Realtime Kernel Implantation in a PIC Microcontroller
Contents
1. thanks to primitives cooperation and interruptions 1 2 Brief outline of the Project The teacher in charge of the project would like to introduce a new series of hands on seminars on real time embedded systems more precisely on a microcontroller PIC within the scope of an 3rd year option in telecommunications In order to prepare these seminars the group worked on a 16F877 PIC integrated in an electronic card see Figure 1 Reset button Interruption button Programming working mode switch Analogical sensor LM35 Digital sensor DS1620 LCD screen 2x16 Figure 1 Electronic board layout The embedded operating system chosen is Salvo which is dedicated to the Microchips PIC family of microcontrollers 1 3 Project Goals The desired outcome is to program the PIC using Salvo functionalities and a package of low level functions lof 8 A schedule of conditions was imposed the software had to enable regular updating of the current analogical and digital temperatures displayed on the LCD provided by two sensors while a LED had to blink as a heartbeat The LED blinking shows the system is operational A change in the kind of displayed temperatures will occur when a user exerts pressure briefly on the interruption button e At the first push both analogical and digital minimal temperatures are displayed print_mode MINI e At the second push the two maximal temperatures are displayed print_mode MAXI One2. variable timeout_print_mode is decreased incrementally and once it has reached 0 the value of print_mode is set to CURRENT Otherwise the timeout is reloaded Interrupt Handler Timer Software Prescaler The timer hardware prescaler is configured through the PS0 PS1 and PS2 bits of the timerO register to trigger off timer interrupts every 512 microseconds Yet it is said in the Salvo documentation that OSTimer that increments Salvo ticks has to be called at a period less than 5 msec That s why we had to generate a software prescaler which divides the hardware rate by 195 The out rate of this scaler is a call to OSTimer every 100 msec Interrupt Handler Reinitialization of Minimal and Maximal Temperatures This is performed using a timeout like for the display mode Every time a timer interrupt occurs the program reads the state of the input pin to which the button is connected this state is inverted reading 0 means the button is pressed and vice versa If it is pushed the counter timeout_longbp is decreased Once it has reached 0 all extrema values are reset and the display mode is set to RAZ_EXTREMA If the button is not pressed the counter is reloaded 6 of 8 Interrupt Handler External Interrupt Pressing the button triggers off an external interrupt It changes the value of the external interrupt flag INTF to 1 As any interrupt leads to calling the interrupt handler function checking the flags is
3. AB Configuration The Integrated Development Environment we used called MPLAB was some trouble to configure A strict and non intuitive order has to be followed in order to allow more than one file to be included in the software project The same kind of trouble made us spend a couple of hours selecting and adding right options for the integrated compiler Moreover the text editor into which we typed the software code was very capricious since most of the time it refused to handle the keystroke necessary for writing or Contrary to smart editors like Emacs or Vim MPLAB doesn t allow features such as syntax highlighting and coloration automatic indentation and auto completion Salvo Configuration Since Salvo was designed to run on many families of PIC microcontrollers and even now on Motorola s or other hardware architectures the configuration files are somewhat complicated The specifications of each microcontroller such as the number of ports the functions assigned to each pin and even the name of the internal 7 of 8 registers have to be handled by them A define SyYSA declaration needed to be added to the compiler command line in order to specify the family of PIC we used The configuration how to is written in the Salvo manual but since the manual is a bit confusing and disorganized we did not see this part on our first reading Salvo is really not easy to configure unless the user reads its manual carefully Long Pr
4. Real time Kernel Implantation in a PIC Microcontroller Yves Gr alou Pierre Gu ant Wilfried Jouve Micka l Le Baillif grealou gueant jouve lebailli enseirb fr Abstract The purpose of the project was to develop a Salvo based program to display analogical and digital temperatures computed by 2 sensors on the LCD screen By successively pressing the button the program has to display the current temperatures the minimal temperatures and then the maximal temperatures with a return to the current temperatures after a few seconds Extended pressure on the button reinitializes the minimal and maximal temperatures 1 Introduction 1 1 Real Time Specifications A system works in real time when there are constraints upon the time which are respected The whole processing time matches up to the time taken to e Acquire data from sensors e Process this data e Give the result displayed on the LCD in the project Thus the following inequality has to be respected T1 T2 T3 lt Tlimit In the project a time lower than 500 milliseconds was required In order to respect the time constraints the real time operating system RTOS has to include among others three features a real time clock an interrupt mechanism and a priority mechanism between tasks The RTOS Salvo provides the minimum requirements to implement this kind of system These are mainly task scheduler synchronization and communication These possibilities are implemented
5. ch as short signed char or unsigned long is meaningless We have preferred to rename them with more comprehensible labels one letter followed by two digits The letter can be u or s meaning unsigned or signed The two digits indicate the memory space required for the storage 08 16 or 32 bits Table 7 lists all the types we have defined Table 7 Variable Typing signed unsigned Friendly type Compiler type Range Friendly type Compiler type Range 8 bits S08 signed char 128 127 u08 unsigned char 0 255 16 bits S16 signed short 32768 32767 u16 unsigned short Q 65535 32 bits 32 signed long 292 QM21 u32 unsigned long 0 27 1 Using understandable names for types is all the more important as we worked on an embedded software program The memory size is very limited so we had to be careful to use the minimal variable size according to its usage A good example of such an optimization is the declaration of the variable used to avoid the bounce phenomenon in the interrupt handler IntVector The maximum value of this variable can be fixed through the macro constant ANTI_BOUNCE A simple test computed by the C preprocessor compares ANTI_BOUNCE with 255 and eventually with 65535 If the first test Is ANTI_BOUNCE inferior or equal to 255 is positive the type of the variable is u08 Otherwise the second test is conducted If it is positive the type is u16 else it is u32 MPL
6. d it is useless to make the lowest priority task wait So all this leads us to set the temperature acquiring task as a lower priority than the LCD task LED Blinking Task and Display Task The LED blinking is useful to inform the user about the actual working of the program Indeed if the program crashes the LED will stop blinking At first glance this task seems to be the least 4of8 important However this task is also used to establish the rhythm of the display Indeed before displaying a new set of temperatures the display task will wait for a semaphore function OS_WaitSem This semaphore is released by the LED blinking task after a delay of 500 milliseconds functions OS_Delay OSSignalSem So every shift of the LED state leads to new values of temperatures on the LCD screen As the display has to wait for a call to the LED blinking task it is obvious that the display task is the most important task LED Blinking Task and Temperature Acquisition Task The LED blinking task is shorter than the temperature acquisition one Moreover if the priority of the LED blinking task was lower than the temperature acquisition one the priority of the LED blinking task would be the lowest of the three and so the LED priority blinking task would not ever be scheduled Indeed the display task will wait for the semaphore and then the acquisition task will take the CPU and will not release it Even if this task calls a oS_yield the scheduler w
7. essure on the Button Catching an interrupt is not sufficient to know how long the button has been pressed Therefore we had to consider the button as an input and not as an interrupt trigger Timer Interrupt Frequency The notion of time in a real time operating system is critical In our system it is managed thanks to periodic interruptions generated by an internal timer timer0 This timer can be configured through a register mainly the hardware prescaler value Indeed the principle of the timer interrupts is as follows the external clock of the PIC is first divided by 4 constant factor to get a 1 MHz input clock Then this periodic signal goes into a software configurable prescaler The output frequency determines the timer interrupt frequency The problem we have encountered is an inexplicable slow down of the frequency We have set the prescaler to its maximum value 1024 Thus the output frequency should have been about 1 KHz and cannot be set to be slower However we notice that it sometimes reached as low as 15 Hz This variation even occurred while the software was running even if it changed most of the time when we recompiled the code and reprogrammed the microcontroller We did not understand how this could happen but we have fixed it by correctly setting the timer register in the Salvo configuration instead of in our own configuration function pic_init 5 Conclusion 5 1 Comments About the Project Developing an embedded sof
8. he tasks is illustrated in Figure 2 t 0 1 Display task 2 LED blinking task 3 Acquisition task g LED state A Semaphore value Q 0 1 OS_WaitSem Waits for the semaphore 2 OSSignalSem Release the semaphore w i 3 OS_Yield The scheduler choose a new task Monopolisation CPU 4 Display new set of temperatures Figure 2 Schedule of the Three Salvo Tasks 1 oS_yield This function allows the current task to call the scheduler in order to release the CPU to the highest priority task among those eligible 5of8 4 Development and Experimentation Details 4 1 Software Explanation The application is written in the file main c divided into eight functions as shown on Table 6 Some useful shareable functions have been written in the file 1ib1 c Table 6 Functions that can be found in file main c Function name and prototype Description int main void The main function launched on power on and reset void Print void The task which displays the temperatures on the LCD void Acquisition void The task which acquire the data from the sensors void Born_to_be_alive void The task which informs the user that the system is running void interrupt IntVector void The interrupt handler used both for external and timer interrupt void pic_init void Initialization of the PIC internal registers void accueil void Friendly message displayed a
9. icrocontroller The human interface is quite light a LCD display 2x16 characters a LED and a button There is also a reset button useful in case of a system crash The microcontroller can be programmed from a PC using a RS 232 serial interface obtained with a MAX232 driver connected to the Parallel Port of the PC 2 of 8 2 2 PIC Microcontroller Overview The core of the electronic card is the Microchip 16F877 PIC microcontroller It is used with a clock rate of 4 MHz It is shipped with 8K of Flash ROM for program storage 368 bytes of RAM for static variables stack internal registers etc and 256 bytes of EEPROM not relevant in our project One input pin is used as an external interrupt linked to the button Another pin is an output for the LED Other pins are used for the LCD the sensors and the PC serial interface 2 3 Salvo Salvo is a cooperative real time operating system RTOS that is designed for single chip microcontrollers with severely limited RAM and ROM The maximum stack level is typically 4 so it can be used with low cost microcontrollers such as Microchip PIC family 12000 14000 16000 17000 or 18000 In fact we used Salvo Lite a freeware version of Salvo Some of Salvo Lite s main features are e Cooperative event driven priority based multitasking RTOS e Designed for processors with severely limited RAM e g less than 256 bytes e Works within a hardware call return stack of 8 levels or fewer 4 is t
10. ill give the CPU to the highest priority task among those eligible the acquisition task So the LED blinking task would never have the CPU and could not wake up the display task It follows that the priority of the LED blinking task has to be higher than the acquisition task Conclusion Priority levels were chosen from 1 to 3 1 given the highest priority me display task 2 LED blinking W temperature acquisition 3 4 Task progression Being the highest priority the display task takes the CPU and then waits for the semaphore the semaphore is initially locked The scheduler attributes the CPU to the second highest priority task the LED blinking task So the LED switches on and the task executes an OS_Delay which makes it unavailable for the scheduler for 500 milliseconds The two highest priority tasks being unavailable the scheduler lets the acquisition task take the CPU After every acquisition this task calls the scheduler Thus once the LED task finishes its delay of 500 milliseconds the LED task can be elected by the scheduler led task priority gt acquisition task priority Then the LED task releases the semaphore by using the function OSSignalSem which calls the scheduler The scheduler chooses the display task which is now eligible thanks to the LED task The temperatures are displayed according to the current mode then the display task waits again for the semaphore The schedule of t
11. k Function Acquisition The goal of this task is to retrieve analogical and digital temperatures computed by the sensors Then the results are stored in the two variables listed in Table 4 Table 4 Variables for Current Temperatures ul6 currentAna the analogical temperature ul6 currentNum the digital temperature The new minimal and maximal temperatures can be inferred from comparing the previous minimal and maximal temperatures with the current temperatures These minimal and maximal temperatures are stored in four variables Table 5 Variables for Extrema Temperatures ul6 miniNum ul6 maxiNum ul6 miniAna ul6 maxiAna Digital temperatures Analogical temperatures LED Blinking Task Function Born_to_be_alive This task deals with the shift of the LED state Every call of this task triggers a shift in the LED state If the LED is on the task will switch it off if not the task will switch it on 3 3 Choice of Priority Temperature Acquisition Task and Display Task The logic is to begin with the acquisition and then to display the temperatures which were computed So it would be tempting to make the acquisition a higher priority than the display task However the display task cannot be considered a background task since it would lead to too swift a refresh rate making the LCD difficult to read Besides the display task waits for a semaphore before displaying a new set of temperatures an
12. lopment of An Acquisition card based on a microcontroller Micropchip PIC from the electronic department of ENSEIRB 2002 2003 4 Datasheet of the PIC 16F87x series 8 of 8
13. more press enables the LCD to come back to the current temperatures print_mode CURRENT If the LCD stays in a mode different from the CURRENT one then a timer enables the system to switch back to the CURRENT mode after a couple of seconds When a user presses the interruption button for a long time the minimal and maximal values will be reset 1 4 Available Hardware and Software The development tools were installed on a PC which was equipped with Windows 98 They include an Integrated Development Environment IDE named MPLAB and a PIC programmer named Universal PIC Programmer v 6 4 0 Moreover some documents were provided such as e User s Guide PIC Ansi C Compiler Ed Hig Tech Software 2004 Salvo User Manual v 3 1 0 Ed Pumpkin 2004 e Documents detailing the development of An Acquisition card based on a microcontroller Microchip PIC from the electronics department of ENSEIRB 2002 2003 It describes the low level functions included in the library given with the subject of the project Each state of PIC pins linked with a component of the circuit is explained e More precisely the datasheet of the PIC 16F87x series enabled us to know the functions of each pin and the description of the internal registers for instance one of the three internal timers has been configured to generate periodical interrupts The Demo Lite version of Salvo and a library of the low level functions fitted to the b
14. oard were provided as well 1 5 Project Development The project was divided into two parts First there was a sequence of tests developed from the low level functions Second the project itself was carried out It includes the use of Salvo structure and the compilation of Salvo In addition a technical manual of the used Salvo functions has been written to allow the 3 year students to understand the code and to update it Tests In a first step the test of the LED was implemented Then the interruption button test used the LED s blinking to see if the program was still running Then the LCD LM35 and DS1620 tests alternated the test of each function meanwhile the LED was blinking to check the activity of the Microchip PIC Salvo First of all functionalities were grouped in three tasks with different priorities Thereafter the implementation began with the use of the previous low level functions library the basic functions of Salvo and the previous tests 2 Project Description 2 1 Acquisition Card Description The purpose of the electronic card is to develop a digital thermometer Therefore two thermal sensors are soldered onto it The LM35 sensor is analogical it delivers a signal whose voltage is linearly proportional to the temperature The PIC has a built in Analog to Digital Converter to which the sensor is connected The second sensor the DS1620 has a digital interface It can be directly initialized and controlled from the m
15. r called LM35 Two functions from the LM35 library are used They are listed in Table 2 Table 2 Analogical Sensor Functions void 1m35_init to initialize LM35 ul6 1lm35_readtemp to read the current analogical temperature The u16 value returned by the function 1m35_readtemp is displayed thanks to the function print_int developed in libl c Test of the DS1620 Digital Sensor This test consists in displaying the temperature computed by the digital sensor called 3 of 8 DS1620 Three functions from the DS1620 library are used They are listed in Table 3 Table 3 Digital Sensor Functions void dsl1620_init to initialize DS 1620 void ds1620_start to start a new conversion ul6 dsl620_readtemp to read the current analogical temperature The u16 value returned by the function ds1620_readtemp is displayed thanks to the function print_int developed in libl c 3 2 Description of the Tasks The system consists of 3 tasks The first task handles the display The second task handles the LED blinking The third task handles the temperatures acquisition Display Task Function Print Depending on the current mode this task displays on the LCD The current analogical and digital temperatures print_mode CURRENT The minimal temperatures print_mode MINI or the maximal temperatures print_mode MAXI recorded until now Temperatures Acquisition Tas
16. t power on time void init_value void Initialization of the global variables Main Interruption Enabling The interruption are allowed by calling OSEi mainly for timer interrupt and by positioning INTE to external interrupt enabled The call to init_value has to be made after allowing these interruptions otherwise a pending external interrupt would change the value of print_mode and the system would start displaying the minimal temperature instead of the current values Acquisition A call to ds1620_start triggers off a new conversion The value stored in the DS1620 sensor is retrieved via ds1620_readtemp the LM35 via 1m35_readtemp These values are compared with the minimal and maximal ones which are updated if necessary Next the task calls OS_Yield to allow pending tasks to be executed Born_to_be_alive Principle Every time this task is scheduled it toggles the LED state It is delayed for 500 msec thanks to OS_Delay 5 As the Salvo tick period is 100 msec see the Interrupt Handler function waiting for 5 Salvo ticks corresponds to 500 msec The task is also used for handling the timeout used for switching back to the display of current temperatures as explained below It is important to delay this task before signaling the semaphore for the first data to be acquired before it is displayed Born_to_be_alive Q Display Timeout If the global variable print_mode is different from CURRENT the
17. the only way to know which interrupt has occurred but it is not sufficient for the external interrupt a mechanical bounces phenomenon generates a huge number of rising edges that produces an interrupt It is possible to avoid it thanks to an active loop into the interrupt handler if an external interrupt has been detected the loop runs until the transitional phenomenon ends The flag INTF has to be manually reset to 0 Next the program can perform actions needed to handle the interrupt incrementing the variable print_mode that is to say changing the temperatures on the display However an external interrupt can occur when a reset of the extrema values is requested by the user In this case the display mode is not changed to ensure that the informational message will be displayed Initialization of the Global Variables The initialization of the minimal temperatures cannot be accomplished by setting them to 0 since 0 is the minimal value the system can compute and therefore the minimal temperatures would not be updated The solution we have chosen is to set them to 1 As the storage variables are unsigned 1 stands for the maximum value all bits equal to 1 These false minimal temperatures will not be displayed as the values that will be printed next will be current temperatures both at startup or at reset requested by the user the print_mode variable is set to CURRENT 4 2 Difficulties and Trouble Variable Typing Manipulating types su
18. tware program is really exciting as some aspects we don t pay attention to in computer software development have to be considered memory usage hardware storage of variables bit manipulation registers configuration and many other low level concepts The development method is also interesting starting with easy to use interfaces such as a LED or an interrupter the system progressively emerges from nothing to finally carry out the desired tasks However problems resulting from the use of the IDE and the programmer as well as Salvo were disarming The IDE required a huge amount of time to configure Moreover the editor in MPLAB was not as smart as Emacs The programmer was very slow under Windows NT as a result the group asked for a new computer with Windows 98 To fit the numerous kinds of PIC the programming tools were adaptable However the configuration was not immediate and not obvious As a consequence we spent a lot of time on mastering our development environment 5 2 Salvo Use in Everyday Life RTOS are commonly used in applications with critical time and reliability requirements such as military and aerospace applications but they are also used by telecommunications equipment manufacturers in data modems communication servers or ATM switches for example References 1 User s Guide PIC Ansi C Compiler Ed Hig Tech Software 2004 2 Salvo User Manual v 3 1 0 Ed Pumpkin 2004 3 Subject of the deve
19. ypical e Number of tasks and events limited to three Events allowed include semaphores messages and message queues 3 Programming Strategy 3 1 Tests Test of the LED The test of the LED involves making it blink First the LED port is initialized then in an infinite loop the LED pin is alternately activated and deactivated RB5 1 or 0 A macro WAIT is implemented to allow users to see the shift between states Test of the Button This test involves shifting the state of the LED by pressing the button Pressure on the button triggers an interruption INTF 1 which is processed by the interrupt handler written in the function void interrupt IntVector void Then the pressure produces a shift only if the interruption is still valid after several milliseconds Indeed the interrupt handler has to avoid generating a quick host of shifts of the LED state on account of the bounce phenomenon Test of the LCD This test consists in displaying characters on the LCD screen Four functions from LCD library are used listed in Table 1 Table 1 LCD Functions led_init to initialize LCD lcd_clear to clear the display lcd_pos line column to position the current cursor line from 0 to 1 and column from 0 to 15 lcd_puts message to display the message at the current cursor location Test of the LM35 Analogical Sensor This test consists in displaying the temperature read from the analogical senso
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