MPLAB XC8 C编译器用户指南：PIC MCUs应用详解

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MPLAB_XC8_C_Compiler_User_Guide_for_PIC是一份Microchip Technology Inc.于2012年至2018年期间发布的官方用户手册，主要针对他们的PIC（微控制器）系列。这份文档详细介绍了MPLAB XC8 C编译器的使用方法和指导，是开发人员在使用Microchip PIC MCU时的重要参考资源。该手册的核心内容包括： 1. **概述**：介绍了MPLAB XC8 C Compiler，它是Microchip为基于PIC微控制器的项目设计提供的高性能C语言编译器。它支持C99标准，旨在简化代码编写、优化性能并提高开发效率。 2. **安装与配置**：手册提供了编译器的安装步骤和配置指南，确保用户能够根据其特定的硬件环境设置合适的编译选项和工具链。 3. **编程语言特性**：展示了C语言在MPLAB XC8中的特性和用法，包括数据类型、控制结构、函数、库函数以及内存管理等，以适应嵌入式系统的特殊需求。 4. **代码优化**：讲解了如何利用编译器的优化选项来减少程序大小、提高运行速度，以及如何平衡代码性能与资源使用。 5. **调试与错误处理**：手册还涉及调试工具的使用，如断点设置、内存检查和错误诊断，帮助开发者快速定位和修复问题。 6. **安全注意事项**：强调了在生命支持和安全应用中使用Microchip设备的风险，并指出使用者需自行确保应用符合其具体规格，同时声明Microchip不对信息的正确性、性能或适用性做出任何保证。 7. **版权与责任声明**：明确表示Microchip不承担任何法律责任，且使用其产品存在风险，购买者需自行承担可能产生的损害、索赔或费用。此外，知识产权许可仅在明确说明的情况下授予。这份文档对于任何希望使用MPLAB XC8 C Compiler进行Microchip PIC MCU项目开发的工程师来说，具有很高的实用价值，提供了一套全面的指南和支持。通过深入学习和理解其中的内容，开发者可以更有效地编写、测试和优化他们的嵌入式代码。

MPLAB

XC8 C Compiler User’s Guide for PIC

MCU

DS50002737A-page 16  2012-2018 Microchip Technology Inc.

2.3.4 How Can I Select Which Compiler I Want to Build With?

The compilation and installation process has been designed to allow you to have more

than one compiler installed at the same time. You can create a project in MPLAB X IDE

and then build this project with different compilers by simply changing a setting in the

project properties.

To select which compiler is actually used when building a project under MPLAB X IDE,

go to the Project Properties dialog. Select the Configuration category in the Project

Properties dialog (Conf: [default]). A list of MPLAB XC8 compilers is shown in the

Compiler Toolchain, on the far right. Select the compiler that you require.

Once selected, the controls for that compiler are then shown by selecting the MPLAB

XC8 global options, MPLAB XC8 Compiler and MPLAB XC8 Linker categories. These

reveal a pane of options on the right. Note that each category has several panes which

can be selected from a pull-down menu that is near the top of the pane.

2.3.5 How Do I Build Libraries?

Use the librarian, xc8-ar, to build libraries from p-code (.p1 extension files). See

Section 3.3.3 “Multi-Step Compilation” for information on building p-code files and

Section 7.2 “Librarian” for the librarian options.

For example:

xc8-cc --chip=16f877a -c lcd.c utils.c io.c

xc8-ar -r myLib.a lcd.p1 utils.p1 io.p1

creates a library file called myLib.a.

The MPLAB X IDE allows you to create a library project which will build a library file as

the final output.

Note that if you intend to step through your library code at a C level in MPLAB X IDE,

you will need to place the library source files so that the relative path between their

location and the project that is using them is the same as the relative path between

where the library build command was executed and where the source files were

located when they were built.

2.3.6 How Do I Know What Compiler Options Are Available and What

They Do?

A list of all compiler options can be obtained by using the --help option on the com-

mand line (see Section 3.7.2.7 “help”).

Alternatively, all options are listed in Section 3.7 “Option Descriptions” of this user’s

guide.

2.3.7 What is Different About an MPLAB X IDE Debug Build?

In MPLAB X, there are distinct build buttons and menu items to build (production build)

a project and to debug (debug build) a project.

When performing a debug build, the IDE will also set the configuration bit to allow

debugging of the project by a debug tool, such as the MPLAB ICD 4.

In MPLAB X IDE, memory is reserved for your debugger (if selected) only when you

perform a debug build. See Section 2.5.4 “What Do I Need to Do When Compiling to

Use a Debugger?”

Another difference is the setting of a preprocessor macro called __DEBUG, which is

assigned 1 when performing a debug build. This macro is not defined for production

builds. You can make code in your source conditional on this macro using #ifdef

directives, etc., (see Section 4.14.1 “Preprocessor Directives”); so that you can have

your program behave differently when you are still in a development cycle.

How To’s

 2012-2018 Microchip Technology Inc. DS50002737A-page 17

2.4 WRITING SOURCE CODE

This section presents issues that pertain to the source code you write. It has been

subdivided into the sections listed below.

• C Language Specifics

• Device-Specific Features

• Memory Allocation

• Variables

• Functions

• Interrupts

• Assembly Code

2.4.1 C Language Specifics

This section discusses commonly asked source code issues that directly relate to the

C language itself.

• When Should I Cast Expressions?

• Can Implicit Type Conversions Change the Expected Results of My Expressions?

• How Do I Enter Non-English Characters Into My Program?

• How Can I Use a Variable Defined in Another Source File?

• How Can I Use a Function Defined in Another Source File?

2.4.1.1 WHEN SHOULD I CAST EXPRESSIONS?

Expressions can be explicitly cast using the cast operator -- a type in round brackets,

e.g., (int). In all cases, conversion of one type to another must be done with caution

and only when absolutely necessary.

Consider the example:

unsigned long l;

unsigned int i;

i = l;

Here, a long type is being assigned to an int type and the assignment will truncate

the value in l. The compiler will automatically perform a type conversion from the type

of the expression on the right of the assignment operator (long) to the type of the value

on the left of the operator (int).This is called an implicit type conversion. The compiler

typically produces a warning concerning the potential loss of data by the truncation.

A cast to type int is not required and should not be used in the above example if a

long to int conversion was intended. The compiler knows the types of both operands

and performs the conversion accordingly. If you did use a cast, there is the potential for

mistakes if the code is later changed. For example, if you had:

i = (int)l;

the code works the same way; but if in the future, the type of i is changed to a long,

for example, then you must remember to adjust the cast, or remove it, otherwise the

contents of l will continue to be truncated by the assignment, which cannot be correct.

Most importantly, the warning issued by the compiler will not be produced if the cast is

in place.

MPLAB

XC8 C Compiler User’s Guide for PIC

MCU

DS50002737A-page 18  2012-2018 Microchip Technology Inc.

Only use a cast in situations where the types used by the compiler are not the types

that you require. For example, consider the result of a division assigned to a floating

point variable:

int i, j;

float fl;

fl = i/j;

In this case, integer division is performed, then the rounded integer result is converted

to a float format. So if i contained 7 and j contained 2, the division yields 3 and this

is implicitly converted to a float type (3.0) and then assigned to fl. If you wanted the

division to be performed in a float format, then a cast is necessary:

fl = (float)i/j;

(Casting either i or j forces the compiler to encode a floating-point division.) The

result assigned to fl now is 3.5.

An explicit cast can suppress warnings that might otherwise have been produced. This

can also be the source of many problems. The more warnings the compiler produces,

the better chance you have of finding potential bugs in your code.

2.4.1.2 CAN IMPLICIT TYPE CONVERSIONS CHANGE THE EXPECTED

RESULTS OF MY EXPRESSIONS?

Yes! The compiler will always use integral promotion and there is no way to disable this

(see Section 4.6.1 “Integral Promotion”). In addition, the types of operands to binary

operators are usually changed so that they have a common type, as specified by the C

Standard. Changing the type of an operand can change the value of the final expres-

sion, so it is very important that you understand the type C Standard conversion rules

that apply when dealing with binary operators. You can manually change the type of an

operand by casting; see Section 2.4.1.1 “When Should I Cast Expressions?”

2.4.1.3 HOW DO I ENTER NON-ENGLISH CHARACTERS INTO MY PROGRAM?

The C standard (and accordingly, the MPLAB XC8 C compiler) does not support

extended characters set in character and string literals in the source character set. See

Section 4.4.7 “Constant Types and Formats” to see how these characters can be

entered using escape sequences.

2.4.1.4 HOW CAN I USE A VARIABLE DEFINED IN ANOTHER SOURCE FILE?

Provided the variable defined in the other source file is not static (see

Section 4.5.2.1.1 “Static Objects”) or auto (see Section 4.5.2.2 “Automatic Storage

Duration Objects”), then adding a declaration for that variable into the current file will

allow you to access it. A declaration consists of the keyword extern in addition to the

type and the name of the variable, as specified in its definition, e.g.,

extern int systemStatus;

This is part of the C language. Your favorite C textbook will give you more information.

The position of the declaration in the current file determines the scope of the variable.

That is, if you place the extern declaration inside a function, it will limit the scope of

the variable to that function. If you place it outside of a function, it allows access to the

variable in all functions for the remainder of the current file.

Often, declarations are placed in header files and then they are #included into the C

source code (see Section 4.14.1 “Preprocessor Directives”).

How To’s

 2012-2018 Microchip Technology Inc. DS50002737A-page 19

2.4.1.5 HOW CAN I USE A FUNCTION DEFINED IN ANOTHER SOURCE FILE?

Provided the function defined in the other source file is not static, then adding a dec-

laration for that function into the current file will allow you to call it. A declaration option-

ally consists of the keyword extern in addition to the function prototype. You can omit

the names of the parameters, e.g.,

extern int readStatus(int);

This is part of the C language. Your favorite C textbook will give you more information.

Often, declarations are placed in header files and then they are #included into the C

source code (see Section 4.14.1 “Preprocessor Directives”).

2.4.2 Device-Specific Features

This section discusses the code that needs to be written to set up or control a feature

that is specific to Microchip PIC devices.

• How Do I Set the Configuration Bits?

• How Do I Use the PIC Device’s ID Locations?

• How Do I Determine the Cause of Reset on Mid-range Parts?

• How Do I Access SFRs?

• How Do I Place a Function Into a Unique Section?

See also the following linked information in other sections.

• What Do I Need to Do When Compiling to Use a Debugger?

2.4.2.1 HOW DO I SET THE CONFIGURATION BITS?

These should be set in your code using a pragma. See Section 4.3.5 “Configuration Bit

Access” for details about how these are set.

2.4.2.2 HOW DO I USE THE PIC DEVICE’S ID LOCATIONS?

There is a pragma that allows these values to be programmed (see Section 4.3.6 “ID

Locations”).

2.4.2.3 HOW DO I DETERMINE THE CAUSE OF RESET ON MID-RANGE

PARTS?

The TO and PD bits in the STATUS register allow you to determine the cause of a

Reset. However, these bits are quickly overwritten by the runtime startup code that is

executed before main(); see Section 4.10.2 “Runtime Startup Code” for more infor-

mation. You can have the STATUS register saved into a location that is later accessible

from C code, so that the cause of Reset can be determined by the application after it is

running again (see Section 4.10.2.4 “STATUS Register Preservation”).

2.4.2.4 HOW DO I ACCESS SFRS?

The compiler ships with header files (see Section 4.3.3 “Device Header Files”) that

define the variables that are mapped over the top of memory-mapped SFRs. Since

these are C variables, they can be used like any other C variables and no new syntax

is required to access these registers.

Bits within SFRs can also be accessed. Individual bit-wide variables are defined that

are mapped over the bits in the SFR. Bit-fields are also available in structures that map

over the SFR as a whole. You can use either in your code (see Section 4.3.7 “Using

SFRs From C Code”).

The name assigned to the variable is usually the same as the name specified in the

device data sheet. See Section 2.4.2.5 “How Do I Find The Names Used to Represent

SFRs and Bits?” if these names are not recognized.

MPLAB

XC8 C Compiler User’s Guide for PIC

MCU

DS50002737A-page 20  2012-2018 Microchip Technology Inc.

2.4.2.5 HOW DO I FIND THE NAMES USED TO REPRESENT SFRS AND BITS?

Special function registers, and the bits within them, are accessed via special variables

that map over the register (see Section 2.4.2.4 “How Do I Access SFRs?”). However,

the names of these variables sometimes differ from those indicated in the data sheet

for the device you are using.

If required, you can examine the <xc.h> header file to find the device-specific header

file that is relevant for your device. This file will define the variables that allow access

to these special variables. However, an easier way to find these variable names is to

look in any of the preprocessed files left behind from a previous compilation. Provided

the corresponding source file included <xc.h>, the preprocessed file will show the

definitions for all the SFR variables and bits for your target device.

If you are compiling under MPLAB X IDE, the preprocessed file(s) are left under the

build/default/production directory of your project for regular builds, or under

build/default/debug for debug builds. They are typically left in the source file

directory if you are compiling on the command line. These files have a .i extension.

2.4.3 Memory Allocation

Here are questions relating to how your source code affects memory allocation.

• How Do I Position Variables at an Address I Nominate?

• How Do I Place a Variable Into a Unique Section?

• How Do I Position a Variable Into an Address Range?

• How Do I Position Functions at an Address I Nominate?

• How Do I Place Variables in Program Memory?

• How Do I Place a Function Into a Unique Section?

• How Do I Position a Function Into an Address Range?

• How Do I Place a Function Into a Unique Section?

See also the following linked information in other sections.

• Why Are Some Objects Positioned Into Memory That I Reserved?

• How Do I Use High-Endurance Flash for Data, Not Code?

2.4.3.1 HOW DO I POSITION VARIABLES AT AN ADDRESS I NOMINATE?

The easiest way to do this is to make the variable absolute by using the

__at(address) construct (see Section 4.5.4 “Absolute Variables”), but you might

consider also placing the variable in a unique section (see Section 4.15.3 “Changing

and Linking the Allocated Section”). Absolute variables use the address you nominate

in preference to the variable’s symbol in generated code.

2.4.3.2 HOW DO I PLACE A VARIABLE INTO A UNIQUE SECTION?

Use the __section() specifier to have the variable positioned in a new section

(psect). After this has been done, the section can be linked to the desired address by

using the -Wl driver option. See Section 4.15.3 “Changing and Linking the Allocated

Section” for examples of both these operations.

2.4.3.3 HOW DO I POSITION A VARIABLE INTO AN ADDRESS RANGE?

You need to move the variable into a unique psect (section), define a memory range,

and then place the new section in that range.

Use the __section() specifier to have the variable positioned in a new section. Use

the -Wl driver option to define a memory range and to place the new section in that

range. See Section 4.15.3 “Changing and Linking the Allocated Section” for examples

of all these operations.

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