Getting video out from your Adafruit Trinket or DigiSpark (or ANY ATTINY85)

While pouring through some videos and posts on just what the tiny little ATTiny85 chip can do, I ran across a HackADay post about someone generating NTSC video on VHF channel 3 (US channel 3.)  I was intrigued.  So, I tried it and got mixed results and, as I have only a couple of old analog sets, I thought it was kind of impractical to pursue, but cool anyway.  So, I wondered if just composite out would be better and, to my delight, the same person who posted the video also had done composite out with an ATTiny84.  It should work on the 85 as well, and, it does.

So, I found his code and downloaded it.  Mr. Lohr is a GENIUS, plain and simple.  Thank you, sir!

The code did not work well at first. I had to tweak it a bit to work with my hardware, especially the little monitor.  It still is not perfect, the video is not centered, but is fairly steady and bright…my first attempts were not very bright and not very steady. I am guessing it needs to be tweaked for the monitor it will be used on, but I have not tested on more than one.

All you need is something like a Trinket from Adafruit or a digispark. I used both, but destroyed the digispark when I attempted to use an external power source.  It does not like more than five volts.  Specs say 3.5 to 12volts, I used 9. My mistake.

The Trinket works great. I am using the 5volt version with USB.  Any ATTiny85 based controller should work, though.

Connecting it is simple: Pin 3 to video ground, Pin 4 to video center pin.  Download the code to your device, connect it to the monitor and reset it. You should see the video demo.

Now, there are some major caveats:

• it, so far, only does text
• you have 13 characters by 6 lines
• character spacing is mono
• there’s no video memory, each frame is drawn on the fly
• the ‘video memory’ is simply a string array
• there is no formatting
• the character set is stored in flash

So, why bother with such limitations? Well, for starters, cheap composite monitors are easy to get and use and the Trinket/digispark are both under ten bucks, so you can have an application with video out for very little money.  If nothing else, this is a great lesson in how video works. Charles Lohr did a fantastic job with the code.  He’s a genius. Did I mention that?

Any way, the code is posted below.  Let us know if you get it working and what you do with it.

 

 
/*
    Copyright (C) 2014 <>< Charles Lohr

    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
    to deal in the Software without restriction, including without limitation
    the rights to use, copy, modify, merge, publish, distribute, sublicense,
    and/or sell copies of the Software, and to permit persons to whom the
    Software is furnished to do so, subject to the following conditions:
    The above copyright notice and this permission notice shall be included
    in all copies or substantial portions of the Software.
    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
    OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
    MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
    IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
    CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
    TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
    SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/sleep.h>
#include <util/delay.h>
#include <avr/pgmspace.h>
#include "ntscfont.h"
 
void delay_ms(uint32_t time) {
  uint32_t i;
  for (i = 0; i < time; i++) {
    _delay_ms(1);
  }
}
#define NOOP asm volatile("nop" ::)
void NumToText( char * c, uint8_t a )
{
    c[0] = (a/100)+'0';
    c[1] = ((a/10)%10)+'0';
    c[2] = (a%10)+'0';
    c[3] = 0;
}
void NumToText4( char * c, uint16_t a )
{
    c[0] = (a/1000)+'0';
    c[1] = ((a/100)%10)+'0';
    c[2] = ((a/10)%10)+'0';
    c[3] = (a%10)+'0';
    c[4] = 0;
}

EMPTY_INTERRUPT(TIM0_OVF_vect );

int main( )
{
    cli();
    CLKPR = 0x80;    /*Setup CLKPCE to be receptive*/
    CLKPR = 0x00;    /*No scalar*/
    PLLCSR = _BV(PLLE) | _BV( PCKE );
//    PLLCSR |= _BV(LSM);
    DDRB = _BV(1);
    DDRB |= _BV(3);
    DDRB |= _BV(4);
    PORTB |= _BV(1);
    TCCR1 = _BV(CS10);// | _BV(CTC1); //Clear on trigger.
    GTCCR |= _BV(PWM1B) |  _BV(COM1B0);// | _BV(COM1B1);
    OCR1B = 2;
    OCR1C = 3;
    DTPS1 = 0;
    DT1B = _BV(0) | _BV(4);
    TCCR0A = 0;
    TCCR0B = _BV(CS01);
    TIMSK |= _BV(TOIE0);
OSCCAL = 215;
//    OSCCAL=186;

#define POWERSET
#ifdef POWERSET
#define NTSC_HI  {    DDRB=0;}
#define NTSC_LOW {    DDRB=_BV(4); }
#define NTSC_VH  {    DDRB=_BV(3); }
#elif defined( POWERSET2 )
#define NTSC_VH  {    DDRB=0;}
#define NTSC_LOW {    DDRB=_BV(4)|_BV(3); }
#define NTSC_HI  {    DDRB=_BV(3); }
#elif defined( POWERSET3 )
#define NTSC_VH  {    DDRB=0; }
#define NTSC_HI   { DDRB=_BV(3); }
#define NTSC_LOW   {    DDRB=_BV(4)|_BV(3); }
#else
//Experimental mechanisms for changing power. Don't work.
#define NTSC_VH  {    OCR1C = 3; TCNT1 = 0; }
#define NTSC_HI  {    OCR1C = 6; TCNT1 = 0;}
#define NTSC_LOW {    OCR1C = 0; TCNT1 = 0;}
#endif
    uint8_t line, i;
    #define TIMEOFFSET .12 //.12
    #define CLKOFS .12
    uint8_t frame = 0, k, ctll;
    char stdsr[8*13];
    sprintf( stdsr, "Fr: " );
    sprintf( stdsr+8,  "HalfByte" );
    sprintf( stdsr+16, "  Blog  " );
    sprintf( stdsr+24, "        " );
    sprintf( stdsr+32, " Trinket" );
    sprintf( stdsr+40, "AdaFruit" );
    sprintf( stdsr+48, "        " );
    sprintf( stdsr+56, "        " );
    sprintf( stdsr+64, "        " );
    sprintf( stdsr+72, "        " );
    sprintf( stdsr+80, "        " );
    sprintf( stdsr+88, "        " );

    ADMUX =/* _BV(REFS1)  |  _BV(ADLAR) | */ 1; //1 = PB2
    ADCSRA = _BV(ADEN) | _BV(ADSC) | _BV(ADATE) | _BV(ADPS2) | _BV(ADPS1);
#define LINETIME 21 //Linetime of 7..20 is barely valid. So, 
//#define WAITTCNT while(TCNT0);
#define RESETCNT {TCNT0 = LINETIME; TIFR|=_BV(TOV0); GTCCR|=PSR0;sei();}
//#define WAITTCNT while(!(TIFR&_BV(TOV0)));
#define WAITTCNT sleep_cpu();
//#define WAITTCNT fintcnt();
     sleep_enable();
    sei();
    uint16_t ovax; //0..1024 = 0...5v
    uint8_t  msd;
    uint8_t  lsd;
    while(1)
    {
        frame++;
        //H = 1./15734.264 = 63.555 / 2 = 31.7775
        for( line = 0; line < 6; line++ )
        { NTSC_LOW; _delay_us(2.3-TIMEOFFSET); NTSC_HI; _delay_us(29.5-TIMEOFFSET-CLKOFS); }
        for( line = 0; line < 6; line++ )
        { NTSC_LOW; _delay_us(27.1-TIMEOFFSET); NTSC_HI; _delay_us(4.7-TIMEOFFSET-CLKOFS); }
        for( line = 0; line < 6; line++ )
        { NTSC_LOW; _delay_us(2.3-TIMEOFFSET); NTSC_HI; _delay_us(29.5-TIMEOFFSET-CLKOFS); }
        for( line = 0; line < 39; line++ )
        {
            RESETCNT;
            NTSC_LOW;
            _delay_us(4.7-TIMEOFFSET);
            NTSC_HI;
            //Do whatever you want.
            //sprintf( stdsr, "%d", frame );
            switch (line)
            {
            case 0:
                NumToText( stdsr+4, frame );
                break;
            case 1:
                ovax = ADC;
                ovax = ovax * 49 + (ovax>>1);
                ovax/=10;
                break;
            case 2:
                NumToText( stdsr+24, ovax/1000 );
                stdsr[27] = '.';
                break;
            case 5:
                NumToText4( stdsr+27, ovax );
                stdsr[27] = '.';
                break;
            }
            WAITTCNT;
            //_delay_us(58.8-TIMEOFFSET-CLKOFS);
        }
        for( line = 0; line < 2; line++ )
        {
            RESETCNT;
            NTSC_LOW;
            _delay_us(4.7-TIMEOFFSET);
            NTSC_HI;
            WAITTCNT;
        }
        for( line = 0; line < 220; line++ )
        {
            RESETCNT;
            NTSC_LOW; _delay_us(4.7-TIMEOFFSET); 
            NTSC_HI; _delay_us(8-TIMEOFFSET-CLKOFS);
//#define LINETEST
#ifdef LINETEST
            NTSC_VH; _delay_us(8-TIMEOFFSET-CLKOFS);
            NTSC_HI; _delay_us(44.5);
#else
            ctll = line>>2;
     
            for( k = 0; k < 8; k++ )
            {
      // draw the character, one pixel at a time
            uint8_t ch = pgm_read_byte( &font_8x8_data[(stdsr[k+((ctll>>3)<<3)]<<3)] + (ctll&0x07) );
            for( i = 0; i < 8; i++ )
            {
                if( (ch&1) ) //if pixel is dark...
                {
                    NTSC_VH;
                }
                else
                {
                    NTSC_HI;  // pixel is lit
                    NOOP;
                }
                ch>>=1;
                NOOP; NOOP; NOOP; NOOP;
            }
                    NTSC_HI;
            }
            NTSC_HI; //_delay_us(4.7-TIMEOFFSET-CLKOFS);
            WAITTCNT;
#endif
//        NTSC_HI; _delay_us(46-TIMEOFFSET-CLKOFS);
//            NTSC_VH; _delay_us(32-TIMEOFFSET-CLKOFS);
//            NTSC_HI; _delay_us(19.8-TIMEOFFSET-CLKOFS);
        }
    }
    
    return 0;
} 

 

Download the original code and the font file here. You will need the font file to make this work.

Mr. Lohr’s YouTube Channel