VGAX Support for ATMega2560 14/04/2019

I've added support for ATMega2560 to my library VGAX! Now the library can be configured to generate 120x90px (with squared pixels) or 120x240px (rectangular pixels).

Video of VGAX on ATMega2560

The wiring is simple, like for ATMega328. You can wire up an Arduino MEGA board with some cheap components:

To enable 120x90px resolution you need to uncomment the constant ATMEGA2560_HIGHRES on VGAX.h header. To enable 120x240px uncomment ATMEGA2560_MAXRES.

The tricky part to add support for ATMega2560 has been adapting the interrupt dejitter code. I've found that the original code, writen by Charles CNLOHR assume to work on an AVR MCU with a 16bit Program Counter. ATMega2560, like others fat MCU, has a 22bit Program Counter! So, when Charles pop PC from stack, code must be modified to pop 3 bytes instead of 2 (22bit PC requires 3 bytes):

#define DEJITTER_OFFSET 1
#define DEJITTER_SYNC -2
asm volatile(
  "     lds r16, %[timer0]    \n\t" //
  #if defined(__AVR_ATmega2560__)
  "     add r16, %[toffset]   \n\t" //
  #endif
  "     subi r16, %[tsync]    \n\t" //
  "     andi r16, 7           \n\t" //
  "     call TL               \n\t" //
  "TL:                        \n\t" //
  #if defined(__AVR_ATmega2560__)
  "     pop r17               \n\t" //ATMEGA2560 has a 22bit PC!
  #endif
  "     pop r31               \n\t" //
  "     pop r30               \n\t" //
  "     adiw r30, (LW-TL-5)   \n\t" //
  "     add r30, r16          \n\t" //
  //"   adc r31, __zero_reg__ \n\t" //
  "     ijmp                  \n\t" //
  "LW:                        \n\t" //
  "     nop                   \n\t" //
  "     nop                   \n\t" //
  "     nop                   \n\t" //
  "     nop                   \n\t" //
  "     nop                   \n\t" //
  "     nop                   \n\t" //
  "     nop                   \n\t" //
  //"   nop                   \n\t" //
  "LBEND:                     \n\t" //
:
: [timer0] "i" (&TCNT0),
  [toffset] "i" ((uint8_t)DEJITTER_OFFSET),
  [tsync] "i" ((uint8_t)DEJITTER_SYNC)
: "r30", "r31", "r16", "r17");

MCU PORTD must be changed to PORTA becouse PORTD does not have 8 pins, so all the assumptions made in VGAX for ATMega328 cannot work with PORTD for ATMega2560. Colors pins must be moved from pin 6 and pin 7 to pin 30 and pin 31 (higher 2 bits of PORTA in ATMega2560).

TIMER0 setup must be also modified to work fine on both ATMega328 and ATMega2560.

That's all! :)

ATMega2560 has 8KB of SRAM, more space for framebuffer to increase vertical resolution from 60 to 90 or 240px. In theory with ATMega2560 a new library can be created to use 256 colors (with RRRGGGBB pixel format) or 16 colors (RGBH-RGBH double pixels in single byte format). I think to try to write, in the future, these variations of VGAX inside new differents libraries (VGAX256 and VGAX16 ?).

Have fun!