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Towards An HD44780-Based Video Mode

The following documentation describes a potential way to drive HD44780-style 16x2 character LCDs in graphics mode by using a synchronized updating scheme and complementary special characters. This concept introduced here can be combined with minimal hardware modifications to re-purpose 20x4 character LCDs as graphic displays.

HD44780 background
Character LCDs used in industrial machinery, test equipment and hobby projects are popular, long-lived and mostly based on Hitachi HD44780-style controllers (+ drivers) from multiple manufacturers:

Hitachi HD44780 (+ HD44100H)
Sitronix ST7066U (+ ST7065C), ST0066, ST7070
Sunplus SPLC870D (+ SPLC100A1)
NXP PCF2119x

With minor but not necessarily documented variations, their block diagram contains

a 6800-style parallel interface
character generator memories (CGROM, CGRAM)
display data RAM (DDRAM)
timing generator and counters
pixel line serializer (40 bit oriented for 8 lines by 5 pixels)
COM and SEG drivers with 4 to 5 voltage levels
CL1, CL2, D and M lines for external drivers.”

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