EE 308 - LAB 6

C Programming Language


The C programming language is used extensively in programming microprocessors. In this lab you will write some simple C programs which do the things you did in assembly language in the last lab.

For example, the following C program increments Port B:

    /* A C Language Program to Increment Port B on a 68HC12
     * Bill Rison
     * February 16, 1999
    #include <hc12.h>            /* Get the HC12 definitions */
    #define D_1MS (8000/4)       /* Inner delay loop takes 4 cycles */
    #define TRUE  1              /* A normal C define */
    void delay(unsigned int ms);
    void main()                  /* The main program */
        DDRB = 0xff;             /* Make all bits of Port B output */
        PORTB = 0;
        while (TRUE)             /* Do forever */
            PORTB = PORTB + 1;   /* Increment Port B */
            delay(100);          /* Wait 100 ms */

    /* Function to delay ms milliseconds */
    void delay(unsigned int ms)
        int i;
        while (ms > 0)
            i = D_1MS;
            while (i >0)
                i = i - 1;
            ms = ms - 1;

Figure 1: A C program to increment Port B.

The Lab

Type in the above C program (or download it from the web) and give it the name inc.c. Open a Command window. Compile the program as described in the TIPS section at the end of this lab.

You should now have the files, inc.h12, inc.s19 and in your directory.

(a) is the assembly language listing generated by the C compiler. Look at the file and try to understand what it does. Note that there may be some things which do not make sense to you. At the very least, find the assembly language code which increments Port B. (Note that the C compiler produces assembly code in decimal rather than hexadecimal.)

Look at the file This shows the addresses of the start of the functions in the program, as well as the addresses of any global variables. (Since the inc.c program does not use any global variables, none will appear in the file. The local variables used in inc.c are allocated on the stack when they are needed.)

Note that the function and variable names are preceded by an underscore. Note also that there is a function _exit. Find the address of this function.

Look at the file inc.s19. This contains the op codes that will be loaded into the HC12. Reverse assemble the _exit function. What does this do?

Load the file inc.h12 into your ZAP simulator and run it. Note that you can see both the assembly code which the HC12 will execute and the C code used to generate the assembly code.

Load the file inc.s19 into your HC12 and run it. Verify that Port B increments.

Using the program inc.c as a model, write a C program to implement the functions from Lab 5.

Compile and run your program. Have an instructor verify that it works.

Look at the file for this week's lab, and determine how many bytes the program takes (the length of the .text segment). Compare this to the length of last week's program written in assembly.

Put your program in the EEPROM at address 0x0D00. Note that you will want the array which stores the turn signal patterns into the EEPROM (so the array will not disappear when you turn off power). You will want variables which will change as the program is executed to be placed in RAM. You can tell the compiler to put an array in EEPROM by defining the array as type const, and telling the linker to put the const section in EEPROM following the text section. An example of setting up an array of type const is
  const char table[] = {0xaa, 0xbb, 0xcc};

For more information on putting your C program into EEPROM, read the TIPS section below.


Bill Rison, < >
Tue Feb 16 1999

© 1999, New Mexico Tech