1. CET360 Microprocessor Engineering J. Sumey
5: Task Management
CET360
Microprocessor Engineering
J. Sumey

2. Smartcar Chaos
by now, we've seen how complex the Smartcar project has become:
line position data acquisition and processing
steering control
speed sensing
throttle control
battery monitoring
UI: LCD, status LEDs, "go" button
data comms
plus, various tasks need to be done at different rates!
CET360: Task Management

3. Observations
modern embedded systems have become correspondingly more complex than in the "early years"
why?
with more capable MCUs at decreasing cost points comes the desire to perform more functions
this trend does not appear to be letting up any time soon!
CET360: Task Management

4. Solution
look at multi-processing / multi-threading models from modern Operating Systems architecture
CET360: Task Management

5. Variable Rate Tasks processing of various tasks at different rates
in order to optimize CPU utilization
ex: it does not make sense to update steering servo output pulse width at any rate above servo update rate (i.e. 50 Hz)
task organization & design
documentation of all tasks to be performed, execution frequency/period, relative priority, constraints/conditions
CET360: Task Management

6. Smartcar Task Design Task Frequency Priority Constraints
Line position processing
50 Hz
Real-time
Limited by steering servo update rate
Steering control
Speed sensing
Variable
Handled by ISR
Throttle control Hz
Limited by mechanical lag
Battery monitoring
5 Hz
Low
Result determines LED color
Data comms
3 Hz
Medium
Limited by comm link bit rate
Status LED / LCD updates
Suitable for human consumption
Note: because of common goal and execution rate, line position processing can be absorbed into steering control task.
CET360: Task Management

7. Implementation
management of multiple tasks requires some form of processor scheduler
either among multiple processes or within a single process (Smartcar project model)
core component of modern Operating System (OS) design
many embedded systems tend to not be complicated enough to warrant a full-blown OS
but still need minimal task management capability
CET360: Task Management

8. Embedded System Solution
enter: a task scheduler
specifically: a variable rate, dynamic priority, cooperative, low overhead task scheduler
variable rate: various tasks are executed at specified & differing periods
dynamic priority: simultaneous tasks are ranked by assigned priority and executed accordingly
cooperative: tasks are aware of and respect other tasks (thus: no CPU hogging!)
low overhead: Coldfire instance requires approx. 2KB code for scheduler and 60 bytes RAM per managed task
CET360: Task Management

9. Scheduler Components
scheduler.c – actual code for scheduler ("implementation")
scheduler.h – "interface" to scheduler functionality
#define constants
enumeration types (task priorities, time units)
function prototypes
of the form scheduler_xxx()
replacement usDelay() and msDelay() functions
CET360: Task Management

10. Scheduler Functions
Function name†
Description
scheduler_init()
Initializes scheduler for execution, must be called first
scheduler_restart()
Resets start time of scheduler
scheduler_getStartTime()
Returns time of scheduler start/restart
scheduler_getUpTime()
Returns amount of time scheduler has been "up"
scheduler_getSystemLoad()
Returns current percent of CPU utilization
scheduler_createTask()
Creates a new managed task
scheduler_deleteTask()
Deletes an existing task
scheduler_getTaskId()
Returns id of task given task name
scheduler_getTaskPeriod()
Returns a task's period in us.
scheduler_setTaskPeriod()
Sets a new period for a task in us.
scheduler_getLatestPeriod()
Returns actual period of latest execution of a task
scheduler_setTaskEnabled()
Enables/disables a task
scheduler_run()
Runs one iteration of the scheduler, must be called as often as possible
† see scheduler.h for parameters & details, minimum needed functions in bold
CET360: Task Management

11. Support Functions Function name Description usDelay()
Delays specified number of microseconds
msDelay()
Delays specified number of milliseconds
CET360: Task Management

12. Typical Use CET360: Task Management task name name of check function
#include "scheduler.h" // add this to include section
// initialization code:
scheduler_init();
scheduler_createTask("taskname", NULL, taskHandler, 10000, TASK_PRIORITY_MEDIUM);
task name
name of check function
name of task function
desired execution period (us)
relative task priority (see TaskPriority_e)
// main loop:
for (;;) {
scheduler_run();
// can have other stuff here, but no long delays
// no direct calls to taskHandler functions! }
CET360: Task Management

13. Example Use† † taken from RGBschedulerTest.c CET360: Task Management
void task1() {
TOGGLE(FB_RGB_RED); }
void task2() {
TOGGLE(FB_RGB_GREEN);
void task3() {
TOGGLE(FB_RGB_BLUE);
void main() {
// TPM1 initialization
TPM1SC = TPM2SC_TOIE_MASK // enable interrupt on timer overflow
+ TPM1SC_CLKSA_MASK // select bus clock for timer clock
+ 4; // divby-16 prescaler (1MHz)
EnableInterrupts; // enable interrupts into CPU
scheduler_init();
scheduler_createTask("task1", NULL, task1, , TASK_PRIORITY_LOW);
scheduler_createTask("task2", NULL, task2, , TASK_PRIORITY_HIGH);
scheduler_createTask("task3", NULL, task3, , TASK_PRIORITY_MEDIUM);
for (;;)
scheduler_run(); }
CET360: Task Management
† taken from RGBschedulerTest.c

14. Coldfire Project Specifics
requires use of a free-running timer (i.e. no set modulo value) clocked at 1 MHz
chosen timer number is set in scheduler.h
look for "#define TIMING_TPMSELECT"
ISR for timer overflow event is supplied in scheduler.c
but requires user to enable timer overflow interrupts! (TOIE bit of TPMSC register)
CET360: Task Management

15. Coldfire Project Application
to integrate scheduler into a Coldfire application developed in CodeWarrior:
copy scheduler.h & scheduler.c into project Sources folder
edit scheduler.h to select TPM to be used
also verify value of MAXTASKS
follow "note" comment at begin of scheduler.h
add #include "scheduler.h" to end of #include section
delete any existing usDelay() and msDelay() definitions
use scheduler-provided version instead
make sure selected timer overflow interrupt is enabled (TOIE)
add scheduler_init() and "createTask" calls to initialization code
task handler functions must be "void(void)"
revise main program loop to call scheduler_run()
very often
CET360: Task Management

16. Smartcar Task Suggestions
taskSteering() – at 50 Hz
taskSpeed() – at 100 Hz
taskPeriodics() – at 5 Hz
acquire new Pot a/d with smoothing
Vbatt monitoring (with smoothing)
updateLCD()
taskDataComms() – at 3 Hz
send 1 status packet
CET360: Task Management

17. Troubleshooting No parameters into/out of task handler functions
but ok to call other functions from handlers with/without parameters
Change usDelay, msDelay prototypes in LCD.h
from unsigned short to plain int
may need to remove scheduler_run() from inside of replacement WAITFOR() macro in scheduler.h?
(credits to CET360 class of SP16)
CET360: Task Management