Lab2 Intro

Introduction to Lab 2Programming in RTOS using LEGO Mindstorms

Martin Stigge <[email protected]>

9. November 2009

Martin Stigge <[email protected]> Lab 2: LEGO 9. November 2009 1 / 20

Lab 2: Programming in RTOS using LEGO Mindstorms

Lab goals:I Basic programming on an embedded deviceI Using the API of an RTOS for current tasks

Lab preparation:I Form groups: 3 students eachI Possibly refresh your C knowledgeI Make sure your Windows login works!I Lab will be done on Fri, 13.11. and 20.11. in rooms 1312D and 1313DI Have a look at the lab homepage

http://www.it.uu.se/edu/course/homepage/realtid/ht09/lab2

Lab report:I C code to all 5 partsI Answers to the questionsI To my mailbox, building 1, floor 4; Deadline: Mon, 14.12. at 10:00

Further:I Demonstrate a working vehicle, participate in car race on 11.12.I Return all hardware you get


http://www.it.uu.se/edu/course/homepage/realtid/ht09/lab2

LEGO Mindstorms

Programmable LEGO brick with sensors and motors

Comes in two generations:

RCX generation (1998) NXT generation (2006)

We will use the RCX platform


LEGO Mindstorms: Components

Package contents:

RCX unit:I LCD display, 5 digitsI 4 ButtonsI Sensor inputs 1, 2, 3I Motor outputs A, B, CI Infrared (IR) portI Can store 8 programs

Two touch sensors

One light sensor

Two motors

RCX Internals:

Hitatchi H8/3292, 32k RAM, 16k ROM, 8/16-bit architecture, 16MHz clock


RTOS: LegOS (BrickOS)

We don’t use the standard firmware

Instead: LegOS (nowadays “BrickOS”)I (Soft) Real-time operating systemI Multitasking OS coreI Provides C/C++ development environmentI Support for threads, priorities, semaphores, event handlingI Comprehensive API for low-level I/O accesses

Rest of this introduction: How toI Upload the firmwareI Compile/upload programsI Write programs/use LegOS API


LegOS: Firmware Upload

1 Start Cygwin (see lab homepage)

2 Compile firmware uploader if necessary (see lab homepage)3 Upload firmware:

(i) Power up RCX unit (six AA batteries, “On/Off” button)(ii) Power up IR transceiver (one 9V block battery)(iii) Connect IR transceiver to COM port(iv) Run firmdl:

Example Run: Firmware upload

$ cd /legos

$ util/firmdl -s boot/legos.srec

Transferring "boot/legos.srec" to RCX ...

100%

$


LegOS: Program Compile/Upload

1 Compile program using make2 Upload program using util/dll

I RCX and IR transceiver need to be running (and idle)

Example Run: Program compile/upload

$ cd /legos/demo

$ make

...

$ cd ..

$ util/dll demo/helloworld.lx

$

To compile own programs:

Edit demo/Makefile and add newprogram.lx to PROGRAMS= row


Remark: Working At Home

You can do all this at home (using Windows/Linux/Mac)

Some hints at lab homepage

Biggest issue: IR transceiver needs COM port (RS-232)

RS-232 port

Only few boxes with USB


Programming For LegOS

“Ordinary” C programs

Have a look at demo/ directory!

Listing: helloworld.c

1 #include <conio.h> // cputs(), cls()

2 #include <unistd.h> // sleep()

3

4 int main(int argc , char **argv) {

5

6 cputs("hello");

7 sleep (1);

8 cputs("world");

9 sleep (1);

10 cls();

11

12 return 0;

13 }


Programming For LegOS

“Ordinary” C programs

Have a look at demo/ directory!

Listing: helloworld.c

1 #include <conio.h> // cputs(), cls()

2 #include <unistd.h> // sleep()

3

4 int main(int argc , char **argv) {

5

6 cputs("hello");

7 sleep (1);

8 cputs("world");

9 sleep (1);

10 cls();

11

12 return 0;

13 }


LegOS API

LegOS C programs can’t use standard C library

But API offers a lot, includingI Input/Output (on LCD)I Reading sensors (light/touch)I Controlling motorsI Time functions (delay)I Random numbersI Multithreading (forking, semaphores)I Wakeup on events

Will do a short walk-through now

See “LEGO Command Reference”!

(Include corresponding header files!)


LegOS API: I/O

Input via buttons

Input via sensors (see next slide)

Output via LCD: Strings, integers and sound

Example: User Input/Output

1 // Input via buttons

2 if (getchar () == KEY_VIEW) { // Blocking call!

3 ...

4 }

5

6 // Output

7 cls(); // Clears LCD

8 cputs("Foo"); // String Output

9 lcd_int (42); // Integer Output

10

11 dsound_system(DSOUND_BEEP); // Short "beep"


LegOS API: Sensors

Touch/light sensors on inputs 1, 2 and 3

Read values from boolean/integer variablesI TOUCH 1, TOUCH 2, TOUCH 3: Touch sensors (boolean)I LIGHT 1, LIGHT 2, LIGHT 3: Light sensors (integer)

Active/passive mode

Example: Sensor input

1 #define DARK_THRESHOLD 42

2 ...

3

4 ds_active (& SENSOR_1); // Set 1st sensor to active mode

5 ...

6 if (LIGHT_1 < DARK_THRESHOLD) {

7 // Handle darkness

8 }


LegOS API: Motors

Motors can go forward/backward

.. at a certain speed (between MIN SPEED and MAX SPEED)

Can also brake (block movements)

Example: Motor Control

1 // Motor A full speed forward

2 motor_a_dir(fwd);

3 motor_a_speed(MAX_SPEED);

4

5 // Everything full stop

6 motor_a_dir(brake);

7 motor_b_dir(brake);

8 motor_c_dir(brake);


LegOS API: Time and Randomness

Delay execution: sleep() (seconds) and msleep() (milliseconds)

Busy waiting: delay()

Generate random number: random() and srandom() (set seed)

Example: Time and randomness

1 sleep (23); // Sleep for 23 seconds

2

3 long int x = sys_time; // Read current time in ms

4 x += 1234; // Set x to time point 1234ms later

5 ...

6 sleep(x - sys_time); // Wait until time point x

7

8 srandom(sys_time); // Initialize random seed with systime

9 if (random () % 42 == 23) {

10 ... // Execute this with (not too) low probability

11 }


LegOS API: Creating Threads

Create new thread from a function: execi()

pid_t execi(&PROCESS_FUNC, int argc, char *argv[],priority_t priority, size_t stack_size);

PROCESS FUNC is function for new threadI Two arguments: int and char**I Returns an int

Pass arguments via global variables or argc/argv

Priorities: Between PRIO LOWEST (1) and PRIO HIGHEST (20)

Example: Creating Threads

1 // Define thread function

2 int foo(int argc , char *argv []) { ... }

3 ...

4 // Create thread (starts running immediately !)

5 execi(foo , 0, NULL , PRIO_NORMAL , DEFAULT_STACK_SIZE);


LegOS API: Events

Suspend thread until wake-up event: wait event()wakeup_t wait_event(wakeup_t(*wakeup) (wakeup_t),

wakeup_t data);wakeup is function for event check

I Argument and return type: wakeup t (i.e., integer)I Is called by OS every 20ms, releases thread if non-zero return value

Example: Event Waiting

1 // Define wake -up function

2 wakeup_t sensor_press_wakeup(wakeup_t data) {

3 return TOUCH_1 || TOUCH_3;

4 }

5 ...

6 // Wait for event

7 wait_event(sensor_press_wakeup , NULL);

8 // Sensor pressed!

Note: sleep() is implemented like this. (Exercise: How?)


LegOS API: Semaphores

Used for signalling between threads, e.g., lockingI Type: sem tI Init: sem init(sem t *sem, int pshared, unsigned int value)I Blocking Wait: sem wait(sem t *sem)I Non-blocking Wait: sem trywait(sem t *sem)I Release: sem post(sem t *sem)

See also: API reference

Example: Locking Using A Semaphore

1 int A;

2 sem_t Asem;

3 sem_init (&Asem , 0, 1); // Semaphore gets initial value 1

4 ...

5 sem_wait (&Asem); // Aquire lock for A

6 A = 42; // Change A

7 sem_post (&Asem); // Release lock for A


Lab Assignment

Part 1: Simple Light DetectorI Attach only light sensorI Write simple “Geiger counter”: More Light = Faster BeepingI Nothing fancy, just to get a soft start

Part 2: Static Cyclic SchedulingI Build a car from LEGO bricks, with motors and collision detectorsI Write four tasks to control it and a cyclic scheduler for them

Part 3: Adding a watchdog taskI Add a watchdog task to Part 2

Part 4: Multitasking in BrickOSI Restructure program to use multiple threads (one for each task)I Add light sensor and line tracking

Part 5: LEGO Car RaceI Tune car so that it can follow a track as fast as possible


LEGO Car Race

Track looks roughly like this:

Competition takes place on Fri, 11.12.I Fastest team wins! (Prize award included)I Timeout is 5 minutes (if more: assignment failed, fix car)

Keep in mind: Demo conditions might differ (different light etc.)

Some hints:I Build a physically robust car

F It should not break by itself!I No hard-coding of light thresholds

F Calibrate at program start!


The End

Questions?

(Now: Get LEGO boxes in groups of 3.)


Documents

Lab2 Intro