Description
Store data in flash (program) memory instead of SRAM. There’s a description of the various types of memory available on an Arduino board.
The PROGMEM
keyword is a variable modifier, it should be used only with the datatypes defined in pgmspace.h. It tells the compiler "put this information into flash memory", instead of into SRAM, where it would normally go.
PROGMEM is part of the pgmspace.h library. It is included automatically in modern versions of the IDE, however if you are using an IDE version below 1.0 (2011), you’ll first need to include the library at the top your sketch, like this:
#include <avr/pgmspace.h>
Syntax
const dataType variableName[] PROGMEM = {data0, data1, data3…};
dataType
- any variable type
variableName
- the name for your array of data
Note that because PROGMEM is a variable modifier, there is no hard and fast rule about where it should go, so the Arduino compiler accepts all of the definitions below, which are also synonymous. However experiments have indicated that, in various versions of Arduino (having to do with GCC version), PROGMEM may work in one location and not in another. The "string table" example below has been tested to work with Arduino 13. Earlier versions of the IDE may work better if PROGMEM is included after the variable name.
const dataType variableName[] PROGMEM = {}; // use this form
const PROGMEM dataType variableName[] = {}; // or this one
const dataType PROGMEM variableName[] = {}; // not this one
While PROGMEM
could be used on a single variable, it is really only worth the fuss if you have a larger block of data that needs to be stored, which is usually easiest in an array, (or another C++ data structure beyond our present discussion).
Using PROGMEM
is also a two-step procedure. After getting the data into Flash memory, it requires special methods (functions), also defined in the pgmspace.h library, to read the data from program memory back into SRAM, so we can do something useful with it.