should be run every k milliseconds
Some tasks are sporadic
Run when needed, not other times
Tasks don’t know about other tasks
Timing demands for example
Tasks need to communicate
Tasks have different priorities
Lecture 17: Advanced Programming
6/30/2019

5. Example Suppose we have 3 tasks How would you code this?
One should be run every 6 ms
One should be run every 10 ms
One should be run when a flag is set
How would you code this?
What would the main loop look like
What would the task prefix code look like
Lecture 17: Advanced Programming
6/30/2019

6. Timed Task Loop Where do the numbers (2000,15,5,3) come from?
void loop() {
long time = micros();
if ((time – last_time) >= PERIOD) {
long last_time = 0;
last_time = time;
int cycle_counter = 0;
cycle_counter = (cycle_counter+1)%15;
boolean run_task = false;
if (cycle_counter%3 == 0) taskA();
const int PERIOD = 2000;
if (cycle_counter%5 == 0) taskB(); }
if (run_task) taskC();
Where do the numbers (2000,15,5,3) come from?
This is one way of doing it. What are other ways.
Each task could maintain its own last_time and time. This is a bit more complex, but allows easy changes to task.
Similarly taskC could always run and could check the run_task flag itself.
Lecture 17: Advanced Programming
6/30/2019

7. Other Alternatives Wait at the end for next cycle to begin
Use a (priority) queue of tasks to run
Tasks can be added/removed from queue
Main loop
Pop top task off queue, execute it
Task can add itself back to queue if periodic
Priority can be when the task needs to be run
This looks a lot like scheduling
Lecture 17: Advanced Programming
6/30/2019

8. Coding a Task Given a FSA model, what should the code do
The code should be designed to be quick
State actions should never wait
The code should take the next step, no more
Any actions it does should be fast
Setting output, reading input, setting flags
It shouldn’t do any waiting
Long computations should be broken up
Lecture 17: Advanced Programming
6/30/2019

9. Coding a Task
enum State { S1, S2, … Sn }
State cur_state = S1;
void taskA() {
switch (cur_state) {
case S1 :
// check for transitions, set cur_state if so
// also, do any action associated with transition
case S2 : … }
// Do any action associated with state
Can simplify this if there are common actions or transitions by state (e.g. error flag for all states goes to error state)
Lecture 17: Advanced Programming
6/30/2019

10. Coding Timing Information
Suppose a state has a timeout transition
How should this be coded
Example: beep for 5 seconds
What’s wrong here:
Turn on tone()
wait(5000)
Turn off tone()
go to next state
Lecture 17: Advanced Programming
6/30/2019

11. Coding a Timer Transition
Keep track of time of state entry
Static variable (global or static for function)
Set to current time when entering a state with a timeout
Or to timeout time
Code for the state checks if current time>last_time+TO
If so, it transitions to the next state
Implies that check is made often enough
Lecture 17: Advanced Programming
6/30/2019

12. Coding a Timer Transition
If task is periodic with fixed period
Runs every 10 ms for example
Use the task timing in place of the real clock
Can model the delay as a set of interim states
Or just have a counter of the number of runs since you entered the current state
Lecture 17: Advanced Programming
6/30/2019

13. Light Array Task What should this task look like?
What is the appropriate model?
Lecture 17: Advanced Programming
6/30/2019

14. Floating Point Problem: computations sometimes need real numbers
Floating point is slow
Integers aren’t accurate enough
Solution
Use fixed point arithmetic
Essentially scaled integers
Lecture 17: Advanced Programming
6/30/2019

15. Fixed Point Suppose we want to add 10.234 + 5.1
We could use integers scaled by 10^3
Add = ===
Can use different scalings (more complex)
Multiplying is slightly more complex
Mutliply * 5100 =
Divide by 10^3 = = ===
Actually want to round ( ) / 1000
What do you do for division?
Lecture 17: Advanced Programming
6/30/2019

16. Fixed Point Arithmetic
Suppose we want to divide / 5.1
Multiple * 10^3 =
Divide by 5100 = 2006 with remainder 3400
3400 > 5100/2 so set result to 2007 === 2.007
Can also multiply by 10^4 to get more accuracy and scale back
Can also take (10234*10^ /2)/5100
Computers don’t do this with powers of 10
Why not?
Do it with powers of two (eg 8 bit fraction, 24 bit int)
Lecture 17: Advanced Programming
6/30/2019

17. Simplifying Computations
Embedded systems deal with limited domain
We can use this to simplify coding
Suppose we know angle to a degree
Want to compute next position in x,y coordinates
This requires sin/cos computation
How to compute sin and cos
These require multiple floating point computations
Taylor series approximation
Lecture 17: Advanced Programming
6/30/2019

18. Look Up Tables Instead of computing it as needed, precompute it
We only need it for 90 values (0..90)
Set up a table sin_data[90] with the result for each degree
sin and cos then can be done with table lookup
cos(X) = sin(X+90)
sin(X+90) = 1-sin(X-90)
sin(X+180) = -sin(X) …
This can often be used for complex computations
How could you use lookup tables in tic-tac-toe?
Lecture 17: Advanced Programming
6/30/2019

19. Look up Tables How could you use look up tables in tic-tac-toe?
Lecture 17: Advanced Programming
6/30/2019

20. Homework Read Chapter 10 Tic-Tac-Toe is due
Exercise 10.4
Tic-Tac-Toe is due
Project Status Report due Next Monday
Suitable for posting on the web site (HTML preferred)
Lecture 17: Advanced Programming
6/30/2019