NAME

avr_pgmspace - <avr/pgmspace.h>: Program Space Utilities

SYNOPSIS

Macros

#define PROGMEM __ATTR_PROGMEM__
#define PGM_P const char *
#define PGM_VOID_P const void *
#define PSTR(s) ((const PROGMEM char *)(s))
#define pgm_read_byte_near(address_short) __LPM((uint16_t)(address_short))
#define pgm_read_word_near(address_short) __LPM_word((uint16_t)(address_short))
#define pgm_read_dword_near(address_short) __LPM_dword((uint16_t)(address_short))
#define pgm_read_float_near(address_short) __LPM_float((uint16_t)(address_short))
#define pgm_read_ptr_near(address_short) (void*)__LPM_word((uint16_t)(address_short))
#define pgm_read_byte_far(address_long) __ELPM((uint32_t)(address_long))
#define pgm_read_word_far(address_long) __ELPM_word((uint32_t)(address_long))
#define pgm_read_dword_far(address_long) __ELPM_dword((uint32_t)(address_long))
#define pgm_read_float_far(address_long) __ELPM_float((uint32_t)(address_long))
#define pgm_read_ptr_far(address_long) (void*)__ELPM_word((uint32_t)(address_long))
#define pgm_read_byte(address_short) pgm_read_byte_near(address_short)
#define pgm_read_word(address_short) pgm_read_word_near(address_short)
#define pgm_read_dword(address_short) pgm_read_dword_near(address_short)
#define pgm_read_float(address_short) pgm_read_float_near(address_short)
#define pgm_read_ptr(address_short) pgm_read_ptr_near(address_short)
#define pgm_get_far_address(var)

Detailed Description

#include <avr/io.h>
#include <avr/pgmspace.h>

The functions in this module provide interfaces for a program to access data stored in program space (flash memory) of the device. In order to use these functions, the target device must support either the LPM or ELPM instructions.

Note

These functions are an attempt to provide some compatibility with header files that come with IAR C, to make porting applications between different compilers easier. This is not 100% compatibility though (GCC does not have full support for multiple address spaces yet).

If you are working with strings which are completely based in ram, use the standard string functions described in <string.h>: Strings.

If possible, put your constant tables in the lower 64 KB and use pgm_read_byte_near() or pgm_read_word_near() instead of pgm_read_byte_far() or pgm_read_word_far() since it is more efficient that way, and you can still use the upper 64K for executable code. All functions that are suffixed with a _P require their arguments to be in the lower 64 KB of the flash ROM, as they do not use ELPM instructions. This is normally not a big concern as the linker setup arranges any program space constants declared using the macros from this header file so they are placed right after the interrupt vectors, and in front of any executable code. However, it can become a problem if there are too many of these constants, or for bootloaders on devices with more than 64 KB of ROM. All these functions will not work in that situation.

For Xmega devices, make sure the NVM controller command register (NVM.CMD or NVM_CMD) is set to 0x00 (NOP) before using any of these functions.

Macro Definition Documentation

#define pgm_get_far_address(var)

Value:.PP

({                                                    \
    uint_farptr_t tmp;                                \
                                                      \
    __asm__ __volatile__(                             \
                                                      \
            "ldi    %A0, lo8(%1)"           "\n\t"    \
            "ldi    %B0, hi8(%1)"           "\n\t"    \
            "ldi    %C0, hh8(%1)"           "\n\t"    \
            "clr    %D0"                    "\n\t"    \
        :                                             \
            "=d" (tmp)                                \
        :                                             \
            "p"  (&(var))                             \
    );                                                \
    tmp;                                              \
})

This macro facilitates the obtention of a 32 bit 'far' pointer (only 24 bits used) to data even passed the 64KB limit for the 16 bit ordinary pointer. It is similar to the '&' operator, with some limitations.

Comments:

The overhead is minimal and it's mainly due to the 32 bit size operation.
24 bit sizes guarantees the code compatibility for use in future devices.
hh8() is an undocumented feature but seems to give the third significant byte of a 32 bit data and accepts symbols, complementing the functionality of hi8() and lo8(). There is not an equivalent assembler function to get the high significant byte.
The returned value is the identifier's VMA (virtual memory address) determined by the linker and falls in the corresponding memory region. The AVR Harvard architecture requires non overlapping VMA areas for the multiple address spaces in the processor: Flash ROM, RAM, and EEPROM. Typical offset for this are 0x00000000, 0x00800xx0, and 0x00810000 respectively, derived from the linker script used and linker options. The value returned can be seen then as a universal pointer.

#define PGM_P const char *

Used to declare a variable that is a pointer to a string in program space.