小弟最近在学习PIC单片机。现在玩的开发板上的单片机型号为PIC24F16KA102。编了个简单的程序，目的是使用外部变量，可怎么弄编译都不通过。
现象如下：
下面是main.c。（不必细看，此文件 #include "U2.h" ，主要是想使用U2.h里定义的变量tt）
#include <p24F16KA102.h>
#include "U2.h"
//函数功能：I/O端口初始化
//入口参数：无
//出口参数：无
void IOInit()
{
AD1PCFG = 0xFFFF; //关闭PortB端口所有引脚的模拟功能
_TRISB8 = 0; //RB8输出
_TRISB15 = 0; //RB15输出
}
int main (void)
{
IOInit();
while(1)
{
_LATB8 = 0;
_LATB15 = 1;
DelaymS(DELAY_NUM);
_LATB8 = 1;
_LATB15 = 0;
DelaymS(DELAY_NUM);
tt++;
}
}
下面是U2.c文件。（不必细看，此文件也包含了U2.h）
#include "U2.h"
//函数功能：延时程序(毫秒)
//入口参数：del(数值范围:16位整型)
//出口参数：无
void DelaymS(unsigned int del)
{
unsigned int j;
while(del--)
for(j=0;j<1600;j++);
}
下面是U2.h文件
#ifndef __U2_H
#define __U2_H
#define DELAY_NUM 100
unsigned char tt; //想在此定义一个变量tt，在main.c和H2.c中都可以引用到
//此处没有声明extern ，有声明extern 的错误情况在下面有说明。
void DelaymS(unsigned int del);
#endif
下面是编译后的结果。
----------------------------------------------------------------------
Debug build of project `F:CZH est est2 est1.mcp' started.
Language tool versions: pic30-as.exe v3.22, pic30-gcc.exe v3.22, pic30-ld.exe v3.22, pic30-ar.exe v3.22
Preprocessor symbol `__DEBUG' is defined.
Target debug platform is `__MPLAB_DEBUGGER_PK3=1'.
Debug platform options are: `__ICD2RAM=1'.
Sat Jun 11 08:56:08 2011
----------------------------------------------------------------------
Clean: Deleting intermediary and output files.
Clean: Deleted file "F:CZH est est2main.o".
Clean: Deleted file "F:CZH est est2U2.o".
Clean: Deleted file "F:CZH est est2 est1.cof".
Clean: Deleted file "F:CZH est est2 est1.hex".
Clean: Done.
Executing: "d:Program FilesMicrochipMPLAB C30inpic30-gcc.exe" -mcpu=24F16KA102 -x c -c "main.c" -o"main.o" -D__DEBUG -D__MPLAB_DEBUGGER_PK3=1 -g -Wall -O1
Microchip MPLAB C30 License Manager Version v3_22 (Build Date Dec 8 2009).
Copyright (c) 2008 Microchip Technology Inc. All rights reserved.
The MPLAB C30 license has expired.
Options have been disabled due to expired license
Visit http://www.microchip.com/ to purchase a new key.Executing: "d:Program FilesMicrochipMPLAB C30inpic30-gcc.exe" -mcpu=24F16KA102 -x c -c "U2.c" -o"U2.o" -D__DEBUG -D__MPLAB_DEBUGGER_PK3=1 -g -Wall -O1
Microchip MPLAB C30 License Manager Version v3_22 (Build Date Dec 8 2009).
Copyright (c) 2008 Microchip Technology Inc. All rights reserved.
The MPLAB C30 license has expired.
Options have been disabled due to expired license
Visit http://www.microchip.com/ to purchase a new key.Executing: "d:Program FilesMicrochipMPLAB C30inpic30-gcc.exe" -mcpu=24F16KA102 "main.o" "U2.o" -o"test1.cof" -Wl,--script="D:Program FilesMicrochipMPLAB C30supportPIC24Fgldp24F16KA102.gld",--defsym=__MPLAB_BUILD=1,--defsym=__MPLAB_DEBUG=1,--defsym=__MPLAB_DEBUGGER_PK3=1,--defsym=__ICD2RAM=1,--heap=0,-Map="test1.map",--report-mem
Microchip MPLAB C30 License Manager Version v3_22 (Build Date Dec 8 2009).
Copyright (c) 2008 Microchip Technology Inc. All rights reserved.
The MPLAB C30 license has expired.
Options have been disabled due to expired license
Visit http://www.microchip.com/ to purchase a new key.U2.o(.nbss+0x0):F:CZH est est2U2.c: multiple definition of `tt'
main.o(.nbss+0x0):F:CZH est est2main.c: first defined here
d:program filesmicrochipmplab c30inin..in/pic30-coff-ld.exe: Link terminated due to previous error(s).
Link step failed.
----------------------------------------------------------------------
Debug build of project `F:CZH est est2 est1.mcp' failed.
Language tool versions: pic30-as.exe v3.22, pic30-gcc.exe v3.22, pic30-ld.exe v3.22, pic30-ar.exe v3.22
Preprocessor symbol `__DEBUG' is defined.
Target debug platform is `__MPLAB_DEBUGGER_PK3=1'.
Debug platform options are: `__ICD2RAM=1'.
Sat Jun 11 08:56:10 2011
----------------------------------------------------------------------
BUILD FAILED
我看了一下错误原因，是说变量tt重复定义了。可是我在U2.h中已经对重复定义进行了条件编译
#ifndef __U2_H
#define __U2_H
...
#endif
然后我把U2.h中的tt定义加了个extern声明。即为：extern unsigned char tt;
编译后的结果如下：
----------------------------------------------------------------------
Debug build of project `F:CZH est est2 est1.mcp' started.
Language tool versions: pic30-as.exe v3.22, pic30-gcc.exe v3.22, pic30-ld.exe v3.22, pic30-ar.exe v3.22
Preprocessor symbol `__DEBUG' is defined.
Target debug platform is `__MPLAB_DEBUGGER_PK3=1'.
Debug platform options are: `__ICD2RAM=1'.
Sat Jun 11 09:33:49 2011
----------------------------------------------------------------------
Clean: Deleting intermediary and output files.
Clean: Deleted file "F:CZH est est2main.o".
Clean: Deleted file "F:CZH est est2U2.o".
Clean: Done.
Executing: "d:Program FilesMicrochipMPLAB C30inpic30-gcc.exe" -mcpu=24F16KA102 -x c -c "main.c" -o"main.o" -D__DEBUG -D__MPLAB_DEBUGGER_PK3=1 -g -Wall -O1
Microchip MPLAB C30 License Manager Version v3_22 (Build Date Dec 8 2009).
Copyright (c) 2008 Microchip Technology Inc. All rights reserved.
The MPLAB C30 license has expired.
Options have been disabled due to expired license
Visit http://www.microchip.com/ to purchase a new key.Executing: "d:Program FilesMicrochipMPLAB C30inpic30-gcc.exe" -mcpu=24F16KA102 -x c -c "U2.c" -o"U2.o" -D__DEBUG -D__MPLAB_DEBUGGER_PK3=1 -g -Wall -O1
Microchip MPLAB C30 License Manager Version v3_22 (Build Date Dec 8 2009).
Copyright (c) 2008 Microchip Technology Inc. All rights reserved.
The MPLAB C30 license has expired.
Options have been disabled due to expired license
Visit http://www.microchip.com/ to purchase a new key.Executing: "d:Program FilesMicrochipMPLAB C30inpic30-gcc.exe" -mcpu=24F16KA102 "main.o" "U2.o" -o"test1.cof" -Wl,--script="D:Program FilesMicrochipMPLAB C30supportPIC24Fgldp24F16KA102.gld",--defsym=__MPLAB_BUILD=1,--defsym=__MPLAB_DEBUG=1,--defsym=__MPLAB_DEBUGGER_PK3=1,--defsym=__ICD2RAM=1,--heap=0,-Map="test1.map",--report-mem
Microchip MPLAB C30 License Manager Version v3_22 (Build Date Dec 8 2009).
Copyright (c) 2008 Microchip Technology Inc. All rights reserved.
The MPLAB C30 license has expired.
Options have been disabled due to expired license
Visit http://www.microchip.com/ to purchase a new key.
Program Memory [Origin = 0x200, Length = 0x29ff]
section address length (PC units) length (bytes) (dec)
------- ------- ----------------- --------------------
.text 0x200 0x90 0xd8 (216)
.text 0x290 0x30 0x48 (72)
.dinit 0x2c0 0x2 0x3 (3)
.isr 0x2c2 0x2 0x3 (3)
Total program memory used (bytes): 0x126 (294) 1%
Data Memory [Origin = 0x800, Length = 0x600]
section address alignment gaps total length (dec)
------- ------- -------------- -------------------
.icd 0x800 0x50 0x50 (80)
Total data memory used (bytes): 0x50 (80) 5%
Dynamic Memory Usage
region address maximum length (dec)
------ ------- ---------------------
heap 0 0 (0)
stack 0x850 0x5b0 (1456)
Maximum dynamic memory (bytes): 0x5b0 (1456)
main.o(.text+0x1a): In function `main':
F:CZH est est2main.c:38: undefined reference to `tt'
Link step failed.
----------------------------------------------------------------------
Debug build of project `F:CZH est est2 est1.mcp' failed.
Language tool versions: pic30-as.exe v3.22, pic30-gcc.exe v3.22, pic30-ld.exe v3.22, pic30-ar.exe v3.22
Preprocessor symbol `__DEBUG' is defined.
Target debug platform is `__MPLAB_DEBUGGER_PK3=1'.
Debug platform options are: `__ICD2RAM=1'.
Sat Jun 11 09:33:51 2011
----------------------------------------------------------------------
BUILD FAILED
报错是说main.c中没有声明tt。
是不是重复定义不是这种格式啊，请大侠指点。谢谢！
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