byte PROGMEM list[100] .. strange results [solved]

Using Arduino Programming Questions
1

Hello !
I'm new to Arduino and i've tried the Code attached.
The result was:

freeMemory()=1635
97

I was expecting the result of my array[0] which is 1 but not 97 ?

At my first attempt i was using double to store in Progmem. I've read about "Floating point numbers in program memory do not appear to be supported." in the Arduino Reference. So i made this simple test with byte. But obviously there is a lack of understanding. When i exchange the line "byte PROGMEM list[100] " with "prog_uchar PROGMEM list[100]" it doesn't change the result.
I'd be happy to get any hint on this issue.

Arduino IDE 1.0.5-r2 / ATmega 368

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

void setup(void) {
    Serial.begin(9600);
}

void loop ()
{
  delay(2000);
  
  Serial.print(F("freeMemory()="));
  Serial.println(freeMemory());
  
  byte PROGMEM list[100] = {
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0
                      };
  Serial.println(pgm_read_byte (&list[0]));
}
2

Hi, welcome to the forum.

I would like to check it, but which "MemoryFree.h" are you using ?

You can try these:
prog_uchar list[100] PROGMEM = ...
prog_uchar PROGMEM list[100] = ...
const byte list[100] PROGMEM = ...

That has been changed a little for the new Arduino 1.5.7 BETA. If you like to try that, the 'prog_uchar' has been discarded, and 'const byte' should be used.

3

I don't think you can use PROGMEM as an automatic variable - one defined inside a function. Move it to the global data area outside of setup().

4 
#include <MemoryFree.h>
#include <avr/pgmspace.h>

void setup(void) {
    Serial.begin(9600);
}

void loop ()
{
  delay(2000);
  
  Serial.print(F("freeMemory()="));
  Serial.println(freeMemory());
  
  static byte PROGMEM list[100] = {
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0
                      };
  Serial.println(pgm_read_byte (&list[0]));
}
5

Or do that. 8^)

6

Thank you all !
Moving the array definition outside the function made it work :slight_smile:
Also the "static byte PROGMEM" worked as well.

7

Test with Arduino 1.5.7 BETA, this version shows the sketch (code in flash) size and the global variables use (in ram).
This is my test sketch:

const byte PROGMEM list[100] = {
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0,
                        1,2,3,4,5,6,7,8,9,0
                      };
                      
void setup(void) {
    Serial.begin(9600);
}

void loop ()
{
  Serial.println( pgm_read_byte (&(list[0])));
}

Sketch uses 2222 bytes, Global variables use 188 bytes.
Without the 'const' and without 'PROGMEM' : Sketch uses 2222 bytes, Global variables use 288 bytes.
Instead of 'byte', now 'uint32_t' : Sketch uses 2522 bytes, Global variables use 188 bytes.
That is perfect, 100 times 4 bytes is 400 bytes, and it was 100 bytes before. 300 byte increase.
Now for the real test with : const float PROGMEM list[100] = {
Sketch uses 2522 bytes, Global variables use 188 bytes.
So even for floating point there is no problem.

It all makes perfect sense.