Hi there,
I am new to this forum so please help me any correct and errors I make.
I am relatively new to C and I would like to make my own POV display.
Rather than just using some code I’d like to try and understand it too.
Here is the code I am testing with which has been taken from this forum post.
#include <Wire.h>
#include <MsTimer2.h>
// font data - each character is 8 pixels deep and 5 pixels wide
byte font [96] [5] PROGMEM = {
{ 0x00, 0x00, 0x00, 0x00, 0x00 }, // space (0x20)
{ 0x00, 0x00, 0x2F, 0x00, 0x00 }, // !
{ 0x00, 0x07, 0x00, 0x07, 0x00 }, // "
{ 0x14, 0x7F, 0x14, 0x7F, 0x14 }, // #
{ 0x24, 0x2A, 0x7F, 0x2A, 0x12 }, // $
{ 0x23, 0x13, 0x08, 0x64, 0x62 }, // %
{ 0x36, 0x49, 0x55, 0x22, 0x50 }, // &
{ 0x00, 0x05, 0x03, 0x00, 0x00 }, // '
{ 0x00, 0x1C, 0x22, 0x41, 0x00 }, // (
{ 0x00, 0x41, 0x22, 0x1C, 0x00 }, // (
{ 0x14, 0x08, 0x3E, 0x08, 0x14 }, // *
{ 0x08, 0x08, 0x3E, 0x08, 0x08 }, // +
{ 0x00, 0x50, 0x30, 0x00, 0x00 }, // ,
{ 0x08, 0x08, 0x08, 0x08, 0x08 }, // -
{ 0x00, 0x30, 0x30, 0x00, 0x00 }, // .
{ 0x20, 0x10, 0x08, 0x04, 0x02 }, // /
{ 0x3E, 0x51, 0x49, 0x45, 0x3E }, // 0 (0x30)
{ 0x00, 0x42, 0x7F, 0x40, 0x00 }, // 1
{ 0x42, 0x61, 0x51, 0x49, 0x46 }, // 2
{ 0x21, 0x41, 0x45, 0x4B, 0x31 }, // 3
{ 0x18, 0x14, 0x12, 0x7F, 0x10 }, // 4
{ 0x27, 0x45, 0x45, 0x45, 0x39 }, // 5
{ 0x3C, 0x4A, 0x49, 0x49, 0x30 }, // 6
{ 0x01, 0x71, 0x09, 0x05, 0x03 }, // 7
{ 0x36, 0x49, 0x49, 0x49, 0x36 }, // 8
{ 0x06, 0x49, 0x49, 0x29, 0x1E }, // 9
{ 0x00, 0x36, 0x36, 0x00, 0x00 }, // :
{ 0x00, 0x56, 0x36, 0x00, 0x00 }, // ;
{ 0x08, 0x14, 0x22, 0x41, 0x00 }, // <
{ 0x14, 0x14, 0x14, 0x14, 0x14 }, // =
{ 0x00, 0x41, 0x22, 0x14, 0x08 }, // >
{ 0x02, 0x01, 0x51, 0x09, 0x06 }, // ?
{ 0x32, 0x49, 0x79, 0x41, 0x3E }, // @ (0x40)
{ 0x7E, 0x11, 0x11, 0x11, 0x7E }, // A
{ 0x7F, 0x49, 0x49, 0x49, 0x36 }, // B
{ 0x3E, 0x41, 0x41, 0x41, 0x22 }, // C
{ 0x7F, 0x41, 0x41, 0x22, 0x1C }, // D
{ 0x7F, 0x49, 0x49, 0x49, 0x41 }, // E
{ 0x7F, 0x09, 0x09, 0x09, 0x01 }, // F
{ 0x3E, 0x41, 0x49, 0x49, 0x7A }, // G
{ 0x7F, 0x08, 0x08, 0x08, 0x7F }, // H
{ 0x00, 0x41, 0x7F, 0x41, 0x00 }, // I
{ 0x20, 0x40, 0x41, 0x3F, 0x01 }, // J
{ 0x7F, 0x08, 0x14, 0x22, 0x41 }, // K
{ 0x7F, 0x40, 0x40, 0x40, 0x40 }, // L
{ 0x7F, 0x02, 0x0C, 0x02, 0x7F }, // M
{ 0x7F, 0x04, 0x08, 0x10, 0x7F }, // N
{ 0x3E, 0x41, 0x41, 0x41, 0x3E }, // O
{ 0x3F, 0x09, 0x09, 0x09, 0x06 }, // P (0x50)
{ 0x3E, 0x41, 0x51, 0x21, 0x5E }, // Q
{ 0x7F, 0x09, 0x19, 0x29, 0x46 }, // R
{ 0x46, 0x49, 0x49, 0x49, 0x31 }, // S
{ 0x01, 0x01, 0x7F, 0x01, 0x01 }, // T
{ 0x3F, 0x40, 0x40, 0x40, 0x3F }, // U
{ 0x1F, 0x20, 0x40, 0x20, 0x1F }, // V
{ 0x3F, 0x40, 0x30, 0x40, 0x3F }, // W
{ 0x63, 0x14, 0x08, 0x14, 0x63 }, // X
{ 0x07, 0x08, 0x70, 0x08, 0x07 }, // Y
{ 0x61, 0x51, 0x49, 0x45, 0x43 }, // Z
{ 0x00, 0x7F, 0x41, 0x41, 0x00 }, // [
{ 0x02, 0x04, 0x08, 0x10, 0x20 }, // backslash
{ 0x00, 0x41, 0x41, 0x7F, 0x00 }, // ]
{ 0x04, 0x02, 0x01, 0x02, 0x04 }, // ^
{ 0x40, 0x40, 0x40, 0x40, 0x40 }, // _
{ 0x00, 0x01, 0x02, 0x04, 0x00 }, // ` (0x60)
{ 0x20, 0x54, 0x54, 0x54, 0x78 }, // a
{ 0x7F, 0x50, 0x48, 0x48, 0x30 }, // b
{ 0x38, 0x44, 0x44, 0x44, 0x20 }, // c
{ 0x38, 0x44, 0x44, 0x48, 0x7F }, // d
{ 0x38, 0x54, 0x54, 0x54, 0x18 }, // e
{ 0x08, 0x7E, 0x09, 0x01, 0x02 }, // f
{ 0x0C, 0x52, 0x52, 0x52, 0x3E }, // g
{ 0x7F, 0x08, 0x04, 0x04, 0x78 }, // h
{ 0x00, 0x44, 0x7D, 0x40, 0x00 }, // i
{ 0x20, 0x40, 0x44, 0x3D, 0x00 }, // j
{ 0x7F, 0x10, 0x28, 0x44, 0x00 }, // k
{ 0x00, 0x41, 0x7F, 0x40, 0x00 }, // l
{ 0x7C, 0x04, 0x18, 0x04, 0x78 }, // m
{ 0x7C, 0x08, 0x04, 0x04, 0x78 }, // n
{ 0x38, 0x44, 0x44, 0x44, 0x38 }, // o
{ 0x7C, 0x14, 0x14, 0x14, 0x08 }, // p (0x70)
{ 0x08, 0x14, 0x14, 0x08, 0x7C }, // q
{ 0x7C, 0x08, 0x04, 0x04, 0x08 }, // r
{ 0x48, 0x54, 0x54, 0x54, 0x20 }, // s
{ 0x04, 0x3F, 0x44, 0x40, 0x20 }, // t
{ 0x3C, 0x40, 0x40, 0x20, 0x7C }, // u
{ 0x1C, 0x20, 0x40, 0x20, 0x1C }, // v
{ 0x3C, 0x40, 0x30, 0x40, 0x3C }, // w
{ 0x44, 0x28, 0x10, 0x28, 0x44 }, // x
{ 0x0C, 0x50, 0x50, 0x50, 0x3C }, // y
{ 0x44, 0x64, 0x54, 0x4C, 0x44 }, // z
{ 0x00, 0x08, 0x36, 0x41, 0x00 }, // {
{ 0x00, 0x00, 0x7F, 0x00, 0x00 }, // |
{ 0x00, 0x41, 0x36, 0x08, 0x00 }, // }
{ 0x30, 0x08, 0x10, 0x20, 0x18 }, // ~
{ 0x7F, 0x55, 0x49, 0x55, 0x7F } // unknown char (0x7F)
};
char displayThis [40] = { 0 };
byte pos, pixel;
void blink()
{
// get current letter
char thisLetter = displayThis [pos];
// look up bit pattern for this pixel (table starts at 0x20 - space)
byte bitPattern = pgm_read_byte ( &font [thisLetter - 0x20] [pixel++]);
// output to LEDs
PORTD = bitPattern;
// move onto next letter if required
if (pixel >= 5)
{
pos++;
if (displayThis [pos] == 0) // back to start
pos = 0;
} // if done all 5 pixels
} // end of blink
void setup()
{
DDRD = 0b11111111; // set digital 0- 7 to output
MsTimer2::set(2, blink);
MsTimer2::start();
} // end of setup
void loop()
{
sprintf (displayThis, "JAMES");
delay (1000);
} // end of loopThe setup and loop sections I understand. I also understand how the font section works to an extent. The part I am having trouble understanding is.
// look up bit pattern for this pixel (table starts at 0x20 - space)
byte bitPattern = pgm_read_byte ( &font [thisLetter - 0x20] [pixel++]);
// output to LEDs
PORTD = bitPattern;
I understand the bytepattern is a variable that will be written with the binary details which chooses which leds to set to 1’s and 0’s which is then sent down PORTD to like the LEDs.
However, I am unsure of the rest of this. I believe the pgm_read_bye reads the correct byte/font from the PROGMEM, although I do not understand what the & is needed for before the font, is it used to specify something?
The table starts at 0x20 because of the ASCII table and the hex values of characters but how does it go find out which letter it is, could somebody please explain to me how this section in particular works. Especially the pgm_read_byte ( &font [thisLetter - 0x20] [pixel++]);
I have tried looking on Google for help before this and it helped me with the other sections but I couldn’t find anything that explained this for me very well. Such as I don’t understand why you subtract the Hex value from the variable.
Sorry if these are really basic questions, I am just trying to further my understanding. I know this probably all sounds really confusing and poorly written so if you need me to explain what I mean differently then please ask
Thanks for any help you guys can give.