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PICkit™ 3 Debug Express Lessons

3.3 LESSON 3: ROTATE LED

This lesson builds on the previous two lessons to introduce defining global variables

and code sections, and to add rotation to the LED display. It will light up LED 0, then

shift it to LED 1, then to LED 2 and on up to LED 7, and back to LED 0.

In this and following lessons, please open the lesson workspace in the MPLAB IDE

upon starting the lesson.

3.3.1 Allocating File Register Memory

In the source code file 03 Rotate LED.c for Lesson 3 the global variable,

LED_Number, is declared as in Figure 3-16.

FIGURE 3-16: LESSON 3 GLOBAL VARIABLE DECLARATION

The directive #pragma udata is used prior to declaring the variable LED_Number to

indicate to the compiler that the following declarations are data variables that should be

placed in the PIC18FXXXX file registers. This differs from PC compilers where

instructions and variables share the same memory space due to the Harvard

architecture of the PIC18FXXXX as discussed in Section 2.1 of this document.

There are two directives for use with #pragma when defining variables:

Data declarations can also be given a section name. The section name may be used

with a Linker Script SECTION entry to place it in a particular area of memory. See

Section 2.9 of the “MPLAB C18 C Compiler User’s Guide” (DS51288) for more

information on using sections with Linker Scripts. Even without a Linker Script section,

the #pragma udata directive may be used to specify the starting address of the data

in the file registers. For example, to place LED_Number at the start of file register Bank

3 declare the udata section as:

#pragma udata mysection = 0x300 unsigned char

LED_Number; // 8-bit variable unsigned int

AnotherVariable;

Other variables declared in a udata or idata section will be placed at subsequent

addresses. For instance, the 16-bit integer

AnotherVariable above would occupy

address 0x301 and 0x302

Note that function local variables will be placed on the software stack.

Key Concepts

- The directives #pragma udata and #pragma idata are used to allocate

memory for static variables in the file registers.

- The directive #pragma code is used to indicate a section of instructions to

be compiled into the program memory of the PIC18FXXXX.

- The directive #pragma romdata is used for constant (read-only) data

stored in the program memory. This is used with the keyword rom.

- Constant data can be stored in program memory so as not to use up file

udata Uninitialized data. The following data is stored uninitialized in the file register

space.

idata Initialized data. The following data is stored in the file register space. The initial-

ization values are stored in program memory, and then moved by the start-up

initialization code into file registers before program execution begins.

/** V A R I A B L E S *************************************************/

#pragma udata // declare statically allocated uninitialized variables

unsigned char LED_Number; // 8-bit variable