Programming%20with%20Lejos

1
Programming with Lejos

• Part 1

2
What is lejos?

• Cut-down version of Java for the RCX platform
• Includes
• Lejos API
• JVM
• API includes functionality needed to control
  motors, sensors, buttons as well as many
  components from the standard Java API

3
How it works

• Lego firmware is replaced with lejos JVM (only
  has to be done once)
• Program source code is written compiled on PC
• Compiled code (Java byte code) is downloaded to
  runs on RCX

4
JVM RCX technical specifications

• RCX has 32k of RAM
• 4K unusable
• 16K for Lejos
• 12K free memory for user code!
• Built-in support for
• Floating-point numbers (useful for
  trig/navigation)
• Limited multithreading
• 2-dimensional Arrays
• Limited recursion

5
Events the lejos API

• Objects their listeners
• Timer TimerListener
• Button ButtonListener
• Sensor SensorListener
• Object acts as event source when event occurs,
  all registered listeners we notified can
  execute appropriate code

6
Exception Handling Garbage Collection

• Lejos follows standard Java exception-handling
  model
• Lejos does NOT have automatic garbage collection
• Important to re-use objects rather the just
  creating new ones
• Forces good memory hygiene

7
Packages in the Lejos API

• josx.platfom.rcx includes classes for
  controlling
• 3 output ports (A, B C)
• 3 input ports (1, 2 3)
• 3 buttons (Prgm, Run, View)
• LCD display
• Speaker output
• IR communication
• There are also classes for accessing system time
  monitoring battery power

8
Packages in the Lejos API

• java.lang Includes some of the most commonly
  used standard classes
• Math
• String
• StringBuffer mutable version of String
• System
• Thread
• Throwable

9
Packages in the Lejos API

• java.util like java.lang, includes some familiar
  and/or useful classes
• Random
• Vector dynamic array
• BitSet series of bits

10
Packages in the Lejos API

• java.io provides support for transmitting data
  streams via the IR port
• Serializable interface
• InputStream, OutputStream, DataInputStream,
  DataOutputStream and IOException classes

11
Packages in the Lejos API

• josx.robotics generic classes methods for
  robot-specific tasks
• Behavior Control classes includes Behavior
  interface Arbitrator class
• Navigation classes

12
Classes in josx.platform.rcx

• Sound class contains a set of static methods that
  can produce predefined or custom sounds
• Sound.beep() // beeps once
• Sound.twoBeeps() // beeps twice
• Sound. buzz() // emits almost inaudible tone
• Sound. beepSequence() // series of
• // downward tones
• Sound. playTone (int frequency, int duration)
• MinSound class is a cut-down version - contains
  only the (deprecated) playTone method

13
Classes in josx.platform.rcx

• Unlike most Java environments, robots running
  Lejos are physical objects
• The RCX brick has 3 programmable buttons, 3
  sensor connections and 3 power output ports
• When programming for the RCX, these components
  are represented by predefined static objects it
  wouldnt make sense to instantiate more sensor
  ports, for example, because physically there are
  just 3

14
Classes in josx.platform.rcx

• The LCD, MinLCD and TextLCD classes relate to
  various aspects of the RCXs LCD display
• MinLCD, like MinSound, is a cut-down version
  primarily used to conserve memory
• LCD is used for number display, while TextLCD can
  display Strings

15
LCD class methods (all are static)

• void refresh()
• void setNumber (int code, int value, int point) -
  parameters
• code value Indicates use of signed or unsigned
  number, use of 5th digit
• value number to display
• point code indicating position of decimal point
• These 2 methods are also found in MinLCD

16
LCD class methods (all are static)

• void clear() clears display
• void showNumber(int i) displays an unsigned, up
  to 4-digit number
• void setSegment (int code) activates the display
  segment indicated by code codes are defined by
  static constants in the Segment interface

17
Example 1 Sounds Display
import josx.platform.rcx. public class
MakeNoise public static void main(String
args) throws InterruptedException int
freq // frequency of tone for (freq
440 freq lt 10000 freq 110 freq / 100)
// ascending tones
LCD.showNumber(freq)
Sound.playTone(freq, 50)
Thread.sleep(500) for
(freq 440 freq gt 20 freq 90 freq / 100)
// descending tones
LCD.showNumber(freq)
Sound.playTone(freq, 50)
Thread.sleep(500)
18
Classes in josx.platform.rcx

• Button class methods include
• public void addButtonListener (ButtonListener b)
• public boolean isPressed()
• public static int readButtons()
• Returns int with bits set 0x02 (view button
  pressed) 0x04 (prgm button pressed), 0x01 (run
  button pressed). If all buttons are released,
  this method returns 0.
• void waitForPressAndRelease () throws
  InterruptedException
• stops execution until button pressed released

19
Classes in josx.platform.rcx

• Button class also contains static fields that
  represent the RCX buttons
• Button.PRGM
• Button.RUN
• Button.VIEW
• ButtonListener interface specifies 2 methods
• public void buttonPressed (Button b)
• public void buttonReleased (Button b)

20
Classes in josx.platform.rcx

• ROM class provides access to embedded routines
  in the RCX ROM includes static method int
  getBatteryPower()
• Motor class includes methods controlling DC
  output to ports A, B and C
• Sensor class includes methods to communicate
  with sensor controllers 1, 2 and 3

21
Motor class methods mutators

• void forward() motor rotates forward
• void backward() motor rotates backward
• void stop() stops with braking action
• void flt() motor floats to a stop
• void reverseDirection() changes from forward to
  backward, or vice-versa
• void setPower(int power) sets motor power 7 is
  high, 0 low

22
Motor class methods accessors

• int getPower() returns current power setting
• char getID() returns motor ID (A, B or C)
• All of the following methods return a boolean
  value
• isBackward()
• isFloating()
• isForward()
• isMoving()
• isStopped()

23
Example
import josx.platform.rcx. public class Patrol
public static void main(String args)
throws InterruptedException
Motor.A.forward() while (true)
Motor.C.forward() // go forward
Thread.sleep (5000) Motor.C.reverse()
// turn around Thread.sleep (1000)

24
Sensor class

• There are several types of sensors available for
  the RCX, including
• Touch
• Light
• Rotation
• Because each operates differently, there is more
  complexity built into this class than in most of
  the rest of the lejos API

25
Sensor Type and Mode

• A predefined set of constants is used to
  describe
• the kind of sensor attached to a particular
  connector (type) and
• the kind of readings that are recorded by the
  sensor (mode)
• These values as used as arguments to the Sensor
  classs setTypeAndMode method, which is used to
  initially set up a sensor

26
Type and Mode examples

• Sensor.S1.setTypeAndMode(1, 0x20)
• // 1 represents touch sensor
• // 0x20 is boolean mode
• Sensor.S1.setTypeAndMode(3, 0x80)
• // 3 represents light sensor
• // 0x80 is percent mode

27
Example
import josx.platform.rcx. public class
GoForward implements SensorConstants public
static void main(String args)
Sensor.S2.setTypeAndMode(SENSOR_TYPE_TOUCH,
SENSOR_MODE_BOOL) // sets up touch
sensor using constants from SensorConstants
interface Motor.A.forward()
Motor.C.forward() while(
Sensor.S2.readValue() ! 1 ) //
loop ends ( robot stops) when bumper is touched

28
Methods of Sensor class

• public void activate ()
• activates sensor
• should be called after setTypeAndMode
• Methods that return Sensor readings
• public final boolean readBooleanValue()
• public final int readValue()
• public final int readRawValue()
• Sensor readings can also be handled by a
  SensorListener as described on the next slide

29
Sensor Listeners

• Sensor Listener interface specifies a single
  method
• public void stateChanged(Sensor s, int old, int
  new)
• s event source
• old previous value
• new new value
• As with the GUI components we have looked at
  before, the steps are
• define a class that implements the interface, and
  with the code for the method
• add the listener to the appropriate Sensor

30
Serial class Methods

• The Serial class is used for RCX to IR and RCX to
  RCX communication
• Methods work on the most basic level of network
  communication, passing around packets in the form
  of arrays of bytes
• A separate communications API provides support
  for higher-level operations

31
One more example
import josx.platform.rcx. public class
ListenerExample implements SensorConstants,
SensorListener public static void
main(String args) ListenerExample
exam new ListenerExample()
Sensor.S1.setTypeAndMode(SENSOR_TYPE_TOUCH, SE
NSOR_MODE_BOOL) Sensor.S3.setTypeAndMode(S
ENSOR_TYPE_TOUCH, SENSOR_MODE_BOOL)
Sensor.S1.addSensorListener(exam)
Sensor.S3.addSensorListener(exam)
exam.patrol()
32
Example continues
public void patrol () try
Motor.A.forward() while (true)
Motor.C.forward()
Thread.sleep(3000)
Motor.C.backward()
Thread.sleep(1000)
catch (Exception e)
33
End of example
public void stateChanged(Sensor source, int
oldValue, int newValue) if (newValue
1) if (source.getId() 0)
// left bumper Sound.playTone(220,
50) else if (source.getId() 2)
// right bumper Sound.playTone(660,
50) // end class ListenerExample