Flash: A new PROGMEM library

Forum 2005-2010 (read only) Software Development
#1

Flash
I have written a new library, Flash, which abstracts away most of the complexity of PROGMEM. It provides String, Array, and Table types that make ROM-based data collections as easy to use as “normal” types. Each overrides the C++ operator, so to get at individual elements, one simply uses familiar array syntax. For example, if you have 1000 floating point temperatures in a flash-based “temperature_table” array, you might write code like this:

      for (int i=0; i<temperature_table.count(); ++i)
        if (temperature_table[i] > 98.6)
          Serial.println(temperature_table[i]);

Font tables are a good application for Flash. Assuming that font_table is a 256x7 table of On/Off values, one row for each character, this simple example displays the number “2” on a 7-segment display:

      int digit = '2';
      for (int i=0; i<7; ++i)
        digitalWrite(pin[i], font_table[digit][i]);

Inline ROM Strings
One of the most useful features of Flash is the ability to use “inline”, or unnamed ROM-based strings. This provides a tremendous convenience for applications that process lots of string data. To print an inline flash string, you use the library’s built-in F() macro:

    Serial << F("A very long ROM-based string?.");
For those uncomfortable with the << streaming notation, you can also write

      Serial.print(F("A very long ROM-based string?"));

but this requires a minor patch to core file Print.h to tell it about Flash objects. (See the file patching_print.txt.)

I welcome feedback and suggestions for improvement. The library is posted here.

Thanks,

Mikal

#2

Huzzah, very nice! I have mostly avoided PROGMEM due to the byte-for-byte problem. I almost made a PROGMEM_readAnything() to make RAM copies, but that defeats the purpose in most cases. I really do wish the ATmega didn't require a completely different instruction to address prom, but oh well.

#3

great work!

Nice idea and good explanation on your site. I will test it soon. I really like the idea of FLASH_TABLE and the patch to Print.h is useful because people are very used to print and println.

#4

Thank you both for the kind feedback. I think Flash is some of my most useful work.

I published a new version (2) Wednesday that reduces the flash space overhead consumed by String objects.

Thanks again!

Mikal

#5

This is very cool. I'll add some pointers to this into my Webduino library and documentation, as it looks to be a better solution than what I did; in that library, I made a P() macro that defined a progmem string then added a printP method to the WebServer class for printing them out.

I'm also going to adopt your template for adding a no-cost stream operator. I was really missing that one!

Thanks.

#6

Thanks, Ben. For a long time people have wanted a way to print() a PROGMEM string as easily as a “normal” string. The problem, as I’m sure you noticed, is that gcc does not consider a prog_char * sufficiently different from a char *, so you can’t override print() on that basis. But it occurred to me one day that if one were to define a lightweight class to wrap PROGMEM strings, that would be sufficiently different.

I appreciate the feedback.

Mikal

#7

i got a compile error using your library.

“error: ‘cosarray’ was not declared in this scope In function ‘void closedoors()’:”

tools:
Arduino 0017
Arduiniana flash 2.0

code snippets:

#include <Flash.h>
#include <EEPROM.h> 
...
void setup(){
 FLASH_ARRAY(float, cosarray,
  cos(0),cos(4.5),cos(9),cos(13.5),
  cos(18),cos(22.5),cos(27),cos(31.5),
  cos(36),cos(40.5),cos(45),cos(49.5),
  cos(54),cos(58.5),cos(63),cos(67.5),
  cos(72),cos(76.5),cos(81),cos(85.5)); //define array of cos(x) in progmem

}

void opendoors(){
...
      stepper(int(minDelay + delayVal + cosarray[trajectCounter]*(maxDelay)));

}

i have no idea what goes wrong. FLASH_ARRAY is red, so is recognized by the ide

the next uses a direct paste from Flash | Arduiniana and yelds the same error. :frowning:

#include <Flash.h>
float result = 0;

void setup(){
  FLASH_ARRAY(float, temperatures, 23.1, 23.1, 23.2, 23.2, 23.4,
    23.7, 25.0, 26.0, 26.8, 28.8, 30.2, 31.9, 33.1, 33.1, 33.2,
    33.2, 33.4, 33.7, 35.0, 36.0, 36.8, 38.8, 40.2, 41.9);
}

void loop(){
  result = temperatures[4];
}
#8

Move the cosarray declaration…

#include <Flash.h>
#include <EEPROM.h>

FLASH_ARRAY(float, cosarray,
  cos(0),cos(4.5),cos(9),cos(13.5),
  cos(18),cos(22.5),cos(27),cos(31.5),
  cos(36),cos(40.5),cos(45),cos(49.5),
  cos(54),cos(58.5),cos(63),cos(67.5),
  cos(72),cos(76.5),cos(81),cos(85.5)); //define array of cos(x) in progmem

void setup()
{
}

void opendoors()
{
...
      stepper(int(minDelay + delayVal + cosarray[trajectCounter]*(maxDelay)));
}
#9

Ah. a macro is just a declaration. of course. :P

#10