How to make the library VGAXUA use different timers (Mega2560)

venixsk · August 21, 2022, 1:11pm

I wanted to use this library in combination with my usb host shield, however I ran into a problem. The host shield uses pin D9 which is needed for VGAXUA to function (it is used for Hsync). So I tried to modify the VGAXUA library to use different pins and timers. I was successful changing the Vsync from pin D11 (timer1) to D2 (timer3), but I was unsuccessful changing the Hsync to D7 (timer4) or D45 (timer5). The closest I got was getting the monitor recognizing the signal and the backlight turning on, but no picture.
During testing I also noticed that the TX light would only stay on when the library was using D9 for Hsync, otherwise it would only blink. And before anyone asks, I was testing these changes without the usb host shield attached.

Link to the library

Only Vsync changed, working :

#include "VGAXUA.h"

#define MSPIMSCK 4

#if defined(__AVR_ATmega2560__)
//HSYNC pin used by TIMER2
#define HSYNCPIN 9 
//VSYNC pin used by TIMER3
#define VSYNCPIN 2
//These two pin cannot be modified without modify the HSYNC assembler code
#define COLORPIN0 30
#define COLORPIN1 31
#define EXTRACOLORPORT PORTC
#else
//HSYNC pin used by TIMER2
#define HSYNCPIN 3
//VSYNC pin used by TIMER1
#define VSYNCPIN 9
//These two pin cannot be modified without modify the HSYNC assembler code
#define COLORPIN0 6
#define COLORPIN1 7
#define EXTRACOLORPORT PORTD
#endif

//Number of VGA lines to be skipped (black lines)
/*These lines includes the vertical sync pulse and back porch.
Minimum value must be 35 (calculate from Nick Gammon)
You can modify this value to center the framebuffer vertically, or not*/
#define SKIPLINES 35

unsigned vtimer;
static byte aline, rlinecnt;
static byte vskip;
byte vgaxfb[VGAX_HEIGHT * VGAX_BWIDTH];
static byte* videoline;
static byte vmask;

//VSYNC interrupt
ISR(TIMER3_OVF_vect) {
    aline = -1;
    vskip = SKIPLINES;
    vtimer++;
    rlinecnt = 0;
    videoline = vgaxfb;
}
//HSYNC interrupt
ISR(TIMER2_OVF_vect) {
    /*
    NOTE: I prefer to generate the line here, inside the interrupt.
    Gammon's code generate the line pixels inside main().
    My versin generate the signal using only interrupts, so inside main() function
    you can do anything you want. Your code will be interrupted when VGA signal
    needs to be generated
    */
    //check vertical porch
    if (vskip) {
        vskip--;
        return;
    }
    if (rlinecnt < VGAX_HEIGHT) {
#define nop asm volatile("nop")
#if defined(__AVR_ATmega2560__)
        nop;
        nop;
#endif

        //interrupt jitter fix (needed to keep signal stable)
        //code from https://github.com/cnlohr/avrcraft/tree/master/terminal
        //modified from VGAX dejitter version (this is faster)    
#define DEJITTER_SYNC -0 //was -3 on ATMEGA2560
        asm volatile(
            "     lds r16, %[timer0]    \n\t" //
            "     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" //
            "     ijmp                  \n\t" //
            "LW:                        \n\t" //
            "     nop                   \n\t" //
            "     nop                   \n\t" //
            "     nop                   \n\t" //
            "     nop                   \n\t" //
            "     nop                   \n\t" //
            "LBEND:                     \n\t" //
            :
        : [timer0] "i" (&TCNT0),
            [tsync] "i" ((uint8_t)DEJITTER_SYNC)
            : "r30", "r31", "r16", "r17");

        register byte* p = videoline;
        register byte c = *p++;;

        //first 8px 
#define drawF UDR0=c; UCSR0B=bit(TXEN0); c=*p++; nop;

//second 8px will turn on Extended Colors PINS
#define drawS UDR0=c; EXTRACOLORPORT=vmask; nop; nop; nop; nop; nop; nop; nop; c=*p++;

//draw 8px
#define draw8 UDR0=c; nop; nop; nop; nop; nop; nop; nop; nop; nop; nop; nop; nop; c=*p++;

#if !defined(__AVR_ATmega2560__)
        nop; nop;
#endif

        //draw 192px
#if defined(__AVR_ATmega2560__)
        drawF draw8 drawS draw8
#else
        drawF drawS draw8 draw8
#endif
            draw8 draw8 draw8 draw8
            draw8 draw8 draw8 draw8
            draw8 draw8 draw8 draw8
            draw8 draw8 draw8 draw8
            draw8 draw8 draw8

#if defined(__AVR_ATmega2560__)
            draw8
#endif

            //draw last 8px
            UDR0 = c;
        UCSR0B = 0;

        //wait to disable EXTRACOLORS with the right timing
        nop;
#if !defined(__AVR_ATmega2560__)
        nop; nop; nop; nop;
        nop; nop; nop; nop;
        nop; nop; nop; nop;
        nop; nop; nop; nop;
#endif

        //disable VMASK
        EXTRACOLORPORT &= ~vmask;
    }
    //increment framebuffer line counter after 6 VGA lines
#if defined(__AVR_ATmega2560__) && defined(ATMEGA2560_MAXRES)
#define CLONED_LINES (2-1)
#else
#define CLONED_LINES (6-1)
#endif
    if (++aline == CLONED_LINES) {
        aline = -1;
        rlinecnt++;
        videoline += VGAX_BWIDTH;
    }
    else {
    }
}
void VGAXUA::begin() {
    //Timers setup code, modified version of the Nick Gammon's VGA sketch
    cli();
    //disable TIMER0 interrupt
    TIMSK0 = 0;
    TCCR0A = 0;
    TCCR0B = (1 << CS00); //enable 16MHz counter (used to fix the HSYNC interrupt jitter)
    OCR0A = 0;
    OCR0B = 0;
    TCNT0 = 0;
    //TIMER3 - vertical sync pulses
    pinMode(VSYNCPIN, OUTPUT);
#if VSYNCPIN==10 //ATMEGA328 PIN 10
    TCCR1A = bit(WGM10) | bit(WGM11) | bit(COM1B1);
    TCCR1B = bit(WGM12) | bit(WGM13) | bit(CS12) | bit(CS10); //1024 prescaler
    OCR1A = 259; //16666 / 64 uS=260 (less one)
    OCR1B = 0; //64 / 64 uS=1 (less one)
    TIFR1 = bit(TOV1); //clear overflow flag
    TIMSK1 = bit(TOIE1); //interrupt on overflow on TIMER1
#else //ATMEGA328 PIN 9 or ATMEGA2560 PIN 11
    TCCR3A = bit(WGM30) | bit(WGM31) | bit(COM3B1);
    TCCR3B = bit(WGM32) | bit(WGM33) | bit(CS32) | bit(CS30); //1024 prescaler
    OCR3A = 259; //16666 / 64 uS=260 (less one)
    OCR3B = 0; //64 / 64 uS=1 (less one)
    TIFR3 = bit(TOV3); //clear overflow flag
    TIMSK3 = bit(TOIE3); //interrupt on overflow on TIMER3
#endif
//TIMER2 - horizontal sync pulses
    pinMode(HSYNCPIN, OUTPUT);
    TCCR2A = bit(WGM20) | bit(WGM21) | bit(COM2B1); //pin3=COM2B1
    TCCR2B = bit(WGM22) | bit(CS21); //8 prescaler
    OCR2A = 63; //32 / 0.5 uS=64 (less one)
    OCR2B = 7; //4 / 0.5 uS=8 (less one)
    TIFR2 = bit(TOV2); //clear overflow flag
    TIMSK2 = bit(TOIE2); //interrupt on overflow on TIMER2
    //Setup USART
    UBRR0 = 0;
    pinMode(MSPIMSCK, OUTPUT);
    UCSR0B = 0;
    UCSR0C = bit(UCPOL0) | bit(UMSEL00) | bit(UMSEL01);
    UCSR0B = 0;
    //pins for outputting the colour information
    pinMode(COLORPIN0, OUTPUT);
    pinMode(COLORPIN1, OUTPUT);
    sei();
}
void VGAXUA::end() {
    //disable TIMER0
    TCCR0A = 0;
    TCCR0B = 0;
    //disable TIMER3
    TCCR3A = 0;
    TCCR3B = 0;
    //disable TIMER2
    TCCR2A = 0;
    TCCR2B = 0;
}
void VGAXUA::clear(byte color) {
    register byte c = color;
    c &= 1;    //0000 0001
    c |= c << 1; //0000 0011
    c |= c << 2; //0000 1111
    c |= c << 4; //1111 1111
    unsigned cnt = VGAX_BSIZE;
    byte* o = (byte*)vgaxfb;
    while (cnt--)
        *o++ = c;
}
void VGAXUA::copy(byte* src) {
    byte* o = (byte*)vgaxfb;
    unsigned cnt = VGAX_BSIZE;
    while (cnt--)
        *o++ = pgm_read_byte(src++);
}
void VGAXUA::setExtendedColorsMask(byte mask) {
    vmask = (mask & 3) << 6;
}
void VGAXUA::delay(int msec) {
    while (msec--) {
        unsigned cnt = 16000 / 32;
        while (cnt--)
            asm volatile("nop\nnop\nnop\nnop\n");
    }
}
static unsigned long rand_next = 1;

unsigned VGAXUA::rand() {
    rand_next = rand_next * 1103515245UL + 12345;
    return rand_next + ((unsigned)(rand_next / 65536) % 32768);
}

Hsync & Vsync changed, doesn't work :

#include "VGAXUA.h"

#define MSPIMSCK 4

#if defined(__AVR_ATmega2560__)
//HSYNC pin used by TIMER4
#define HSYNCPIN 7 
//VSYNC pin used by TIMER3
#define VSYNCPIN 2
//These two pin cannot be modified without modify the HSYNC assembler code
#define COLORPIN0 30
#define COLORPIN1 31
#define EXTRACOLORPORT PORTC
#else
//HSYNC pin used by TIMER2
#define HSYNCPIN 3
//VSYNC pin used by TIMER1
#define VSYNCPIN 9
//These two pin cannot be modified without modify the HSYNC assembler code
#define COLORPIN0 6
#define COLORPIN1 7
#define EXTRACOLORPORT PORTD
#endif

//Number of VGA lines to be skipped (black lines)
/*These lines includes the vertical sync pulse and back porch.
Minimum value must be 35 (calculate from Nick Gammon)
You can modify this value to center the framebuffer vertically, or not*/
#define SKIPLINES 35

unsigned vtimer;
static byte aline, rlinecnt;
static byte vskip;
byte vgaxfb[VGAX_HEIGHT * VGAX_BWIDTH];
static byte* videoline;
static byte vmask;

//VSYNC interrupt
ISR(TIMER3_OVF_vect) {
    aline = -1;
    vskip = SKIPLINES;
    vtimer++;
    rlinecnt = 0;
    videoline = vgaxfb;
}
//HSYNC interrupt
ISR(TIMER4_OVF_vect) {
    /*
    NOTE: I prefer to generate the line here, inside the interrupt.
    Gammon's code generate the line pixels inside main().
    My versin generate the signal using only interrupts, so inside main() function
    you can do anything you want. Your code will be interrupted when VGA signal
    needs to be generated
    */
    //check vertical porch
    if (vskip) {
        vskip--;
        return;
    }
    if (rlinecnt < VGAX_HEIGHT) {
#define nop asm volatile("nop")
#if defined(__AVR_ATmega2560__)
        nop;
        nop;
#endif

        //interrupt jitter fix (needed to keep signal stable)
        //code from https://github.com/cnlohr/avrcraft/tree/master/terminal
        //modified from VGAX dejitter version (this is faster)    
#define DEJITTER_SYNC -0 //was -3 on ATMEGA2560
        asm volatile(
            "     lds r16, %[timer0]    \n\t" //
            "     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" //
            "     ijmp                  \n\t" //
            "LW:                        \n\t" //
            "     nop                   \n\t" //
            "     nop                   \n\t" //
            "     nop                   \n\t" //
            "     nop                   \n\t" //
            "     nop                   \n\t" //
            "LBEND:                     \n\t" //
            :
        : [timer0] "i" (&TCNT0),
            [tsync] "i" ((uint8_t)DEJITTER_SYNC)
            : "r30", "r31", "r16", "r17");

        register byte* p = videoline;
        register byte c = *p++;;

        //first 8px 
#define drawF UDR0=c; UCSR0B=bit(TXEN0); c=*p++; nop;

//second 8px will turn on Extended Colors PINS
#define drawS UDR0=c; EXTRACOLORPORT=vmask; nop; nop; nop; nop; nop; nop; nop; c=*p++;

//draw 8px
#define draw8 UDR0=c; nop; nop; nop; nop; nop; nop; nop; nop; nop; nop; nop; nop; c=*p++;

#if !defined(__AVR_ATmega2560__)
        nop; nop;
#endif

        //draw 192px
#if defined(__AVR_ATmega2560__)
        drawF draw8 drawS draw8
#else
        drawF drawS draw8 draw8
#endif
            draw8 draw8 draw8 draw8
            draw8 draw8 draw8 draw8
            draw8 draw8 draw8 draw8
            draw8 draw8 draw8 draw8
            draw8 draw8 draw8

#if defined(__AVR_ATmega2560__)
            draw8
#endif

            //draw last 8px
            UDR0 = c;
        UCSR0B = 0;

        //wait to disable EXTRACOLORS with the right timing
        nop;
#if !defined(__AVR_ATmega2560__)
        nop; nop; nop; nop;
        nop; nop; nop; nop;
        nop; nop; nop; nop;
        nop; nop; nop; nop;
#endif

        //disable VMASK
        EXTRACOLORPORT &= ~vmask;
    }
    //increment framebuffer line counter after 6 VGA lines
#if defined(__AVR_ATmega2560__) && defined(ATMEGA2560_MAXRES)
#define CLONED_LINES (2-1)
#else
#define CLONED_LINES (6-1)
#endif
    if (++aline == CLONED_LINES) {
        aline = -1;
        rlinecnt++;
        videoline += VGAX_BWIDTH;
    }
    else {
    }
}
void VGAXUA::begin() {
    //Timers setup code, modified version of the Nick Gammon's VGA sketch
    cli();
    //disable TIMER0 interrupt
    TIMSK0 = 0;
    TCCR0A = 0;
    TCCR0B = (1 << CS00); //enable 16MHz counter (used to fix the HSYNC interrupt jitter)
    OCR0A = 0;
    OCR0B = 0;
    TCNT0 = 0;
    //TIMER3 - vertical sync pulses
    pinMode(VSYNCPIN, OUTPUT);
#if VSYNCPIN==10 //ATMEGA328 PIN 10
    TCCR1A = bit(WGM10) | bit(WGM11) | bit(COM1B1);
    TCCR1B = bit(WGM12) | bit(WGM13) | bit(CS12) | bit(CS10); //1024 prescaler
    OCR1A = 259; //16666 / 64 uS=260 (less one)
    OCR1B = 0; //64 / 64 uS=1 (less one)
    TIFR1 = bit(TOV1); //clear overflow flag
    TIMSK1 = bit(TOIE1); //interrupt on overflow on TIMER1
#else //ATMEGA328 PIN 9 or ATMEGA2560 PIN 11
    TCCR3A = bit(WGM30) | bit(WGM31) | bit(COM3B1);
    TCCR3B = bit(WGM32) | bit(WGM33) | bit(CS32) | bit(CS30); //1024 prescaler
    OCR3A = 259; //16666 / 64 uS=260 (less one)
    OCR3B = 0; //64 / 64 uS=1 (less one)
    TIFR3 = bit(TOV3); //clear overflow flag
    TIMSK3 = bit(TOIE3); //interrupt on overflow on TIMER3
#endif
//TIMER2 - horizontal sync pulses
    pinMode(HSYNCPIN, OUTPUT);
    TCCR4A = bit(WGM40) | bit(WGM41) | bit(COM4B1); //pin3=COM4B1
    TCCR4B = bit(WGM42) | bit(WGM43) | bit(CS41); //8 prescaler
    OCR4A = 63; //32 / 0.5 uS=64 (less one)
    OCR4B = 7; //4 / 0.5 uS=8 (less one)
    TIFR4 = bit(TOV4); //clear overflow flag
    TIMSK4 = bit(TOIE4); //interrupt on overflow on TIMER4
    //Setup USART
    UBRR0 = 0;
    pinMode(MSPIMSCK, OUTPUT);
    UCSR0B = 0;
    UCSR0C = bit(UCPOL0) | bit(UMSEL00) | bit(UMSEL01);
    UCSR0B = 0;
    //pins for outputting the colour information
    pinMode(COLORPIN0, OUTPUT);
    pinMode(COLORPIN1, OUTPUT);
    sei();
}
void VGAXUA::end() {
    //disable TIMER0
    TCCR0A = 0;
    TCCR0B = 0;
    //disable TIMER3
    TCCR3A = 0;
    TCCR3B = 0;
    //disable TIMER4
    TCCR4A = 0;
    TCCR4B = 0;
}
void VGAXUA::clear(byte color) {
    register byte c = color;
    c &= 1;    //0000 0001
    c |= c << 1; //0000 0011
    c |= c << 2; //0000 1111
    c |= c << 4; //1111 1111
    unsigned cnt = VGAX_BSIZE;
    byte* o = (byte*)vgaxfb;
    while (cnt--)
        *o++ = c;
}
void VGAXUA::copy(byte* src) {
    byte* o = (byte*)vgaxfb;
    unsigned cnt = VGAX_BSIZE;
    while (cnt--)
        *o++ = pgm_read_byte(src++);
}
void VGAXUA::setExtendedColorsMask(byte mask) {
    vmask = (mask & 3) << 6;
}
void VGAXUA::delay(int msec) {
    while (msec--) {
        unsigned cnt = 16000 / 32;
        while (cnt--)
            asm volatile("nop\nnop\nnop\nnop\n");
    }
}
static unsigned long rand_next = 1;

unsigned VGAXUA::rand() {
    rand_next = rand_next * 1103515245UL + 12345;
    return rand_next + ((unsigned)(rand_next / 65536) % 32768);
}

.ino program i use to test this :

#include <VGAXUA.h>

VGAXUA vga;

void setup() {
  vga.begin();
}
void loop() {
  vga.putpixel(vga.rand()%VGAX_WIDTH, vga.rand()%VGAX_HEIGHT, vga.rand()%2);
  vga.setExtendedColorsMask(1);
}

system · February 17, 2023, 1:11pm

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How to make the library VGAXUA use different timers (Mega2560)

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