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PROGMEM

[Utilities]

Description

Keep constant data in flash (program) memory only, instead of copying it to SRAM when the program starts. 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 "keep this information in flash memory only", instead of copying it to SRAM at start up, like it would normally do.

PROGMEM is part of the pgmspace.h library. It is included automatically in the Arduino IDE.

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 a two-step procedure. Once a variable has been defined with PROGMEM, it cannot be read like a regular SRAM-based variable: you have to read it using specific functions, also defined in pgmspace.h.

==== Important Note!

PROGMEM is useful only when working with AVR boards (Uno Rev3, Leonardo etc.). Newer boards (Due, MKR WiFi 1010, GIGA R1 WiFi etc.) automatically use the program space when a variable is declared as a const. However, for retro compatibility, PROGMEM can still be used with newer boards. This implementation currently lives here.

Syntax

const dataType variableName[] PROGMEM = {data0, data1, data3…​};

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

Parameters

dataType: Allowed data types: any variable type.
variableName: the name for your array of data.

Example Code

The following code fragments illustrate how to read and write unsigned chars (bytes) and ints (2 bytes) to PROGMEM.

// save some unsigned ints
const PROGMEM uint16_t charSet[] = { 65000, 32796, 16843, 10, 11234};

// save some chars
const char signMessage[] PROGMEM = {"I AM PREDATOR,  UNSEEN COMBATANT. CREATED BY THE UNITED STATES DEPART"};

unsigned int displayInt;
char myChar;


void setup() {
  Serial.begin(9600);
  while (!Serial);  // wait for serial port to connect. Needed for native USB

  // put your setup code here, to run once:
  // read back a 2-byte int
  for (byte k = 0; k < 5; k++) {
    displayInt = pgm_read_word_near(charSet + k);
    Serial.println(displayInt);
  }
  Serial.println();

  // read back a char
  int signMessageLength = strlen_P(signMessage);
  for (byte k = 0; k < signMessageLength; k++) {
    myChar = pgm_read_byte_near(signMessage + k);
    Serial.print(myChar);
  }

  Serial.println();
}

void loop() {
  // put your main code here, to run repeatedly:
}

Arrays of strings

It is often convenient when working with large amounts of text, such as a project with an LCD, to setup an array of strings. Because strings themselves are arrays, this is actually an example of a two-dimensional array.

These tend to be large structures so putting them into program memory is often desirable. The code below illustrates the idea.

/*
  PROGMEM string demo
  How to store a table of strings in program memory (flash),
  and retrieve them.

  Information summarized from:
  http://www.nongnu.org/avr-libc/user-manual/pgmspace.html

  Setting up a table (array) of strings in program memory is slightly complicated, but
  here is a good template to follow.

  Setting up the strings is a two-step process. First, define the strings.
*/

#include <avr/pgmspace.h>
const char string_0[] PROGMEM = "String 0"; // "String 0" etc are strings to store - change to suit.
const char string_1[] PROGMEM = "String 1";
const char string_2[] PROGMEM = "String 2";
const char string_3[] PROGMEM = "String 3";
const char string_4[] PROGMEM = "String 4";
const char string_5[] PROGMEM = "String 5";


// Then set up a table to refer to your strings.

const char *const string_table[] PROGMEM = {string_0, string_1, string_2, string_3, string_4, string_5};

char buffer[30];  // make sure this is large enough for the largest string it must hold

void setup() {
  Serial.begin(9600);
  while (!Serial);  // wait for serial port to connect. Needed for native USB
  Serial.println("OK");
}


void loop() {
  /* Using the string table in program memory requires the use of special functions to retrieve the data.
     The strcpy_P function copies a string from program space to a string in RAM ("buffer").
     Make sure your receiving string in RAM is large enough to hold whatever
     you are retrieving from program space. */


  for (int i = 0; i < 6; i++) {
    strcpy_P(buffer, (char *)pgm_read_ptr(&(string_table[i])));  // Necessary casts and dereferencing, just copy.
    Serial.println(buffer);
    delay(500);
  }
}

Notes and Warnings

Please note that variables must be either globally defined, OR defined with the static keyword, in order to work with PROGMEM.

The following code will NOT work when inside a function:

const char long_str[] PROGMEM = "Hi, I would like to tell you a bit about myself.\n";

The following code WILL work, even if locally defined within a function:

const static char long_str[] PROGMEM = "Hi, I would like to tell you a bit about myself.\n"

The F() macro

When an instruction like :

Serial.print("Write something on  the Serial Monitor");

is used, the string to be printed is normally saved in RAM. If your sketch prints a lot of stuff on the Serial Monitor, you can easily fill the RAM. This can be avoided by not loading strings from the FLASH memory space until they are needed. You can easily indicate that the string is not to be copied to RAM at start up using the syntax:

Serial.print(F("Write something on the Serial Monitor that is stored in FLASH"));

See also