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Add font unpacking documentation

This commit is contained in:
Alex Voinea 2023-10-03 08:10:14 +02:00
parent c479848497
commit 9606c4f00b
1 changed files with 19 additions and 1 deletions
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20
Firmware/lcd.cpp
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@ -389,7 +389,25 @@ void lcd_set_cursor_column(uint8_t col)
// with custom characters
void lcd_createChar_P(uint8_t location, const CustomCharacter *char_p)
{
uint8_t charmap[8];
uint8_t charmap[8]; // unpacked font data
// The LCD expects the CGRAM data to be sent as pixel data, row by row. Since there are 8 rows per character, 8 bytes need to be sent.
// However, storing the data in the flash as the LCD expects it is wasteful since 3 bits per row are don't care and are not used.
// Therefore, flash can be saved if the character data is packed. For the AVR to unpack efficiently and quickly, the following scheme was used:
//
// colbyte data0 data1 data2 data3
// a b c d e
//
// ** ** ** b7 b6 b5 b4 a0
// ** ** ** b3 b2 b1 b0 a1
// ** ** ** c7 c6 c5 c4 a2
// ** ** ** c3 c2 c1 c0 a3
// ** ** ** d7 d6 d5 d4 a4
// ** ** ** d3 d2 d1 d0 a5
// ** ** ** e7 e6 e5 e4 a6
// ** ** ** e3 e2 e1 e0 a7
//
// The bits marked as ** in the unpacked data are don't care and they will contain garbage.
uint8_t temp;
uint8_t colByte;