Atmel CAVR-4 Manual De Usuario

CAVR-4

Selecting data types and placing data in memory

Reference Guide

This can be done on individual strings or for the whole application/file using the option 

--string_literals_in_flash

. Examples on how to put individual strings into 

flash:

_ _flash char str1[] = "abcdef";

_ _flash char str2[] = "ghi";

_ _flash char _ _flash * pVar[] = { str1, str2 };

String literals cannot be put in flash automatically, but you can use a local static variable 
instead:

#include <pgmspace.h>

void f (int i)

{

  static _ _flash char sl[] = "%d cookies\n";

  printf_P(sl, i);

}

This does not result in more code compared to allowing string literals in flash.

To use flash strings, you must use alternative library routines that expect flash strings. 
A few such alternative functions are provided in the 

pgmspace.h

 header file. They are 

flash alternatives for some common C library functions with an extension 

_P

. For your 

own code, you can always use the 

_ _flash

 keyword when passing the strings between 

functions.

For reference information about the alternative functions, see AVR–specific library 
functions, page 249.

The data types you use should be considered carefully, because this can have a large 
impact on code size and code speed.

●

Use small and unsigned data types, (

unsigned char

 and 

unsigned short

) 

unless your application really requires signed values.

●

Try to avoid 64-bit data types, such as 

double

 and 

long long

.

●

Bitfields with sizes other than 1 bit should be avoided because they will result in 
inefficient code compared to bit operations.

●

When using arrays, it is more efficient if the type of the index expression matches 
the index type of the memory of the array. For 

_ _near

 the index type is 

int

.

●

Using floating-point types on a microprocessor without a math co-processor is very 
inefficient, both in terms of code size and execution speed.

●

Declaring a pointer to 

const

 data tells the calling function that the data pointed to 

will not change, which opens for better optimizations.