3. Sample Programs

A series of sample programs is provided in the Dynamic C Samples/JackRab folder. You can load a sample program by using the File Open menu in Dynamic C. The sample programs are listed in Table 1.

Table 1. Jackrabbit Sample Programs

DEMOJR1.C

DEMOJR2.C

DEMOJR3.C

DEMOJR6.C

JRIOTEST.C

JRIO_COF.C

RABDB01.C

RABDB02.C

The first five sample programs provide a step-by-step introduction to the Jackrabbit board. Additional sample programs illustrate more advanced topics.

Each sample program has comments that describe to the purpose and function of the program.

3.1 Running Sample Program DEMOJR1.C

This sample program can be used to illustrate some of the functions of Dynamic C.

First, open the file DEMOJR1.C, which is in the Samples/JackRab folder. The program will appear in a window, as shown in Figure 4 below (minus some comments). Use the mouse to place the cursor on the function name WrPortI in the program and type <ctrl-H>. This will bring up a documentation box for the function WrPortI. In general, you can do this with all functions in Dynamic C libraries, including libraries you write yourself. Close the documentation box and continue.

Figure 4. Sample Program DEMOJR1.C

To run the program DEMOJR1.C, load it with the File menu, compile it using the Compile menu, and then run it by selecting Run in the Run menu. The LED on the Prototyping Board should start flashing if everything went well. If this doesn't work review the following points.

• The target should be ready, which is indicated by the message "BIOS successfully compiled..." If you did not receive this message or you get a communication error, recompile the BIOS by typing <ctrl-Y> or select Recompile BIOS from the Compile menu.
• A message reports that "No Rabbit processor detected" in cases where the Jackrabbit and Prototyping Board are not connected together, the wall transformer is not connected, or is not plugged in. (The red power LED lights whenever power is connected.)
• The programming cable must be connected to the Jackrabbit board. (The colored wire on the programming cable is closest to pin 1 on header J3 on the Jackrabbit board, as shown in Figure 3 on page 7.) The other end of the programming cable must be connected to the PC serial port, possibly, using the 9- to 25-pin adapter if necessary. The COM port specified in the Dynamic C Options menu must be the same as the one the programming cable is connected to.
• To check if you have the correct serial port, select Compile, then Compile BIOS, or type <ctrl-Y>. If the "BIOS successfully compiled ..." message does not display, try a different serial port using the Dynamic C Options menu until you find the one you are plugged into. Don't change anything in this menu except the COM number. The baud rate should be 115,200 bps and the stop bits should be 1.

3.2 Single-Stepping

Compile or re-compile DEMOJR1.C by clicking the Compile button on the task bar. The program will compile and the screen will come up with a highlighted character (green) at the first executable statement of the program. Use the F8 key to single-step. Each time the F8 key is pressed, the cursor will advance one statement. When you get to the for(j=0, j< ... statement, it becomes impractical to single-step further because you would have to press F8 thousands of times. We will use this statement to illustrate watch expressions.

3.2.1 Watch Expression

Type <ctrl-W> or chose Add/Del Watch Expression in the Inspect menu. A box will come up. Type the lower case letter j and click on add to top and close. Now continue single-stepping with F8. Each time you step, the watch expression (j) will be evaluated and printed in the watch window. Note how the value of j advances when the statement j++ is executed.

3.2.2 Break Point

Move the cursor to the start of the statement:

   for(j=0; j<1000; j++);

To set a break point on this statement, type F2 or select Breakpoint from the Run menu. A red highlight will appear on the first character of the statement. To get the program running at full speed, type F9 or select Run on the Run menu. The program will advance until it hits the break point. Then the break point will start flashing and show both red and green colors. Note that LED DS3 is now solidly turned on. This is because we have passed the statement turning on LED DS3. Note that j in the watch window has the value 25000. This is because the loop above terminated when j reached 25000.

To remove the break point, type F2 or select Toggle Breakpoint on the Run menu. To continue program execution, type F9 or select Run from the Run menu. Now the LED should be flashing again since the program is running at full speed.

You can set break points while the program is running by positioning the cursor to a statement and using the F2 key. If the execution thread hits the break point, a break point will take place. You can toggle the break point off with the F2 key and continue execution with the F9 key. Try this a few times to get the feel of things.

3.2.3 Editing the Program

Click on the Edit box on the task bar. This will set Dynamic C into the edit mode so that you can change the program. Use the Save as choice on the File menu to save the file with a new name so as not to change the demo program. Save the file as MYTEST.C. Now change the number 25000 in the for (.. statement to 10000. Then use the F9 key to recompile and run the program. The LED will start flashing, but it will flash much faster than before because you have changed the loop counter terminal value from 25000 to 10000.

3.2.4 Watching Variables Dynamically

Go back to edit mode (select edit) and load the program DEMOJR2.C using the File menu Open command. This program is the same as the first program, except that a variable k has been added along with a statement to increment k each time around the endless loop. The statement:

runwatch();

has been added. This is a debugging statement that makes it possible to view variables while the program is running.

Use the F9 key to compile and run DEMOJR2.C. Now type <ctrl-W> to open the watch window and add the watch expression k to the top of the list of watch expressions. Now type <ctrl-U>. Each time you type <ctrl-U>, you will see the current value of k, which is incrementing about 5 times a second.

As an experiment add another expression to the watch window:

k*5

Then type <ctrl-U> several times to observe the watch expressions k and k*5.

3.2.5 Summary of Features

So far you have practiced using the following features of Dynamic C.

• Loading, compiling and running a program. When you load a program it appears in an edit window. You can compile by selecting Compile on the task bar or from the Compile menu. When you compile the program, it is compiled into machine language and downloaded to the target over the serial port. The execution proceeds to the first statement of main where it pauses, waiting for you to command the program to run, which you can do with the F9 key or by selecting Run on the Run menu. If want to compile and start the program running with one keystroke, use F9, the run command. If the program is not already compiled, the run command will compile it first.
• Single-stepping. This is done with the F8 key. The F7 key can also be used for single-stepping. If the F7 key is used, then descent into subroutines will take place. With the F8 key the subroutine is executed at full speed when the statement that calls it is stepped over.
• Setting break points. The F2 key is used to turn on or turn off (toggle) a break point at the cursor position if the program has already been compiled. You can set a break point if the program is paused at a break point. You can also set a break point in a program that is running at full speed. This will cause the program to break if the execution thread hits your break point.
• Watch expressions. A watch expression is a C expression that is evaluated on command in the watch window. An expression is basically any type of C formula that can include operators, variables and function calls, but not statements that require multiple lines such as for or switch. You can have a list of watch expressions in the watch window. If you are single-stepping, then they are all evaluated on each step. You can also command the watch expression to be evaluated by using the <ctrl-U> command. When a watch expression is evaluated at a break point, it is evaluated as if the statement was at the beginning of the function where you are single-stepping. If your program is running you can also evaluate watch expressions with a <ctrl-U> if your program has a runwatch() command that is frequently executed. In this case, only expressions involving global variables can be evaluated, and the expression is evaluated as if it were in a separate function with no local variables.

3.3 Cooperative Multitasking

Cooperative multitasking is a convenient way to perform several different tasks at the same time. An example would be to step a machine through a sequence of steps and at the same time independently carry on a dialog with the operator via a human interface. Cooperative multitasking differs from a different approach called preemptive multitasking. Dynamic C supports both types of multitasking. In cooperative multitasking each separate task voluntarily surrenders its compute time when it does not need to perform any more activity immediately. In preemptive multitasking control is forcibly removed from the task via an interrupt.

Dynamic C has language extensions to support multitasking. The major C constructs are called costatements, cofunctions, and slicing. These are described more completely in the Dynamic C Reference Manual. The example below, sample program DEMOJR3.C, uses costatements. A costatement is a way to perform a sequence of operations that involve pauses or waits for some external event to take place. A complete description of costatements is in the Dynamic C Reference Manual. The DEMOJR3.C sample program has two independent tasks. The first task flashes LED DS4 once a second. The second task uses button S1 on the Prototyping Board to toggle the logical value of a virtual switch, vswitch, and flash DS1 each time the button is pressed. This task also debounces button S1.

int vswitch;             // state of virtual switch controlled by button S1
main(){                  // begin main program
                         // set up parallel port A as output
    WrPortI(SPCR,NULL,0x84);
    WrPortI(PADR,&PADRShadow,0xff);       // turn off all LEDs
    vswitch=0;                            // initialize virtual switch off
(1) while (1) {                           // Endless loop
      BigLoopTop();                     // Begin a big endless loop

    // first task flash LED DS4 every second for 200 milliseconds

(2)   costate {                           // begin a costatement
        BitWrPortI(PADR,&PADRShadow,0,3); // LED DS4 on
(3)     waitfor(DelayMs(200));            // light on for 200 ms
        BitWrPortI(PADR,&PADRShadow,1,3); // LED DS4 off
        waitfor(DelayMs(800));            // light off for 800 ms
(4)   }                                   // end of costatement

    // second task - debounce switch #1 and toggle virtual switch vswitch

    // check button 1 and toggle vswitch on or off

      costate {
(5)     if(BitRdPortI(PBDR,2)) abort;  // if button not down skip out
        waitfor(DelayMs(50));          // wait 50 ms
        if(BitRdPortI(PBDR,2)) abort;  // if button not still down skip out
        vswitch=!vswitch;   // toggle virtual switch- button was down 50 ms
        while (1) {                    // wait for button to be off 200 ms
          waitfor(BitRdPortI(PBDR,2)); // wait for button to go up
          waitfor(DelayMs(200));       // wait for 200 milliseconds
          if(BitRdPortI(PBDR,2)) break;// if button up break
        }                              // end of while(1)
      }                                // end of costatement

    // make LED agree with vswitch if vswitch has changed

(6)   if( (PADRShadow & 1) == vswitch) {
        BitWrPortI(PADR,&PADRShadow,!vswitch,0);
      )
(7) }                              // end of while loop, go back to start
}                                   // end of main, never come here

The numbers in the left margin are reference indicators and are not a part of the code. Load and run the program. Note that LED DS4 flashes once per second. Push button S1 several times and note how LED DS1 is toggled.

The flashing of LED DS4 is performed by the costatement starting at the line marked (2). Costatements need to be executed regularly, often at least every 25 ms. To accomplish this, the costatements are enclosed in a while loop. The term while loop is used as a handy way to describe a style of real-time programming in which most operations are done in one loop. The while loop starts at (1) and ends at (7). The function BigLoopTop() is used to collect some operations that are helpful to do once on every pass through the loop. Place the cursor on this function name BigLoopTop() and hit <ctrl-H> to learn more.

The statement at (3) waits for a time delay, in this case 200 ms. The costatement is being executed on each pass through the big loop. When a waitfor condition is encountered the first time, the current value of MS_TIMER is saved and then on each subsequent pass the saved value is compared to the current value. If a waitfor condition is not encountered, then a jump is made to the end of the costatement (4), and on the next pass of the loop, when the execution thread reaches the beginning of the costatement, execution passes directly to the waitfor statement. Once 200 ms has passed, the statement after the waitfor is executed. The costatement has the property that it can wait for long periods of time, but not use a lot of execution time. Each costatement is a little program with its own statement pointer that advances in response to conditions. On each pass through the big loop, as little as one statement in the costatement is executed, starting at the current position of the costatement's statement pointer. Consult the Dynamic C Reference Manual for more details.

The second costatement in the program debounces the switch and maintains the variable vswitch. Debouncing is performed by making sure that the switch is either on or off for a long enough period of time to ensure that high-frequency electrical hash generated when the switch contacts open or close does not affect the state of the switch. The abort statement is illustrated at (5). If executed, the internal statement pointer is set back to the first statement within the costatement, and a jump to the closing brace of the costatement is made.

At (6) a use for a shadow register is illustrated. A shadow register is used to keep track of the contents of an I/O port that is write only - it can't be read back. If every time a write is made to the port the same bits are set in the shadow register, then the shadow register has the same data as the port register. In this case a test is made to see the state of the LED and make it agree with the state of vswitch. This test is not strictly necessary, the output register could be set every time to agree with vswitch, but it is placed here to illustrate the concept of a shadow register.

To illustrate the use of snooping, use the watch window to observe vswitch while the program is running. Add the variable vswitch to the list of watch expressions. Then toggle vswitch and the LED. Then type <ctrl-U> to observe vswitch again.

3.4 Advantages of Cooperative Multitasking

Cooperative multitasking, as implemented with language extensions, has the advantage of being intuitive. Unlike preemptive multitasking, variables can be shared between different tasks without having to take elaborate precautions. Sharing variables between tasks is the greatest cause of bugs in programs that use preemptive multitasking. It might seem that the biggest problem would be response time because of the big loop time becoming long as the program grows. Our solution for that is a device caused slicing that is further described in the Dynamic C Reference Manual.