Untitled Document

mr. city's arduino section

Project 1: Output VGA Signals to a CRT Monitor

This is code for the Arduino microcontroller that will display vertical lines on a VGA screen at a resolution of approximately 376x282 @ 60 Hz. You will probably need a CRT monitor to get this to work correctly; it doesn't work on my LCD monitor. (If you can figure out the timings that'll work on an LCD, be my guest!)

The schematic is shown at left. Click on it in order to see it in full size. As you can see, Arduino pins 13, 12, 7, 4, and 2 are connected to a female D-SUB connector, but not before going through some resistors first. 13 & 12 (sync) go out to one 220-ohm resistor each, and 7, 4, & 2 (the colors) each go to their own voltage divider with R1 = 1200 ohms and R2 = 180 ohms.

Addenda/Notes:

The pins have been remapped since the firmware version presented below was written; unfortunately the updated firmware was lost in a hard drive crash. The RED which used to come out of pin 2 is now expected to be from pin 4; GREEN from pin 4 should now come from pin 5; and BLUE from pin 7 should now originate from pin 6. It's a shame; the new code was really cool since you could display a custom image.
Pin 9 on the D-SUB connector should really be wired to ground. However, my circuit managed to work without doing this, so it's not in the schematic.
I can't remember the rhyme or reason for putting a resistance on the sync lines of specifically 220 ohms.
The 1200 ohm and 180 ohm resistors are in place as a voltage divider, since the maximum voltage the RGB pins in a VGA monitor can take is 0.7V. These resistor choices lead to (180 / (1200 + 180)) * 5V ~= 0.65V. You can use any other combination as long as it leads to the correct voltage.

Here are a few places I got guidance from on my quest to write this firmware:

http://arcanebolt.net/ - My code is based off theirs, but heavily modified.
http://www.cs.unc.edu/Research/stc/FAQs/Video/GTF_V1R1.xls - A GTF spreadsheet you can use for finding out what frequency and timings to use given the desired VGA resolution and refresh rate you specify. Not quite the same one I used, but this one is probably better.
Lots of Google searches for "vga pinout" and "vga timing specification", etc.

#include <avr/interrupt.h>
// horizontal line counter (negative is during vertical blanking)
int hzCount = -10;

unsigned short int OCR1AMEM = 0;
  
// ==============
// Pinout:
#define HSYNC 0b00100000;   // 13: HSYNC (Port B5)
#define VSYNC 0b00010000;   // 12: VSYNC (Port B4)
#define BLACK   0b00000000;   // Black
#define BLUE    0b10000000;   // 7:  Blue  (Port D7)
#define GREEN   0b00010000;   // 4:  Green (Port D4)
#define RED     0b00000100;   // 2:  Red   (Port D2)
#define CYAN    0b10010000;   // Cyan
#define YELLOW  0b00010100;   // Yellow
#define MAGENTA 0b10000100;   // Magenta
#define WHITE   0b10010100;   // White
// Others go to GND
// =======

// hurry up & wait
#define NOM    asm("nop");
#define NOM5   asm("nop\nnop\nnop\nnop\nnop");
#define NOM10  asm("nop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop");
#define NOM20  asm("nop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop\nnop");

void hLine(void)
{
  PORTB = PORTB ^ HSYNC;         // invert HSYNC - negative polarity
  NOM; NOM; NOM; NOM; NOM; NOM; NOM;
  delayMicroseconds(3);    // wait for "sync length" time
  PORTB = PORTB ^ HSYNC;          // invert HSYNC again
  NOM; NOM; NOM; NOM; NOM; NOM; NOM;
  delayMicroseconds(4);   // wait for back porch time
  // draw a line on-screen 4 pixels wide (the "eq" operation + 3 nops)
  PORTD = RED;
  NOM; NOM; NOM;
  PORTD = BLACK;
  NOM; NOM; NOM;
  PORTD = GREEN;
  NOM; NOM; NOM;
  PORTD = BLACK;
  NOM; NOM; NOM;
  PORTD = BLUE;
  NOM; NOM; NOM;
  PORTD = BLACK;
  NOM; NOM; NOM;
  PORTD = CYAN;
  NOM; NOM; NOM;
  PORTD = BLACK;
  NOM; NOM; NOM;
  PORTD = MAGENTA;
  NOM; NOM; NOM;
  PORTD = BLACK;
  NOM; NOM; NOM;
  PORTD = YELLOW;
  NOM; NOM; NOM;
  PORTD = BLACK;
  NOM; NOM; NOM;
  PORTD = WHITE;
  NOM; NOM; NOM;
  PORTD = BLACK;
  NOM; NOM; NOM;
  
  // three pixels at a time
  PORTD = RED;
  NOM; NOM;
  PORTD = BLACK;
  NOM; NOM;
  PORTD = GREEN;
  NOM; NOM;
  PORTD = BLACK;
  NOM; NOM;
  PORTD = BLUE;
  NOM; NOM;
  PORTD = BLACK;
  NOM; NOM;
  PORTD = CYAN;
  NOM; NOM;
  PORTD = BLACK;
  NOM; NOM;
  PORTD = MAGENTA;
  NOM; NOM;
  PORTD = BLACK;
  NOM; NOM;
  PORTD = YELLOW;
  NOM; NOM;
  PORTD = BLACK;
  NOM; NOM;
  PORTD = WHITE;
  NOM; NOM;
  PORTD = BLACK;
  NOM; NOM;
  
  // two pixels at a time
  PORTD = RED;
  NOM;
  PORTD = BLACK;
  NOM;
  PORTD = GREEN;
  NOM;
  PORTD = BLACK;
  NOM;
  PORTD = BLUE;
  NOM;
  PORTD = BLACK;
  NOM;
  PORTD = CYAN;
  NOM;
  PORTD = BLACK;
  NOM;
  PORTD = MAGENTA;
  NOM;
  PORTD = BLACK;
  NOM;
  PORTD = YELLOW;
  NOM;
  PORTD = BLACK;
  NOM;
  PORTD = WHITE;
  NOM;
  PORTD = BLACK;
  NOM;
  
  // one pixel at a time, with black spaces
  PORTD = RED;
  PORTD = BLACK;
  PORTD = GREEN;
  PORTD = BLACK;
  PORTD = BLUE;
  PORTD = BLACK;
  PORTD = CYAN;
  PORTD = BLACK;
  PORTD = MAGENTA;
  PORTD = BLACK;
  PORTD = YELLOW;
  PORTD = BLACK;
  PORTD = WHITE;
  PORTD = BLACK;
  
  // one pixel at a time, no black
  PORTD = RED;
  PORTD = GREEN;
  PORTD = BLUE;
  PORTD = CYAN;
  PORTD = MAGENTA;
  PORTD = YELLOW;
  PORTD = WHITE;
  PORTD = BLACK;
}

ISR(TIMER1_COMPA_vect)
{ 
  // do vsync with positive polarity
  if (hzCount == -10 || hzCount == -6) PORTB = PORTB ^ VSYNC
  // do hsync when the time is right
  if (hzCount > -1 || hzCount < 282) hLine();

  hzCount++;
  if (hzCount > 285) hzCount = -10;
}

void setup(void)
{
  // pins
  pinMode(2, OUTPUT);
  pinMode(7, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(12, OUTPUT);
  pinMode(13, OUTPUT);
  digitalWrite(13, LOW);  // start HSYNC low
  digitalWrite(12, HIGH);  // start VSYNC high
  digitalWrite(7, LOW);  // start blue low
  digitalWrite(4, LOW);  // start green low
  digitalWrite(2, LOW);  // start red low

  // hsync timer, see ATmega328 datasheet for more details
  
  // TIMSK0: Timer/Counter1 Interrupt Mask Register
  // TOIE0 = 1: when set, enables the Timer/Counter1 Overflow interrupt
  // This enables everything already set EXCEPT for TOIE0
  TIMSK0 &= !(1 << TOIE0);
  // temporarily disable interrupts
  cli();
  // Timer/Counter Control Registers
  // Timer/Counter1 Control Register A
  // Ignore any functionality provided by this Register
  TCCR1A = 0;
  // Timer/Counter1 Control Register B
  // WGM22 = 8: Set timer to CTC mode with TOP = OCR1A (WGM12 may also work?)
  // CS10 = 1: Do not prescale the clock
  TCCR1B = 1 << WGM12 | 1 << CS10;
  // Sets the TOP (highest) value the Counter will count to
  // F_ocr1a = 16MHz / (2 * prescale * OCR1A); F = 1/192 MHz: usu. 0x5FF; F = 1/57 MHz; usu. 1C7
  OCR1A = 0x01C8;
  // TIMSK1: Timer/Counter1 Interrupt Mask Register
  // OCIE1A = 2: when set, enables the Timer1 Output Compare A Match interrupt
  TIMSK1 = 1 << OCIE1A;
  // re-enable interrupts
  sei();
}
void loop()
{
  //twiddle thumbs
}

Project 2: Writing the Arduino Bootloader using an Arduino Uno

This simple workup was derived from this tutorial from Sparkfun. The only difference is I eschewed the command-line stuff they talk about and instead opted for a simple GUI approach provided by the Arduino SDK. The end goal here is to write the Arduino bootloader to a chip on a Duemilanove using an Arduino Uno SMD edition as the ISP. (Whether or not it's the SMD version shouldn't make a difference.)

One other note is I'm often driven to do things that aren't always obvious with the stuff I have. I didn't have the header sockets necessary to bridge the Arduino Uno output pins to the ICSP headers on the Duemilanove board. However, considering my age, I have an inordinate amount of old computer parts dating from the late 1980s to about 2000, and since a lot of this old stuff happens to have the same pin size as many of the DIY kits like Arduino, I leveraged an old "game port" (15-pin 2-row D-SUB connector) I had lying around to use as a bus for the ICSP signals. One end is the 15-pin female jack, and the other end looks like an old IDE cable, but skinnier (as it should be; less pins!). I used regular prototype wire to tenuously connect the 15-pin jack to the Uno's output pins. The Sparkfun tutorial describes the exact pinout you need to use. I'll go over the basic formula here for using the Arduino SDK to initiate the burning:

Do the wiring as described above, but don't plug anything into the ICSP header pins in the Duemilanove just yet.
Plug your Uno into the USB port on your computer, and make sure the correct COM port is selected.
Load the "ArduinoISP" sketch onto the Uno.
Unplug the Uno from the USB port.
Now, you can plug the appropriate header sockets of the game port into the ICSP header pins of the Duemilanove.
Select the correct "Board" type from within the Tools menu of the Arduino SDK. The board type should correspond to where the chip on the Duemilanove will ultimately end up. For instance, if you intend to mount it on a breadboard using its own internal clock, select "ATmega328 on a breadboard." Or, if you're going to leave it on the Duemilanove, then select the appropriate Duemilanove option given your processor type.
Plug your Uno into the USB port again, and make sure the correct COM port is still selected.
In the "Tools" menu, go to "Burn Bootloader" -> "w/ Arduino As ISP." It should take one minute or less to finish burning the bootloader.
If you want to burn a program onto your newly bootloaded chip, unplug the Uno from USB and unplug the Duemilanove's ICSP header pins. Plug the Duemilanove into the USB, set the correct COM port, and proceed to upload your desired sketch.

Note that you can probably flip around the instructions for your board arrangement. For instance, you can treat the word "Uno" as the device you intend to use as the programmer.

Project 3: TOP SECRET! Currently in progress. More to come once it's ready, commercially available, or whatever.

Go back to Mr. City's Web Life - Home Page. (However, you'll be hard-pressed to find anything technical on the rest of the site. :-P)