Array of bytes in PROGMEM

Using Arduino Programming Questions
#1

Hello!
this is my first post here :slight_smile: i hope i posted it in the right section.
I am a beginner with Arduino too

I am trying for two long days to learn how to store variables in PROGMEM and use them, but i can’t get this to work. The following example works fine if i remove PROGMEM and switch back to read the struct array directly.

How do i set it to read those bytes from PROGMEM right?
It compiles but read weird/wrong data

#include <avr/pgmspace.h>
#include "LedControl.h"

//DataIn, CLK, LOAD, single MAX72XX.
LedControl lc=LedControl(12,11,10,1);


void setup() {
  Serial.begin(9600);
  lc.shutdown(0,false);
  lc.setIntensity(0,8);
  lc.clearDisplay(0);
}

typedef struct  {
  String charName;
  byte charBytes[8];
} CustomCharacter;

static const CustomCharacter mxlChr[] PROGMEM = {
{"smiley",{B01000010,B10100101,B10100101,B01000010,B00000000,B10000001,B01000010,B00111100}},//0
{"arrow_L",{0b00011000,0b00110000,0b01100000,0b11111111,0b11111111,0b01100000,0b00110000,0b00011000}},//1
{"arrow_R",{0b00011000,0b00001100,0b00000110,0b11111111,0b11111111,0b00000110,0b00001100,0b00011000}},//2
{"M",{B10000001,B11000011,B10100101,B10011001,B10000001,B10000001,B10000001,B10000001}},
{"A",{B00011000,B00100100,B01000010,B01000010,B01111110,B11000011,B10000001,B10000001}},
{"X",{B10000001,B01000010,B00100100,B00011000,B00011000,B00100100,B01000010,B10000001}},
{"sound",{B00001100,B00000010,B01001001,B11000101,B11000101,B01001001,B00000010,B00001100}},
};

 
void writeCustomChar(int c) {  
    lc.clearDisplay(0);
    for(int r=0; r<8; r++){
      byte lcdrow = pgm_read_byte(mxlChr[c].charBytes[r]);
      Serial.println(lcdrow);
      //byte lcdrow = mxlChr[c].charBytes[r];
      lc.setColumn(0,r,lcdrow);
    }
}


//examples
//lc.setRow(0,row,B10100000);
//lc.setColumn(0,col,B10100000);
//lc.setLed(0,row,col,false);


void loop() { 
  for(int c=0; c<7; c++){
    writeCustomChar(c);
    delay(1000);
  }
}

The purpose: I am trying to do a little code to write custom glyphs on a 8x8 led matrix, that i previously defined in that array, because nothing i found so far works so i decided to make my own.
…and also because i want to learn how to store large amounts of organized data.

Any advice or fix for this is most welcome

Thank you very much!

#2

This doesn’t show anything in the serial monitor.
Where is the & in byte lcdrow = pgm_read_byte(mxlChr
```c
**.charBytes[r]);[/b]

This however, does.
byte lcdrow = pgm_read_byte(&mxlChr[c].charBytes[r]);

If you want to see exactly what you’re getting, you can do this.
Serial.println(lcdrow, BIN); // this will show the value in binary and it should also match what you have in the array.**
```

#3

Thank you for the reply.
I am still fighting with this. Eventually i made the code without using progmem , its working, but eventually i will run out of memory and will need to find ugly alternate ways to avoid that (like dividing the functionality over two arduinos)

If anyone could tell me how i can actually use PROGMEM in the above example , it would be absolutely awesome. The way you sugested HazardMinds is showing on serial "0" but does nothing useful. I tried various other ways in the meantime and i didn't get anything better.

Basically what i want to do is access a structures values that are stored in progmem

Thank you a lot

#4

Please post your new code and your attempt to solve the issue in your first code.

#5