Ada User Guide for LEGO MINDSTORMS NXT
Contents
1. Decodes the buttons so that the main processor is able to tell which buttons are pressed and which are not Note that the co processor does not carry out any button debouncing If it is not handled at driver level the programmer should take care of it To handle all of the above it is necessary for main pro cessor and co processor to periodically exchange infor mation The communication between the two microcon trollers is set up as two memory allocations that on the original LEGO firmware are updated on both microcon trollers every 2 ms The communication interface op erates at 380 Kbit s using the I C hardware interface in both microcontrollers with the ARM main processor functioning as master 2 3 Output ports MO M1 GND POWER TACHOO TACHO1 Figure 2 Output port generic schematic All of the three output ports work in the same man ner see figure 2 They have a ground GND and a 4 3 V supply output POWER Two output signals M0 amp M1 that come from an internal H bridge motor driver that controls the motor standby forward reverse or brake modes This motor driver is governed by the PWM pulses generated by the co processor It also has two input sig nals TACHOO amp TACHO1 that are connected to the main processor s parallel input output controller PIO using a Schmitt trigger for noise suppression Within the Ada drivers these two last signals are used for the motor encoder The encoder has a resolut2. In this section we describe a way to remotely debug Ada C programs for the NXT using the GNU debugger GDB and the ARM Embed dedICE In circuit Emulator technology The ARM Em beddedICE is a JTAG based debugging channel avail able on the ARM main processor Debugging the NXT from a host computer through the available JTAG inter face is therefore possible RAM and Flash programming is also available using this method This solution has been adapted to work on GNU Linux x86 hosts but it could be easily ported to a Windows platform 6 1 Overview The JTAG based debugging channel provides real time access to memory addresses and data dependent watch points step by step execution full control of the central processing unit and other related debugging features It requires no use of memory unlike debugging monitor so lutions The ARM featured EmbeddedICE compatible macro cell from the NXT includes an ARM7 core a small amount of control logic a TAP Test Access Port con troller for the JTAG interface and an EmbeddedICE macrocell see figure 7 This EmbeddedICE macrocell has two real time watchpoint registers as well as control and status registers Each watchpoint register can be config ured as a watchpoint for data access or a breakpoint for instruction fetch If a match occurs between the values programmed into the watchpoint registers and the val ues of the address bus and data busses or some specific control signal the
3. The fourth section gives an overview of the development environment with a description of the tools required to work with the NXT As an example the development of a prototype vehicle is presented in section five Finally section six describes how the internal JTAG interface of the NXT is accessed and used to debug Ada programs Throughout this guide the AdaCore GNAT GPL for LEGO MINDSTORMS NXT 2011 hosted in GNU Linux for x86 available from http polaris dit upm es str projects mindstorms will be used but note that the Windows version is also available http libre adacore com libre tools mindstorms 2 MINDSTORMS NXT 2 1 Architecture overview The NXT kit comes with a programmable controller also called Intelligent Brick This Brick see figure 1 for its block diagram features a 32 bit ARM main processor AT91SAM7S256 with 64 KB of RAM and 256 KB of Flash memory that runs at 48 MHz To assist the main processor an 8 bit AVR co processor ATmega48 is also included Main processor and co processor periodically communicate through an I C bus Power supply Display SPI Bus ae Sound Main Processor Atmel ARM7 Co Processor Atmel AVR Bluetooth Bluecore 4 0 Output circuit Input circuit Figure 1 NXT block diagram It also has three output ports which are bidirectional to connect and control actuators such as electrical motors or linear actuators and four input ports that support both digital
4. 3 Uploading a program With no firmware when the orange button of the NXT Brick is pressed the ARM main processor executes the de fault Boot Program SAM BA Boot Assistant located in the first two sectors of the Flash memory The SAM BA Boot Assistant supports serial communications through the USB Device Port LibNXT is a utility library for communicating with the NXT Brick from a POSIX compliant host computer using USB When the ARM processor is in SAM BA mode ORK is an open source real time kernel that implements the Ravenscar profile for the GNAT compiler system on a bare LEON2 processor LibNXT is able upload the binary image file of the NXT executable to RAM and then execute it For Windows host platforms the Atmel SAM BA software is available 5 Vehicle Prototype This section describes the steps to have a working NXT vehicle prototype using Ada 5 1 Functionality The vehicle has a front castor wheel free to turn and two back wheels each driven by an independent motor To control the vehicle a hardwired joystick made with a touch sensor to start stop drive and a motors encoder to control operation is used Depending on the angle of the joystick encoder different speed commands are sent to the vehicle motors thus controlling vehicle motion see figure 4 touch sensor motor N Figure 4 Vehicle s joystick 5 2 Design and assembly Next step is to assemble a prototype that achieves the abov
5. 30 SWG single core polyurethane insulated cable 6 2 3 NXT Brick disassembly Take out the battery pack or batteries to gain access to the four Philips head screws Unscrew them and remove the front cover Remove the silicon rubber buttons as sembly Figure 8 NXT without front cover Find the two screws that hold down the LCD display located on each side of it the two small squares of figure 8 Loosen these screws and carefully lift the LCD display to get access to the battery terminals that are soldered to the main PCB Note that the LCD display cannot be removed from the PCB board on some models Once the two display screws have been removed the two battery terminals must be de soldered the two small circles of figure 8 To do this remove the solder with the soldering iron and the de soldering pump When the terminals are free of solder separate the PCB from the battery case and remove the input and output connector supports Note that there is a small silicone rubber push button between the battery case and the PCB 6 2 4 JTAG connection Since there was no short delivery 1 27 pitch connectors at the time The hard wired option presented below was used Cut 8 equal lengths at least 100 mm of the single core cable and strip 3 mm of insulation on one side Identify both ends with an indelible marker The JTAG interface J17 on the PCB is located below the loudspeaker beside the quartz crystal the big square of
6. analog sensors that do not need the special power measurement timing of the active sensors The power needs of these sensors are not covered via the analog pin ANA but via a specific pin POWER Note that the sampling of all the AVR A D converters occurs simultaneously so active and passive sensors must be sampled at the same rate 333 Hz All of the sensors packed with the LEGO MIND STORMS NXT are passive with the exception of the ultrasonic sensor e Digital sensors These sensors contain all the nec essary logic and processing resources to work inde pendently Thus they perform their function au tonomously and send or receive information to from the ARM via an I C channel DIGIO amp DIGI1 run ning at 9600 bit s where the ARM functions as mas ter These sensors are mapped as external mem ory areas from to which the programmer can read or write to control the behaviour of the sensor and harvest data For a memory arrangement that opti mizes read and write access refer to LEGO MIND STORMS NXT Hardware Developer Kit 2 The ultrasonic sensor is the only digital sensor packed in the NXT kit If a higher sampling rate is required by an analog in put the hardware allows configuring DIGI1 as an analog input Port 4 can also function as a high speed communication port It has a RS485 IC that allows for high speed bi directional multipoint communications 2 5 Bluetooth features The NXT Brick can be connected using Blue
7. scheduled This allows execution of a low prior ity task deferring execution of higher priority tasks thus minimizing priority inversion More information can be found in Annex D Real Time Systems of the Ada 2005 Reference Manual 5 When writing an Ada application for NXT you should bear in mind that only the Ada subset defined by the Ravenscar profile can be used for tasking These are some of the restrictions requeue statement e abort statements e Task entries e Dynamic priorities e Relative delays e Protected types with more than one entry e Protected entries with barriers other than a single boolean variable declared within the same protected type e Entry calls to a protected entry with a call already queued e select statements e Task termination For a full and detailed list refer to Guide for the use of the Ada Ravenscar Profile in high integrity systems 1 3 2 NXT Ada drivers The NXT drivers developed by AdaCore are completely coded in Ada These drivers are based on those of the LeJOS Project The LeJOS Project is a tiny Java vir tual machine ported to the NXT Brick in 2006 http lejos sourceforge net These drivers have undergone major updates in the last two versions of GNAT GPL for MINDSTORMS 2010 amp 2011 so 2010 programs might not compile with the 2011 compiler Unfortunately AdaCore does not supply API documentation with the drivers It is convenient to revise th
8. with embedded systems it uses a sequential Ada subset where not all language features are available For example attributes Image and Value are not included Moreover there is no exception propagation Unhandled exceptions jump to a last chance handler that can be reprogrammed as desired as long as the application then terminates it must not return to the caller Note that you must explicitly include the package NXT Last_Chance using a with clause for it to be part of your application If you do not a default handler is included that only displays an address for the exception on the NXT LCD screen For a full description of the Ravenscar SFP pro file refer to GNAT User s Guide Supplement for High Integrity Edition Platforms 4 The purpose of the Ravenscar profile is to restrict the use of many tasking facilities so that the outcome of the program is predictable For this purpose the profile is restricted to a fixed priority and pre emptive scheduling With fixed priority pre emptive scheduling the scheduler ensures that at any given time the processor executes the highest priority task of all those tasks that are currently ready to be executed Also the Immediate Ceiling Prior ity Protocol ICPP is enforced by the Ravenscar profile This means that when a task locks the resource its pri ority is temporarily raised to the priority ceiling of the resource thus no task that may lock the resource is able to get
9. ARM7 core ceases to read instructions from the data bus and isolates itself from the memory system entering debug state Access to the processor s state and memory system is then possible through the JTAG interface using the TAP controller GDB provides the remote serial protocol RSP for re mote debugging RSP is a GDB protocol used to send debugging commands through a serial or ethernet link Using a localhost TCP connection on the developer s host computer an OpenOCD daemon processes the commands issued by GDB OpenOCD The Open On Chip Debugger is an open source tool initially developed by Dominic Rath as part 4JTAG as defined by the IEEE Std 1149 1 standard is an in tegrated method for testing interconnects on printed circuit boards PCBs that are implemented at the integrated circuit IC level a TAP is the core of the JTAG standard It is a finite state machine that controls JTAG operations executable GDB cilia OpenOCD debugger SSS daem n USB protocol JTAG protocol a ege 1 01 U09 dY L ARM7 core ARM7 core EmbeddedICE __ macrocell _ Figure 7 ICE debugging solution for NXT of his diploma thesis at the University of Applied Sci ences Augsburg 9 This software provides debugging in system programming and boundary scan testing for embedded targets such as the NXT OpenOCD essen tially allows GDB to talk through a JTAG adapter to the EmbeddedICE com
10. Ada User Guide for LEGO MINDSTORMS NXT Peter J Bradley Juan A de la Puente Juan Zamorano Universidad Polit cnica de Madrid Madrid Spain http polaris dit upm es str Abstract The purpose of this guide is to introduce the robotics kit LEGO MINDSTORMS NXT to the Ada community All the steps required to complete a working Ada application running under the LEGO MINDSTORMS NXT are covered Keywords LEGO MINDSTORMS Ada Ravenscar Real Time Embedded Robotics 1 Introduction The LEGO MINDSTORMS NXT from now on NXT is a simple and flexible robotics kit that allows Ada pro grammers to develop applications that interact with the outside world by means of sensors actuators etc The dynamic features associated to this interaction with the physical environment require that the actions of the con trol software are executed at a specified time rate There fore real time constraints must be generally met Ada s concurrency and real time integrated features together with the use of the Ravenscar profile 1 makes it the ideal language for the NXT This guide is organised as follows The first section is this introduction Then the second section shows some fundamental aspects of the NXT hardware that should be kept in mind for NXT Ada development Section three briefly introduces Ada programming for the NXT tak ing into consideration the Ravenscar compliant NXT run time system and the NXT Ada drivers library
11. adapter information will probably have to be loaded also once it is plugged in sudo modprobe v ftdi_sio vendor Ox product Ox When the NXT has no firmware the orange button must be pressed Then when a clicking sound is heard the JTAG adapter must be plugged to the NXT Next arm eabi openocd must be run with a specific configu ration script arm eabi openocd f debug ram cfg This configuration file is a setup for OpenOCD that es tablishes communications with the NXT EmbeddedICE macrocell The script usually contains the daemon config uration that establishes communications with GDB the configuration for the adapter the board the target and some init commands JTAG adapter vendors usually pro vide this OpenOCD script and in case they do not the share openocd scripts folder from the install directory contains generic configuration files For further informa tion refer to Open On Chip Debugger OpenOCD User s Guide 10 When OpenOCD handshakes with the NXT success fully GDB must be run with the executable as parameter not with the binary image arm eabi gdb executable_name Any breakpoints should be added at this point After the gdbinit script see listing 2 must be run gdb gt source gdbinit Cross debugging is now possible Listing 2 GDB init script Init command target remote localhost 3333 Open0CD command to halt the processor and wait monitor soft_reset_halt OpenOCD com
12. and analog sensors Communications with the Brick are possible using ei ther USB via a full speed USB 2 0 port or Bluetooth available through a CSR BlueCore 4 chip that is con nected to the ARM s USART The USB 2 0 port is usu ally used to connect to a PC and Bluetooth to commu nicate with other NXT Bricks or any other Bluetooth enabled devices such as smartphones tablets etc On the top of the Brick there is a 100 x 64 pixel LCD display connected to the main processor via a SPI bus serial peripheral interface bus and four rubber buttons controlled by the co processor for interacting with the Brick The NXT Brick also comes with an audio amplifier connected to the ARM PWM pulse width modulation controller and a 16 Q speaker with a bandwidth of 2 16 KHz For schematics and further information refer to LEGO MINDSTORMS NXT Hardware Developer Kit 2 2 2 Processor and co processor The AVR co processor handles the following low level tasks for the main processor e Power management Turns the NXT Brick off and wakes it up when the center orange button is pressed It also monitors the battery status sending information to the ARM processor e PWM generation Generates pulses for the three output ports at a frequency of 8 KHz with the duty cycle specified by the ARM processor e A D conversion Performs a 10 bit digital conver sion of the analog signals at the input ports every 3 ms e Button decoding
13. de IAR Installing the JTAG connector IAR Kick Start for LEGO MINDSTORMS NXT 2009 Avail able from http www iar com website1 1 0 1 0 1483 1 Lynch JP Using Open Source Tools for AT9ISAM7S Cross Development Grand Island New York USA 2007 Revision C
14. dulability analysis of the system Ada development on the NXT presents a whole perspec tive of an embedded system with real time constraints At a reasonable price the NXT kit offers all kinds of sensors and mechanisms to work with even custom made sensors can be developed Development and sharing of Ada projects with the NXT would be of great interest in the same way as other complex models like Rubik s cube solvers Segway robots scanners etc have been developed using other program ming languages and shared The Ada community is encouraged to use this develop ment platform that besides the fun can be an interesting teaching asset It is important to note that all of the tools used except LDD are open source and therefore there is no dependance on software vendors All of the source code is available and can by modified Acknowledgements The authors would like to thank AdaCore for their work adapting the Ravenscar run time system and de veloping the Ada drivers for the LEGO MINDSTORMS NXT platform References 1 Burns A Dobbing B Vardanega T Guide for the use of the Ada Ravenscar Profile in high integrity systems Ada Lett 2004 June XXIV 1 74 Avail able from http doi acm org 10 1145 997119 997120 2 LEGO LEGO MINDSTORMS NXT Hardware De veloper Kit Version 1 00 Available from http mindstorms lego com 3 LEGO LEGO MINDSTORMS NXT ARM7 Blue tooth Developer Kit Version 1 00 Ava
15. e drivers code to understand how they work A full description of the drivers is out of the scope of this guide For every Ada NXT program the NXT AVR package must always be imported even if its functions are not required The body of this package contains a periodic task called Pump with the highest priority executed ev ery 20 ms that handles the co processor communications explained in subsection 2 2 using a circular buffer By adding a with clause to the main program and importing NXT AVR the execution of this task within the program is guaranteed It is also advisable to import NXT Display and NXT Last_Chance for exception handling High level access to motors and sensors is available through a series of object oriented interfaces that provide a tagged type a constructor function and some opera tions NXT Motors and NXT I2C_Sensors packages pro vide abstract types and primitive operations This ob ject oriented structure eases extending the code with new drivers for third party sensors For AVR connected pe ripherals analog sensors motors buttons etc the low level package NXT AVR can also be used Note that these drivers provide user transparent but ton debouncing through the NXT Filtering package Both AVR and Bluetooth interfaces perform checksum analysis for all data exchanged with the main processor to discard inconsistent data When using the concurrency features available with the Ravenscar profile it
16. e mentioned functionality The best way to do so especially if dealing with a complex design is to model it using a CAD tool LEGO offers a freeware software to develop NXT models LEGO Digital Designer 8 The vehicle prototype for this guide was modelled with LDD see figure 5 Although it can initially be somehow frustrating using these kind of tools decreases assembly time by allowing the development of several prototypes It lists the bricks used and generates a step by step building guide for the 2Example modified from Bradley et al 7 3This software is available for Windows and Mac OS LDraw and LeoCAD are other CAD software alternatives vehicle ixf LEGO Digital Designer ASF 2 OGixI a Bie A Bricks Templates HF crous lz o IIE lea B O O P Q D la A S S5 ONO OO X 5 XO ANN XO OxO 0 XO 0x0 0 0O O Figure 5 LDD model for the vehicle prototype model Figure 6 shows the vehicle prototype fully assem bled using the generated building guide from the LDD model 5 3 Software Architecture The following are the tasks involved in the software ar chitecture of the vehicle prototype e Control_Task Periodic task that executes every 20 ms It checks if the touch sensor is pressed a 20 ms period to detect a man operated touch sensor is considered sufficient In case it is it gets the value of the joystick motor encoder to determine the s
17. figure 8 Pin 1 has a square pad and the remaining pins have round pads Insert one by one the stripped ends of the 8 cables in pins 1 8 and solder them to the board This type of wire is used because unlike PVC insulation it supports high temperatures 155 C and makes soldering easy With the magnifying glass inspect each solder for bridges between pins See left picture from figure 9 for the final result Figure 9 Soldered JTAG interface amp front cover drilled hole Drill a 4 mm hole on the front cover of the NXT Brick directly above the J17 connection as shown in the right picture of figure 9 As a strain relief bundle the eight wires together and tie a knot with them 20 mm from the PCB Take them through the hole of the front cover and cut them to length for the connection to the ribbon cable connector according to figure 10 As the wire used has a smaller gauge than the connector it is advisable to solder the connections after inserting them Therefore strip the wires insert them and solder them Try to use as little solder as possible to allow inserting the header in the connector A R89 R90 RA4D RA4C RA4B RA4A 10K 10K 10K 10K 10K 10K a N TP84 783 l T TP82 O To TP85 O NRST TP 86 O T GND PULL UP 10K VCC 3V CONOOBRWDND Small connector sin
18. gle row 1 27 pitch 20 pin ribbon cable connector ARM JTAG connector VCC 3V VCC 3V GND 2 1 PULL UP 10K GND j 3 TDI GND 6 5 TMS GND A TCK GND h ik GND e 13 TDO GND NRST 16 15 GND 18 17 GND 20 19 Figure 10 NXT JTAG hardware schematic Note that the GND connection is only connected to pin 6 because the JTAG adapter used has all the GND pins internally connected 6 2 5 Testing the connections Locate on the NXT Brick PCB resistor R89 check for continuity with the multimeter in Q between the top of R89 and pin 2 of the ribbon cable connector VCC 3V Check that the other end of R89 is connected to pin 3 of the ribbon cable connector PULL UP 10K Next check the GND connection between pin 6 of the ribbon cable connector and the negative battery terminal PCB connection J5 Locate test points TP82 TP86 on the solder side of the PCB and check with the multimeter for continuity between them and the corresponding pins of the ribbon cable connector Also check for short circuits between connections Finally once the connections have been checked re assemble the NXT Brick For a more graphical guide on the modification of the NXT Brick refer to Installing the JTAG connector 11 6 3 A debugging session In order to remotely debug programs under GNU Linux libusb 0 1 libusb dev libftdii and libftdi dev are required The FTDI module with the JTAG
19. ilable from http mindstorms lego com 4 AdaCore GNAT Pro User s Guide Supplement for High Integrity Edition Platforms 2011 The GNAT Ada Compiler GNAT GPL Edition Version 2011 Document revision level 175263 5 Std 8652 1995 Amd 1 2007 Ada 2005 Refer ue ence Manual Language and Standard Libraries 2007 Published by Springer Verlag ISBN 978 3 540 69335 2 de la Puente JA Ruiz JF Zamorano J An Open Ravenscar Real Time Kernel for GNAT In Pro ceedings of the 5th Ada Europe International Con ference on Reliable Software Technologies Ada Europe 00 London UK Springer Verlag 2000 p 5 15 Available from http portal acm org citation cfim id 646579 697613 Bradley PJ de la Puente JA Zamorano J Real time system development in Ada using LEGO MIND STORMS NXT In Proceedings of the ACM SIGAda annual international conference on SIGAda SIGAda 10 New York NY USA ACM 2010 p 37 40 Available from http doi acm org 10 1145 1879063 1879077 8 LEGO LEGO Digital Designer 4 1 User Manual 9 10 11 12 ue 2011 Available from http ldd lego com Rath D Open On Chip Debugger Design and Im plementation of an On Chip Debug Solution for Em bedded Target Systems based on the ARM7 and ARM9 Family University of Applied Sciences Augs burg 2005 Brownell D Open On Chip Debugger OpenOCD User s Guide 2011 Available from http openocd berlios
20. ion of 360 counts per revolution When the motor rotates the ARM proces sor receives an interrupt in order to update the encoder counter through the parallel I O controller Notice that clockwise and counterclockwise operation is detected by the counter s increments or decrements 2 4 Input ports Depending on the type of sensor connected to the NXT Brick the input ports behave differently The input ports allow both digital and analog interfaces see figure 3 N Vcc 5 V Figure 3 Input NXT generic schematic LEGO considers three types of sensors e Active sensors These kind of sensors belong to the previous version of LEGO MINDSTORMS the RCX They require an NXT adapter cable NXT firmware provides the same functionality available in the RCX Bricks by using an extra current source This current source delivers power approximately 18 mA to the active sensors It supplies power to the sensor through the analog pin ANA during 3 ms and then measures the analog value during the following 0 1 ms The AVR sends the 10 bit digital conversion of the analog value to the main processor using the scheme presented in section 2 2 When using these kind of sensors e g RCX light sensor RCX rotation sensor be sure to set the appropriate input power settings by calling Set_Input_Power sensor_id RCX_9V from NXT AVR driver package where sensor_id is the input port used for the active sensor e Passive sensors These kind are
21. laration tasks ads and its body tasks adb The Tasks pack age includes the two control tasks Control_Task and Display_Task the empty procedure Background and some auxiliary functions Listing 1 shows a fragment of tasks adb containing the declaration of the two tasks and the background procedure When declaring a task besides using pragma Priority to establish the static pri ority pragma Storage_Size is used Pragma Storage_Size specifies the amount of memory to be allocated for the task stack Notice that this pragma is required because of the small amount of memory available 64KB of RAM memory The stack size must not be exceeded If it does a Storage_Error will be raised If this Storage_Size pragma is not used a compiling error about RAM over flowing could be prompted It must be remembered that the clock resolution de fined by the run time system is of 1 ms Listing 1 Specification of tasks procedure Background is begin loop null end loop end Background task Control_Task is pragma Priority System Priority First 2 pragma Storage Size 4096 end Control_Task task Display_Task is pragma Priority System Priority First 1 pragma Storage Size 4096 end Display_Task 6 Debugging Solution A remote debugger is an extremely useful tool for an em bedded system developer It can drastically decrease de velopment time There is no open source Ada C debug ging solution for the NXT
22. mand to select the core state monitor arm core_state arm set flash wait state AT91C_MC_FMR monitor mww Oxffffff60 0x00320100 watchdog disable AT91C_WDTC_WDMR monitor mww Oxfffffd44 Oxa0008000 enable main oscillator AT91C_PMC_MOR monitor mww Oxfffffc20 OxaQ000601 watt 100 ms monitor sleep 100 set PLL register AT91C_PMC_PLLR monitor mww Oxfffffc2c 0x00480a0e watt 200 ms monitor sleep 200 set master clock to PLL AT91C_PMC_MCKR monitor mww Oxfffffc30 Ox7 wait 100 ms monitor sleep 100 enable user reset AT91C_RSTC_RMR monitor mww Oxfffffd08 Oxab5b000401 force a peripheral RESET AT91C_RSTC_RCR monitor mww Oxfffffd00 Oxa5000004 toggle the remap register to place RAM at Ox00000000 monitor mww OxffffffOO 0x01 set the PC to Ox00000000 monitor reg pc 0x00000000 enable use of software breakpoints monitor gdb_breakpoint_override soft monitor arm 7_9 dbgrq enable upload the application load resume execution from reset vector continue This GDB script basically sets the ARM processor to ex ecute the application and set some debugging features The script used is a modified version of that presented in Using Open Source Tools for AT91SAM7S Cross Devel opment by James P Lynch 12 7 Conclusions This guide shows the basics for Ada development using LEGO MINDSTORMS NXT The Ravenscar profile run time system offers concurrency Ada programming while making possible a sche
23. must be considered that the display and AVR drivers do not implement a thread safe environ ment LCD data and the circular buffer with the outgo ing messages to the AVR are defined as global variables with no access control For concurrent access to the dis play the NXT Display Concurrent package provided can be used For AVR concurrent access a thread safe solution must be provided by the user to avoid race conditions when calling Power Down Set_Power and Set_Input_Power procedures Notice that because of the periodic task that handles ARM AVR communications every time a motor is used or a power down to the NXT is set race condition issues are present The 2010 GNU Linux GNAT version provided modified drivers that addressed this issue but since the 2011 GNU Linux version changed its interface the solution has not yet been adapted 4 Development Environment 4 1 Tools overview A cross compiler toolchain is a set of tools essentially a compiler an assembler and a linker that create exe cutable code for a platform in this case the NXT main processor ARMv3 architecture other than the one on which the tools run that is GNU Linux x86 Cross compiler toolchains are used to compile code for a plat form upon which it is not feasible to do the compiling AdaCore has ported the GNAT compiler toolchain to the ARM architecture by porting part of the LEON based Open Ravenscar Real Time Kernel ORK developed by a team of the Depar
24. patible macrocell on the NXT A JTAG adapter is a piece of hardware that connects the host computer with the JTAG interface of the remote target The JTAG adapter is in charge of adapting the serial electric signalling received from OpenOCD using in this case an FTDI chip to send the JTAG operations to the TAP controller Figure 7 shows the debugging scheme 6 2 Modifying the NXT Brick To connect GDB in the host computer with the JTAG interface of the NXT a JTAG adapter is required Also The NXT Brick PCB has the provision for mounting a JTAG connector but this has not been mounted to save cost The NXT Brick must be opened in order to ac cess the JTAG interface Note that by performing this modification warranty will be lost 6 2 1 EFTDI based JTAG adapter An FTDI based JTAG adapter that is both compati ble with OpenOCD and the main processor of the NXT AT91SAM7S256 is required For this guide the ARM USB TINY H adapter by Olimex http www olimex com was used Open On Chip Debugger OpenOCD User s Guide 10 offers other vendor options 6 2 2 Tools and materials e Small Philips head screwdriver e Fine wire cutter Hardware solution to interface with USB peripherals e Wire stripper e Soldering iron with a fine tip and solder e De soldering pump e Magnifying glass e Drill with 4 mm diameter bit e Digital multimeter e 20 pin 2 54 pitch ribbon cable male connector ARM JTAG connector e
25. peed commands that are then stored in the circular buffer These speed commands are later sent to the AVR by the Pump task The task takes the position of the joystick motor at the beginning of its execution as reference point It also checks if the orange button is pressed to switch off the NXT Brick e Display_Task Periodic task that executes every 500 ms with a lower priority than Control_Task This task shows the joystick s position the execution time and the battery s mV on the LCD screen Background procedure This is just a background procedure that executes every time the ARM pro cessor is free Although the application performs as expected the cir cular buffer global variable used for the ARM AVR com munications is not thread safe and a race condition exist This race condition may or may not happen and if it happens it does not necessarily mean the performance of the vehicle will be affected Nevertheless it is not a good programming practice to rely on non controlled access to a global variable There is a thread safe vehicle version using the 2010 modified AVR drivers that can be downloaded from http polaris dit upm es str projects mindstorms 2010 front castor re Figure 6 Vehicle prototype fully assembled 5 4 Software Implementation Three compilation units are used for the Ada vehi cle application The main procedure vehicle adb that calls Background procedure a package dec
26. tment of Telematics Engineering from the Technical University of Madrid DIT UPM 6 4 2 Compiling a program The NXT s original firmware for the main processor is completely removed this invalidates the warranty and replaced by a binary image of the user s Ada application that is executed from RAM Flash memory is not used This means that every time a program is executed it must first by uploaded to RAM Instead of using the widespread ELF as executable file format the EABI format is used by the GNAT cross toolchain EABI has been created as a common binary interface so that object code and libraries created with one toolchain can be linked to a project created with a different one To generate an executable NXT file from the user s Ada application the GNAT cross toolchain needs first to compile and then link to RAM all compiled code using kernel_samba 1d linker script The code that needs to be compiled is the user s Ada code the run time sys tem the Ada NXT required drivers nxt_main C func tion main c a low level routine to initialise the system init s a low level interrupt handler routine irq s a vector table that is remapped to RAM vectors s by init s and the elaboration code generated by the GNAT binder A GNU make script Makefile inc is in charge of building the binary image that is uploaded This script compiles the run time libraries every time since precom piled library units are not used 4
27. tooth to any other Bluetooth device that implements the Serial Port Profile SPP a serial cable emulation profile The ef fective working Bluetooth range for the NXT Brick is approximately 10 m Bluetooth Class IT device The NXT Brick provides a master slave communica tion scheme with four channels Channel 0 is used when working as slave and the other three when working as master The NXT Brick can either work as master or slave This means that when the NXT Brick works as master it can communicate with three more devices The CSR BlueCore 4 firmware is implemented as a vir tual machine with an integrated command interpreter Thus communication between the main ARM processor and the Bluetooth chip is handled by a set of defined commands and data streams that are exchanged through the USART channel Refer to LEGO MINDSTORMS NXT ARM7 Bluetooth Developer Kit 3 for a full speci fication 3 Ada programming for NXT 3 1 NXT run time system The AdaCore GNAT GPL for LEGO MINDSTORMS NXT 2011 cross compiler toolchain relies on a Raven scar small footprint run time system Ravenscar SFP It is really a superset of the zero footprint profile It adds the specification of a secondary stack mechanism for unconstrained objects and the Ravenscar tasking fea tures to the zero footprint profile This means that Ada applications for the NXT should comply with the Raven scar profile for tasking purposes Also as it is targeted for use
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