Craft

Craft is a demo running on its own minimalistic demo platform. The demo platform is based on an ATmega88 microcontroller.

Having successfully built a soundchip out of a microcontroller together with my friends in kryo, I wanted to tackle the greater challenge of generating a realtime video signal along with the sound. This is the result:

Download

• lft_craft (Original video clip, Xvid, 89.4 MB)
• lft_craft_capture (Full screen video capture, H.264, 85.5 MB)
• Linus Akesson - Craft (Soundtrack, MP3, 3.3 MB)
• lft_craft_src (Schematics, firmware binaries and source code, 70.5 kB)

Scene reaction

Craft won the Console / Real Wild compo at Breakpoint 2008. I will never forget the overwhelming reaction from the audience!

Here's the Pouët page for Craft.

How does it work?

Just like your average 80's home computer, the entire design is centered around the timing of the video signal.

A typical VGA-based, low-resolution CRT monitor will redraw the screen 60 times per second using an electron beam, which is sweeping across the screen one line at a time. TFT monitors work differently, but the VGA signal is still based on the idea of an electron beam. Timing is crucial: One display line takes 24 μs, and is followed by a 7.75 μs break called the horizontal blanking period. After 480 such lines, there's a longer break (1428.75 μs, equal to 45 full display lines) before it all starts over. Two digital signals are used to synchronize the sender (graphics card, custom demo hardware etc.) and the receiver (monitor). They are called the horizontal sync and the vertical sync signals. It's OK to deviate a bit from the standard timing values as long as you keep the sync signals steady.

The microcontroller is clocked at 20 MHz. If we convert the figures above into clock cycles, we get 480 cycles of visible pixels, 155 cycles of horizontal blanking and 45 full display lines worth of vertical blanking — but during those lines you need to keep generating the horizontal sync pulses. Due to rouding errors, we get a frame rate of 59.99 Hz, but that is well within the tolerance range of a computer monitor.

Music

Sound is generated during the horizontal blanking periods. That gives a sample rate of 31.496 kHz. Of course, only the really timing critical part (waveform generation) is performed during the horizontal blanking. Melody, rhythm, amplitude envelopes, arpeggios etc. are handled by a playroutine which gets called once for every video frame, during vertical blanking.

There are four sound channels in total, each with its own fixed waveform. The waveforms are 4-bit triangle, 50% pulse, 75% pulse and white noise. The noise is generated by means of a 15-bit shift register. The volume of each channel can be individually controlled, except for the triangle channel, which is always playing at full volume. This minimalistic softsynth was of course inspired by the NES sound chip.

Video

Apart from the sync signals mentioned above, a VGA signal contains three analog voltage lines — red, green and blue — that vary between ground and 0.7 V during the visible parts of the video frame. As you can see in the schematic below, I perform a two-bit digital-to-analog conversion for each of these signals, using R-2R resistor ladders. That way, the software can change the current colour in a single clock cycle using the out PORTC,register instruction.

In addition to this, I've hooked up a couple of diodes and a PNP transistor in such a way, that when the MOSI pin is low while the OC2B pin is high, the three color signals will be pulled to a high voltage, corresponding to white. This is used to generate high resolution scroll text: The MOSI pin is connected to a shift register internally in the AVR (it is typically used for serial data transmission), and this shift register can be programmed to emit a sequence of 8 bits with a single instruction, thus offloading the CPU. Smooth scrolling is then implemented by inserting variable delays before and after each display line, but that alone is not enough, because characters would appear and disappear suddenly at the edges of the screen. So additionally, the output comparator connected to timer 2 in the chip is set up to create a visibility window using the OC2B pin.

Schematics

This is how it all fits together, hardware-wise. If you are interested in the physical layout used on the breadboard, you can find it along with firmware and source code near the top of the page.

The variable 1K resistor can be used to adjust the brightness of the scroll texts.

                                 .---[ 1K ]-+------- E (PNP) C -------------------+---+---.
                                 |    /|\   |            B                       _|_ _|_ _|_
                                 |     `----'            |                       \ / \ / \ /
                                 |                       |                       --- --- ---
                                 |       .---__---.      |                        |   |   |
               (to programmer) --- RESET |        | PC5 ----[ 442 ]-----------+---+---|---|--- Red
                                 |       |        |      |          .-[ 220 ]-'       |   |
                              (n.c.) PD0 |        | PC4 ----[ 442 ]-+-[ 442 ]--- GND  |   |
                                 |       |        |      |                            |   |
            .----------+-[ 2K ]----- PD1 |        | PC3 ----[ 442 ]-----------+-------+---|--- Green
            | .-[ 1K ]-'         |       |        |      |          .-[ 220 ]-'           |
            | `--+-------[ 2K ]----- PD2 |        | PC2 ----[ 442 ]-+-[ 442 ]--- GND      |
            |    `-[ 2K ]-- GND  |       |        |      |                                |
            |                    `- OC2B |        | PC1 ----[ 442 ]-----------+-----------+--- Blue
            `---[ 1K ]-.                 |        |      |          .-[ 220 ]-'
              .--------+-[ 2K ]----- PD4 |        | PC0 ----[ 442 ]-+-[ 442 ]--- GND
              |                          |        |      |
              |                      VCC |        | GND  `-[ 1K ]-.
              |                          |        |               |
              |                      GND |        | AREF (n.c.)   |
              |            22pF          |        |               |
              |        GND -||--+- XTAL1 |        | AVCC (n.c.)   |
              |         20 MHz [ ]       |        |               |
              |        GND -||--+- XTAL2 |        | SCK -------------- (to programmer)
              `-[ 1K ]-.   22pF          |        |               |
              .--------+-[ 2K ]----- PD5 |        | MISO ------------- (to programmer)
              `-[ 1K ]-.                 |        |               |
              .--------+-[ 2K ]----- PD6 |        | MOSI ---------+--- (to programmer)
           +  `-[ 1K ]-.                 |        |
     ----)|------------+-[ 2K ]----- PD7 |        | OC1B ------------------------------------- HSync
Audio   10uF                             |        |
     --------- GND            (n.c.) PB0 |        | PB1 -------------------------------------- VSync
                                         `--------'

If you want to learn more, I suggest you dive into the source code, starting with boot.S, mainloop.S and asm.S. The most interesting part is probably the cycle-accurate code in asm.S.