Convert String to Const Char XYZ[] PROGMEM {}

Question

How to convert a String to Const Char XYZ[] PROGMEM ?

Specific issue:

I am working on Arduino + OLED project (using the squix library ) in which I am displaying image defined by WiFi_Logo_bits[] using the following code

void drawImageDemo() {
    display.drawXbm(0, 0, WiFi_Logo_width, WiFi_Logo_height, WiFi_Logo_bits); // variables are defined below
}

Everything works as it as, but now I want to dynamically get the image data from internet and store in WiFi_Logo_bits[] . I am successfully able to retrieve the data for ex: {0xFE, 0xDE, 0x01......} as String , but cannot assign the value to WiFi_Logo_bits[] PROGMEM as I am having problem with datatypes conversion.

Variables above are defined as below:
#define WiFi_Logo_width 119
#define WiFi_Logo_height 63
const char WiFi_Logo_bits[] PROGMEM = {
   0xF2, 0xFF, 0xE7, 0xFF, 0xFF, 0xFE, 0x1F, 0x04, 0x91, 0xF8, 0x01, 0x00, 
  0x00, 0x00, 0x00, 0xF2, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x03, 0x04, 0x91, 
  0xF0, 0x03, 0x00, 0x00, 0x10, 0x00, 0xF2, 0xFF, 0xFF, 0xFF, 0xFF, 0x3F, 
  0x0C, 0x86, 0x81, 0xF8, 0x00, 0x00, 0x00, 0x10, 0x00, 0xE3, 0xFF, 0xFF, 
  0xFF, 0xFF, 0x17, 0x04, 0x83, 0xE4, 0xF9, 0x06, 0x00, 0x00, 0x00, 0x00, 
  0xE1, 0xFF, 0xFD, 0xFF, 0xFF, 0x01, 0x82, 0xC0, 0xC9, 0xF8, 0x04, 0x00, 
  0x00, 0x00, 0x00, 0xD0, 0xFF, 0xFB, 0xFF, 0xBF, 0x00, 0x40, 0xE0, 0x20, 
  0xFA, 0x00, 0x00, 0x00, 0x00, 0x00, 0xE2, 0xFF, 0xFF, 0xFF, 0x1D, 0x20, 
  0x20, 0xE0, 0x70, 0xFC, 0x09, 0x00, 0x00, 0x00, 0x40, 0xFC, 0xFF, 0x7B, 
  0xFF, 0x0B, 0x00, 0x18, 0x70, 0x14, 0xF9, 0x04, 0x00, 0x00, 0x00, 0x00, 
  0xFE, 0xFF, 0x9F, 0xBF, 0x02, 0x00, 0x04, 0x18, 0xB1, 0xF8, 0x6C, 0x00, 
  0x00, 0x00, 0x00, 0xFF, 0xFF, 0xFF, 0xDF, 0x01, 0x04, 0x02, 0x86, 0x88, 
  0xFB, 0x3E, 0x00, 0x00, 0x00, 0x00, 0xFF, 0xFF, 0xFF, 0x9F, 0x00, 0x00, 
  0xC1, 0x01, 0x3A, 0x60, 0x1E, 0x00, 0x00, 0x00, 0x00, 0xFE, 0xFF, 0xFF, 
  0x1F, 0x00, 0xC1, 0x20, 0xA0, 0x42, 0x72, 0x0E, 0x00, 0x00, 0x00, 0x00, 
  0xFE, 0xFF, 0xB7, 0x9F, 0x00, 0x40, 0x10, 0xB0, 0x61, 0xF4, 0x8E, 0x00, 
  0x00, 0x00, 0x00, 0xFC, 0xFF, 0xE7, 0x2F, 0x00, 0x30, 0x08, 0x0A, 0x05, 
  0x01, 0x8E, 0x00, 0x00, 0x00, 0x00, 0x7F, 0xFF, 0xFD, 0x0F, 0x00, 0x18, 
  0x8C, 0x02, 0x87, 0x66, 0x1F, 0x03, 0x00, 0x00, 0x00, 0xBF, 0xFF, 0xFF, 
  0x1F, 0x00, 0x08, 0x03, 0x80, 0xAE, 0x24, 0x3E, 0x0F, 0x00, 0x00, 0x20, 
  0xBF, 0x7F, 0xFF, 0x2D, 0x00, 0x04, 0x83, 0x80, 0x8D, 0x24, 0xBF, 0x1B, 
  0x00, 0xA0, 0x14, 0x9F, 0xFF, 0xBE, 0x03, 0x00, 0x82, 0x61, 0x70, 0xA2, 
  0x25, 0xFE, 0xB5, 0x00, 0x10, 0x63, 0xFF, 0x2F, 0xFE, 0x05, 0x00, 0x83, 
  0x10, 0x0C, 0x54, 0x51, 0xFE, 0x63, 0x00, 0x00, 0x00, 0xFF, 0x83, 0xCF, 
  0x11, 0x80, 0x41, 0x00, 0x02, 0xDA, 0x0B, 0xFE, 0xB7, 0x00, 0x00, 0x00, 
  0x7E, 0xF2, 0x3B, 0x0A, 0x81, 0x60, 0x80, 0x01, 0xD7, 0x63, 0xFE, 0x3F, 
  0x02, 0x00, 0x00, 0x1F, 0xF0, 0x5E, 0x08, 0x60, 0x30, 0x40, 0x00, 0x6F, 
  0x07, 0xFC, 0x7F, 0x0C, 0x00, 0x00, 0x1F, 0xFE, 0x6F, 0x00, 0x20, 0x10, 
  0x60, 0xC0, 0xFF, 0x4A, 0xFC, 0x77, 0x0B, 0x00, 0x08, 0x2D, 0xFC, 0x5F, 
  0x80, 0x10, 0x08, 0x10, 0xE0, 0xDF, 0x01, 0xF2, 0xA4, 0x1D, 0x00, 0x00, 
  0x1A, 0x7E, 0x9F, 0x82, 0x18, 0x04, 0x08, 0xF8, 0x6F, 0x04, 0xD0, 0xF5, 
  0x3F, 0x00, 0x00, 0x18, 0xFE, 0x3F, 0x62, 0x08, 0x06, 0x06, 0xFC, 0x9F, 
  0x00, 0x00, 0xE8, 0x7F, 0x00, 0x00, 0xAC, 0xBF, 0x3F, 0x0A, 0x0C, 0x03, 
  0x01, 0xFC, 0x0B, 0x11, 0x00, 0x60, 0xFF, 0x00, 0x00, 0xDA, 0xFF, 0xBF, 
  0x20, 0x04, 0x80, 0x00, 0xFC, 0x0A, 0x04, 0x00, 0xC0, 0xE3, 0x00, 0x00, 
  0x39, 0xFF, 0x1F, 0x30, 0x82, 0x50, 0x20, 0xFF, 0x10, 0x1B, 0x00, 0x00, 
  0xD0, 0x03, 0x00, 0xDF, 0xFF, 0x3F, 0x28, 0x42, 0x20, 0x00, 0xBF, 0x01, 
  0x05, 0x00, 0x00, 0xA0, 0x07, 0x00, 0xDF, 0xFF, 0x7F, 0x30, 0x63, 0x30, 
  0x80, 0x1F, 0x80, 0x6E, 0x01, 0x00, 0x00, 0x07, 0x00, 0xAE, 0xFB, 0xFF, 
  0x20, 0x23, 0x1C, 0xC0, 0x0F, 0x00, 0x17, 0x00, 0x00, 0x00, 0x0E, 0x00, 
  0xBB, 0xF8, 0xFD, 0x19, 0x21, 0x0C, 0xE0, 0x07, 0x40, 0x07, 0x02, 0x00, 
  0x00, 0x0E, 0x00, 0xA7, 0xFA, 0xF9, 0x8B, 0x11, 0x0A, 0xF0, 0x03, 0xC0, 
  0x7F, 0x0C, 0x00, 0x00, 0x38, 0x00, 0x19, 0xFE, 0x79, 0x0F, 0x13, 0x08, 
  0xE4, 0x01, 0xF8, 0xBF, 0x02, 0x00, 0x00, 0x60, 0x00, 0x73, 0xFE, 0x77, 
  0x1F, 0x01, 0x0D, 0xEE, 0x00, 0xFC, 0x7F, 0x05, 0x00, 0x00, 0xE0, 0x00, 
  0x33, 0xFA, 0xFF, 0x7E, 0x12, 0x05, 0x6E, 0x00, 0xFE, 0x7D, 0x06, 0x00, 
  0x00, 0xE0, 0x00, 0xE5, 0xFF, 0xFD, 0x7C, 0x12, 0x04, 0xFD, 0x00, 0x23, 
  0x7E, 0x00, 0x00, 0x00, 0xF0, 0x00, 0xF7, 0xF3, 0xFF, 0xF8, 0x13, 0x04, 
  0x3C, 0x00, 0x19, 0xFC, 0x00, 0x00, 0x00, 0xF0, 0x01, 0xE7, 0xF3, 0xFF, 
  0xF9, 0x12, 0x0C, 0x3C, 0x80, 0x38, 0xFC, 0x00, 0x00, 0x00, 0x78, 0x01, 
  0x4C, 0xF3, 0xDF, 0xF7, 0x33, 0x08, 0x18, 0x80, 0x7C, 0x7C, 0x01, 0x00, 
  0x00, 0x7E, 0x07, 0xCE, 0xF9, 0xFF, 0x77, 0x27, 0x10, 0x14, 0x40, 0xFE, 
  0xFC, 0x00, 0x00, 0x00, 0x3E, 0x06, 0xCE, 0xE9, 0x7B, 0xEF, 0x27, 0x00, 
  0x18, 0x40, 0xFE, 0xFC, 0x04, 0x00, 0x40, 0x7F, 0x07, 0x8C, 0xF4, 0xFF, 
  0xEF, 0x07, 0x00, 0x0C, 0x20, 0xFF, 0x5E, 0x10, 0x00, 0x60, 0x3F, 0x0E, 
  0x8C, 0xE2, 0xFB, 0xFF, 0x0B, 0x00, 0x00, 0x20, 0xFF, 0xDE, 0x10, 0x00, 
  0x60, 0x3F, 0x1E, 0x18, 0xDB, 0xEF, 0xFF, 0x27, 0x00, 0x08, 0x30, 0x6E, 
  0xF6, 0x00, 0x04, 0x60, 0x7A, 0x23, 0x78, 0xCA, 0xF7, 0xEF, 0x1B, 0x00, 
  0x08, 0xB0, 0x3C, 0xFB, 0xA0, 0x0D, 0x60, 0x1D, 0x2F, 0xB2, 0x9F, 0xFF, 
  0xFF, 0x17, 0x00, 0x00, 0xD0, 0x10, 0x65, 0xA0, 0x0D, 0x70, 0x1C, 0x2D, 
  0x44, 0x1B, 0xFF, 0xFF, 0x1D, 0x00, 0x00, 0xF0, 0x52, 0x74, 0x60, 0x73, 
  0xD8, 0x8C, 0x6B, 0x8C, 0x1F, 0xFF, 0xFF, 0x2F, 0x00, 0x00, 0xB4, 0x99, 
  0x6B, 0x80, 0x8C, 0x04, 0x82, 0x67, 0x08, 0xB7, 0xFF, 0xFF, 0x9F, 0x00, 
  0x00, 0xE0, 0x20, 0x6A, 0x40, 0x1A, 0x11, 0xC2, 0x69, 0x11, 0x3F, 0xFF, 
  0xFF, 0x1F, 0x00, 0x00, 0xE0, 0x85, 0x50, 0x00, 0x59, 0x48, 0xE1, 0x64, 
  0x32, 0x7F, 0xFF, 0xFF, 0x9F, 0x00, 0x00, 0xF0, 0x01, 0x20, 0x80, 0x14, 
  0xD6, 0xE0, 0x71, 0x62, 0xFF, 0xFF, 0xFF, 0x1F, 0x00, 0x00, 0x64, 0x15, 
  0x00, 0x80, 0x65, 0x10, 0x30, 0x60, 0xC5, 0xFE, 0xFF, 0xFF, 0x3F, 0x00, 
  0x00, 0x28, 0x05, 0x00, 0x00, 0x12, 0xAB, 0x70, 0x70, 0x89, 0xFD, 0xFF, 
  0xFF, 0x1B, 0x00, 0x00, 0x20, 0x02, 0x20, 0x80, 0xD8, 0x84, 0x58, 0x70, 
  0x09, 0xFB, 0xFF, 0xFF, 0x1F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x29, 
  0x24, 0x0C, 0x70, 0x0B, 0xFF, 0xFF, 0xFF, 0xBF, 0x00, 0x00, 0x00, 0x00, 
  0x00, 0x00, 0x60, 0x1B, 0x1C, 0x70, 0x16, 0xFE, 0xFF, 0xFF, 0x7F, 0x00, 
  0x00, 0x00, 0x00, 0x00, 0x00, 0xA8, 0x0A, 0x0E, 0x78, 0x04, 0xFC, 0xFF, 
  0xFF, 0x7F, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x50, 0x28, 0x0E, 0x78, 
  0x4D, 0xF4, 0xFF, 0xFF, 0xFF, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xA0, 
  0x8A, 0x07, 0x7C, 0x4C, 0xF8, 0xFF, 0xFF, 0xFF, 0x01, 0x00, 0x00, 0x00, 
  0x00, 0x00, 0x40, 0x91, 0x03, 0x21, 0x90, 0xFD, 0xFF, 0xFF, 0xFF, 0x02, 
  0x00, 0x00, 0x00, 0x00, 0x00, 0xA0, 0x95, 0x83, 0x1A,};

Answer 1

The PROGMEM type modifier tells the compiler the array values should be put into flash memory - that makes the variable not only logically const , but also even physically (even if your program would allow modifying the const variable, it couldn't, because flash can't simply be written to like other memory. It would need to re-flash the device on the fly)

The short answer is: If something is declared const PROGMEM , you cannot change it.

Answer 2

In theory you can modify PROGMEM, but:

• It must be stored in one flash memory block (it must be erased and programmed as block)

• It can't be erased/programmed from regular program sections but from bootloader section (doesn't apply for most ATTinys as they don't have bootloader section). This requires your own bootloader to support it.

Or you can use EEPROM to store this array:

• It is not such a big problem to change it. It just needs similar functions to read/write EEMEM variables.

• But draw methods in the library are pretty much relying on PROGMEM, so you have to write your own drawXbm function (method if you extend current library).