久一网络科技

在WinAVR中，有一个delay.h文件，里面包含了_delay_us和_delay_ms两个函数，分别可以延时微秒和毫秒时间。看下面一个例子：

#include <avr/io.h>
#include <util/delay.h>

unsigned char t = 10;

int main()
{
  while(1)
  {
    _delay_ms(10);
  }

  return 0;
}

编译后，程序空间占用为：

AVR Memory Usage
----------------
Device: atmega168

Program: 180 bytes (1.1% Full)
(.text + .data + .bootloader)

Data: 2 bytes (0.2% Full)
(.data + .bss + .noinit)


Build succeeded with 0 Warnings...


如果将上面程序稍作修改：

#include <avr/io.h>
#include <util/delay.h>

unsigned char t = 10;

int main()
{
  while(1)
  {
    _delay_ms(t);
  }

  return 0;
} 

Device: atmega168

Program: 3844 bytes (23.5% Full)
(.text + .data + .bootloader)

Data: 266 bytes (26.0% Full)
(.data + .bss + .noinit)

Build succeeded with 0 Warnings...

发现RAM和ROM的占用突然就急剧增加了。这是为什么呢？从程序直接看，没有什么问题。打开delay.h文件，我们发现有一段说明：

In order for these functions to work as intended, compiler optimizations must be enabled, and the delay time must be an expression that is a known constant at compile-time. If these requirements are not met, the resulting delay will be much longer (and basically unpredictable), and applications that otherwise do not use floating-point calculations will experience severe code bloat by the floating-point library routines linked into the application.

上面这段话说明delay函数需要使用常数作为延时的参数，否则延时的结果可能会很长（不可预知），同时会因为链接了浮点库到程序，所以程序大小会急剧膨胀。


参考文献：
http://blog.ednchina.com/shaoziyang/139750/message.aspx