Arduino PROGMEM byte array

Question

Consider this snippet, compiled with Arduino IDE:

PROGMEM  char charSet[]  = { 0x0, 0x8, 0x4, 0xc, 0x2, 0xA , 0x6, 0xE, 0x1, 0x9,0x5, 0xD, 0x3,0xB,0x7,0xF };
char reversed[]          = { 0x0, 0x8, 0x4, 0xc, 0x2, 0xA , 0x6, 0xE, 0x1, 0x9,0x5, 0xD, 0x3,0xB,0x7,0xF };
char ff;

for (int i=0; i < 16; i++) {
    Serial.print(" ");
    Serial.print(reversed[i], HEX);
}

Serial.println(" ");
for (int i=0; i < 16; i++) {
    Serial.print(" ");
    ff = pgm_read_byte(&charSet[i]);
    Serial.print(ff);
}

I would expect the two for loops to produce same output. But the output is:

  0 8 4 C 2 A 6 E 1 9 5 D 3 B 7 F
  FFFFFF94 FFFFFFB0 6 FFFFFF80 FFFFFF91 FFFFFFC7 3 62 FFFFFFE3 E FFFFFF94 5E 29 FFFFFF99 23 39

What am I missing?

Answer 1

Yes, the const keyword is missing.

Probably it is not a good idea to use prog_char, as it is marked as deprecated in gcc-avr, see avr/pgmspace.h

Answer 2

Well the answer was the PROGMEM declaration. I had to move it out from function, adding const keyword. Other changes as datatypes were not working.

The full answer is on Arduino forum.

Answer 3

You have the wrong datatype. From http://arduino.cc/en/Reference/PROGMEM:

Some cryptic bugs are generated by using ordinary datatypes for program memory calls.

You can't use char — you have to use prog_char

So:

PROGMEM prog_char charSet[] = {0x0, 0xB}

That should work.

by the way: You don't have to do Serial.println(" "); You can just do Serial.println();

Answer 4

PROGMEM  char charSet[] = ...

This is not good - for PROGMEM data, you can't use ordinary in-memory types. What you should do is

PROGMEM  prog_char charSet[] = ...