PIC单片机C语言编程入门与实战指南

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"《pic8位单片机C语言编程》是一本全面介绍PIC单片机C语言编程的教程，适合不同阶段的用户学习。随着嵌入式微控制器在日常生活中的广泛应用，掌握微控制器的编程技能变得越来越重要。如今，无论是在手机、计算器还是MP3播放器等常见设备中，甚至在洗衣机、车库门开启器、卷发棒或电动牙刷等家用电器内部，都能找到微控制器的身影。随着技术的进步，学习和使用微控制器的门槛已大大降低，过去昂贵的编程器和开发工具现在已经被价格更为亲民的Flash基编程内存所取代，使得调试工作不再仅限于专业人士。" 本文档将深入探讨8位PIC单片机的C语言编程，帮助读者理解和掌握如何利用C语言对这些微控制器进行编程。PIC单片机以其高效能、低功耗和广泛应用而著称，是许多嵌入式系统设计者首选的微控制器平台。C语言作为一种高级程序设计语言，具有易读性强、可移植性好等特点，使得它成为编写微控制器程序的理想选择。在教程中，读者将学习到以下关键知识点： 1. **PIC单片机基础**：了解PIC单片机的架构，包括寄存器、I/O端口、中断系统以及定时器/计数器等基本组件。 2. **C语言编程**：学习C语言的基本语法，如变量、控制结构（循环、条件语句）、函数等，以及如何在嵌入式环境中应用这些概念。 3. **嵌入式系统开发环境**：熟悉建立开发环境，包括选择合适的IDE（集成开发环境）、编译器和仿真器，以及如何使用它们进行项目配置和编译。 4. **编程实践**：通过实例，学习如何编写、烧录和测试代码，包括LED闪烁、按键检测、串行通信等基本实验。 5. **中断处理**：理解中断的工作原理，学习如何在C语言中实现中断服务程序，以响应外部事件。 6. **硬件接口**：学习如何控制和通信各种外设，如LCD显示器、ADC（模数转换器）和DAC（数模转换器）。 7. **电源管理与低功耗设计**：了解如何优化代码以降低功耗，这对于电池供电的设备尤其重要。 8. **调试技巧**：掌握使用内置调试器或外部工具进行程序调试的方法，包括断点设置、变量查看和性能分析。 9. **应用案例**：通过实际项目，如温度监测、电机控制或无线通信，将理论知识转化为实际工程经验。通过这本教程的学习，无论是初学者还是有经验的开发者，都能提升在PIC单片机上的C语言编程能力，从而更好地适应和应对嵌入式系统设计中的各种挑战。

6 Part 1

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Table 1.1 : PIC16F877 Simplified File Register Map

Bank 0 (000–07F) Bank 1 (080–0FF) Bank 2 (100–180) Bank 3 (180–1FF)

Address Register Address Register Address Register Address Register

000 h

Indirect

080 h

Indirect

100 h

Indirect

180 h

Indirect

001 h

Timer0

081 h

Option

101 h

Timer0

181 h

Option

002 h

Prog.

count.

low

082 h

Prog.

count.

low

102 h

Prog.

count.

low

182 h

Prog.

count.

low

003 h

Status reg

083 h

Status reg

103 h

Status reg

183 h

Status reg

004 h

File select

084 h

File select

104 h

File select

184 h

File select

005 h

Port A

data

085 h

Port A

direction

105 h

—

185 h

—

006 h

Port B

data

086 h

Port B

direction

106 h

Port B

data

186 h

Port B

direction

007 h

Port C

data

087 h

Port C

direction

107 h

—

187 h

—

008 h

Port D

data

088 h

Port D

direction

108 h

—

188 h

—

009 h

Port E

data

089 h

Port E

direction

109 h

—

189 h

—

00A h

Prog.

count.

high

08 Ah

Prog.

count.

high

10 Ah

Prog.

count.

high

18 Ah

Prog.

count.

high

00 Bh

Interrupt

control

08 Bh

Interrupt

control

10 Bh

Interrupt

control

18 Bh

Interrupt

control

00Ch–

01Fh

peripheral

control

registers

08Ch–

09Fh

peripheral

control

registers

10Ch–

10Fh

peripheral

control

registers

18Ch–

18Fh

peripheral

control

registers

110h–

11Fh

16 general

purpose

registers

190h–

19Fh

16 general

purpose

registers

020h–

06Fh

80 general

purpose

registers

0A0h–

0EFh

80 general

purpose

registers

120h–

16Fh

80 general

purpose

registers

1A0h–

1EFh

80 general

purpose

registers

070h–

07Fh

common

access

GPRs

0F0h–

0FFh

Accesses

070h–

07Fh

170h–

17Fh

Accesses

070h–

07Fh

1F0h–

1FFh

Accesses

070h–

07Fh

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PIC Microcontroller Systems 7

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RAM File Registers

The main RAM block ( Table 1.1 ) is a set of 368 8-bit file registers, including the special

function registers (SFRs), which have a dedicated function, and the general purpose

registers (GPRs). When variables are created in C, they are stored in the GPRs, starting at

address 0020 h. The file registers are divided into four blocks, register banks 0 to 3. The

SFRs are located at the low addresses in each RAM bank.

Some registers are addressable across the bank boundaries; for example, the status

are addressable in only a specific page, for example, Port A data register. Some register

addresses are not physically implemented. Since some registers are accessible in multiple

banks, bank switching can be minimized by the compiler when assembling the machine

code, thus saving program code space and execution time. For full details of the file

The program counter uses two 8-bit registers to store a 13-bit program memory address.

Only the low byte at address 002 h is directly addressable. The status register 003 h

records results from ALU (arithmetic and logic unit) operations, such as zero and carry/

borrow. The indirect and file select registers are used for indexed addressing of the GPRs.

Timer0 is the timer/counter register available in all PIC MCUs, while Timer1 and Timer2

registers are in the peripheral block. The port registers are located in Bank 0 at addresses

05 h (Port A) to 09 h (Port E) with the data direction register for each at the corresponding

location in bank 1. We can see that a total of 80 ⫹ 1 6 ⫹ 8 0 ⫹ 9 6 ⫹ 9 6 ⫽ 368 GPRs are

available for use as data RAM. Note that the number of registers used for each C variable

depends on the variable type and can range from 1 to 32 bits (1–4 GPRs).

Other PIC Chips

In any embedded design, the features of the MCU need to be matched to the application

requirements. The manufacturer needs to make sure that, as applications become more

demanding, a more powerful device of a familiar type is available. We can see this

process at work where Microchip started out producing basic chips such as the 16C84,

then developed the product range to meet the growing market. PIC microcontrollers are

currently available in distinct groups, designated the 10, 12, 16, 18, and 24 series. Their

general characteristics are outlined in Table 1.2 .

The original 16 series CMOS devices were designated as 16CXX. When flash memory

was introduced, they became 16FXXX. Currently, a limited number of devices are

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PIC Microcontroller Systems 9

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configuration register separated from the main memory block. The main options are as

follows.

Clock Options

The ‘ 877 chip has two main clock modes, CR and XT. The CR mode needs a simple

capacitor and resistor circuit attached to CLKIN, whose time constant (C ⫻ R)

determines the clock period. R should be between 3 k and 100 k, and C greater than 20 pF.

For example, if R ⫽ 1 0 k Ω and C ⫽ 10 nF, the clock period will be around 2 ⫻ C ⫻

R ⫽ 2 0 0 μ s (calculated from the CR rise/fall time) and the frequency about 5 kHz. This

option is acceptable when the program timing is not critical.

The XT mode is the one most commonly used, since the extra component cost is small

compared with the cost of the chip itself and accurate timing is often a necessity. An

external crystal and two capacitors are fitted to CLKIN and CLKOUT pins. The crystal

frequency in this mode can be from 200 kHz to 4 MHz and is typically accurate to better

than 50 ppm (parts per million) or 0.005%. A convenient value is 4 Mz, as this is the

maximum frequency possible with a standard crystal and gives an instruction execution

time of 1.000 μ s (1 million instructions per second, or 1 Mip).

A low-speed crystal can be used to reduce power consumption, which is proportional to

clock speed in CMOS devices. The LP (low-power) mode supports the clock frequency

range 32–200 kHz. To achieve the maximum clock speed of 20 MHz, a high-speed (HS)

crystal is needed, with a corresponding increase in power consumption.

The MCU configuration fuses must be set to the required clock mode when the chip is

programmed. Many PIC chips now have an internal oscillator, which needs no external

components. It is more accurate than the RC clock but less accurate than a crystal. It

typically runs at 8 MHz and can be calibrated in the chip configuration phase to provide a

more accurate timing source.

Configuration Options

Apart from the clock options, several other hardware options must be selected.

Watchdog Timer

When enabled, the watchdog timer (WDT) automatically resets the processor after a

given period (default 18 ms). This allows, for example, an application to escape from

an endless loop caused by a program bug or run-time condition not anticipated by the

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10 Part 1

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software designer. To maintain normal operation, the WDT must be disabled or reset

within the program loop before the set time-out period has expired. It is therefore

important to set the MCU configuration bits to disable the WDT if it is not intended to

use this feature. Otherwise, the program is liable to misbehave, due to random resetting of

the MCU.

Power-up Timer

The power-up timer (PuT) provides a nominal 72 ms delay between the power supply

voltage reaching the operating value and the start of program execution. This ensures

that the supply voltage is stable before the clock starts up. It is recommended that it be

enabled as a precaution, as there is no adverse effect on normal program execution.

Oscillator Start-up Timer

After the power-up timer has expired, a further delay allows the clock to stabilize before

program execution begins. When one of the crystal clock modes is selected, the CPU

waits 1024 cycles before the CPU is enabled.

Brown-out Reset (BoR)

It is possible for a transitory supply voltage drop, or brown-out, to disrupt the MCU

program execution. When enabled, the brown-out detection circuit holds the MCU in

reset while the supply voltage is below a given threshold and releases it when the supply

has recovered. In CCS C, a low-voltage detect function triggers an interrupt that allows

the program to be restarted in an orderly way.

Code Protection (CP)

The chip can be configured during programming to prevent the machine code being read

back from the chip to protect commercially valuable or secure code. Optionally, only

selected portions of the program code may be write protected (see WRT_X% later).

In-Circuit Programming and Debugging

Most PIC chips now support in-circuit programming and debugging (ICPD), which

allows the program code to be downloaded and tested in the target hardware, under the

control of the host system. This provides a final test stage after software simulation has

been used to eliminate most of the program bugs. MPLAB allows the same interface to be

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