ATmega168 Snake

Tuesday, 24th November 2009

In addition to the Tetris game from the previous post, I've added an implementation of snake to the ATmega168 project.

Either game can be selected from a menu that appears when the circuit is powered on. To exit menus I've added a second fire button; this allows you to step back to the main menu to pick a different game if need be. The source code and binary can be downloaded as before.

I've written a number of different Snake implementations in the past. The early versions used a single array to represent every cell that the snake's body lay in (head as the first element, tail as the last element) that I would manually shift every frame and resize when the snake ate some food. This gets slower and slower as the snake gets longer, which isn't very good. When I wrote a version for the TI-83+ calculator in BBC BASIC, I switched to using a ring buffer with a pointer to the head element and another to the tail element that would be shunted along every frame, unless the snake ate some food in which case the tail pointer would stay where it was.

As I have even less memory on the ATmega168, I went for a different tactic again; by using "pretty" graphics for the various parts of the snake in the tilemap, I didn't need to store the snake's path anywhere other than this tilemap. That is, if I wanted to advance the tail one unit, I merely need to look at the current tile graphic being used to represent the tail (which will be pointing up, down, left or right) and follow it along to the tile in front of it. By inspecting this tile, I can see if the snake turned a corner at that point or went straight ahead and so adjust the tail position and graphic accordingly.

void advance_tail(void) {
    // Find the current snake tail graphic.
    char tail = tvtext_buffer[tail_y * TVTEXT_BUFFER_WIDTH + tail_x];
    // Where is the body in relation to the tail?
    int8_t body_x = tail_x, body_y = tail_y;
    switch (tail) {
        case FONT_SNAKE_TAIL_UP:
        case FONT_SNAKE_TAIL_DOWN:
        case FONT_SNAKE_TAIL_LEFT:
    // Ensure the body is on the buffer.
    if (body_x < WORLD_LEFT) body_x = WORLD_RIGHT;
    if (body_x > WORLD_RIGHT) body_x = WORLD_LEFT;
    if (body_y < WORLD_TOP) body_y = WORLD_BOTTOM;
    if (body_y > WORLD_BOTTOM) body_y = WORLD_TOP;
    // Find the current body graphic.
    char body = tvtext_buffer[body_y * TVTEXT_BUFFER_WIDTH + body_x];
    // Is it a bend? If so, we'll need to rotate the tail graphic.
    switch (body) {
    // Erase the old tail.
    tvtext_buffer[tail_y * TVTEXT_BUFFER_WIDTH + tail_x] = tvtext_cleared;
    // Draw the new tail.
    tail_x = body_x;
    tail_y = body_y;
    tvtext_buffer[tail_y * TVTEXT_BUFFER_WIDTH + tail_x] = tail;

Similar code is used to advance the head and draw the correct tile behind it.

On an unrelated note, I've released a version of BBC BASIC that should run on the Nspire. The Nspire has an emulator on it to run applications for other calculators, but this emulator doesn't implement undocumented instructions. The TI-83+/TI-84+ BBC BASIC host interface makes use of the sl1 instruction, which shifts a register left one bit and sets the least significant bit to 1. Unfortunately, when this code is run on an Nspire it triggers a crash. Apparently the quick fix I've implemented seems to have done the trick, so unless I hear any further bug reports I'll release the latest version formally soon!

ATmega168 Tetris

Sunday, 22nd November 2009

The tvText library I discussed last entry allows you to display text on a PAL TV in black and white using a 20MHz ATmega168 and a pair of resistors. If this doesn't sound terribly exciting, it's probably because it isn't. However, if you bear some limitations in mind and change the font, you can use this text output as a more general tile-mapping system and use it for games that employ simple graphics.

The new circuit, featuring five sloppily-wired input buttons.
The new circuit, featuring five sloppily-wired input buttons.

I added five buttons to the test circuit — up, down, left, right and fire — to act as game input. This circuit is shown in the photograph above. I also added support for 8×8 characters alongside the existing 6×8 characters to the library, set as a compile-time option. This drops the number of characters per line from 32 to 24, but having square tiles makes producing graphics much easier. The reduction in size of the text buffer also frees up more of the precious 1KB of SRAM for the game!

Diagram of the game circuit.

Even though it was always recommended as an excellent game for beginners to write, I don't believe I've ever written a Tetris clone before. Its simple block graphics makes it an ideal candidate for this system, and it always helps to work on a game that's fun to play. Armed with a Game Boy and a stopwatch I attempted to recreate a moderately faithful version of what is probably the most popular rendition of the game.

I think the result plays pretty well, but don't take my word for it — if you have an ATmega168 lying around, you can download the source and binaries here.

USB joypads and text on your TV courtesy of an ATmega168

Saturday, 14th November 2009

Nearly a month since my last update - my, how time flies when you're having fun (or a heavy workload).

I ended up building myself a cheap and cheerful SI Prog programmer for AVR development. After installing the development tools, scanning through the documentation and writing the microcontroller equivalent of Hello, World (flashing an LED on and off) I needed to find a suitable project. The first one was getting to grips with V-USB, a software USB implementation for AVRs. All you need for this are a couple of I/O pins, a few configuration file changes to set your USB device's vendor ID, product ID and device class, and a few lines of C code to actually implement your device. I attached six tactile switches to an ATmega168 and made the most uncomfortable USB joypad I've ever used. I managed two levels of Sonic the Hedgehog before my thumbs admitted defeat, but it's nice to know that building USB devices is very easy with an AVR.

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/wdt.h>
#include <util/delay.h>
#include <avr/sleep.h>

#include "usbdrv.h"

/* Joystick port bits */
#define JOY_1     (1<<0)
#define JOY_2     (1<<1)
#define JOY_UP    (1<<2)
#define JOY_DOWN  (1<<3)
#define JOY_LEFT  (1<<4)
#define JOY_RIGHT (1<<5)

/* USB HID report descriptor */
    0x05, 0x01,        // USAGE_PAGE (Generic Desktop)
    0x09, 0x05,        // USAGE (Game Pad)
    0xa1, 0x01,        // COLLECTION (Application)
    0x09, 0x01,        //   USAGE (Pointer)
    0xa1, 0x00,        //   COLLECTION (Physical)
    0x09, 0x30,        //     USAGE (X)
    0x09, 0x31,        //     USAGE (Y)
    0x15, 0x00,        //   LOGICAL_MINIMUM (0)
    0x26, 0xff, 0x00,  //     LOGICAL_MAXIMUM (255)
    0x75, 0x08,        //   REPORT_SIZE (8)
    0x95, 0x02,        //   REPORT_COUNT (2)
    0x81, 0x02,        //   INPUT (Data,Var,Abs)
    0xc0,              // END_COLLECTION
    0x05, 0x09,        // USAGE_PAGE (Button)
    0x19, 0x01,        //   USAGE_MINIMUM (Button 1)
    0x29, 0x02,        //   USAGE_MAXIMUM (Button 2)
    0x15, 0x00,        //   LOGICAL_MINIMUM (0)
    0x25, 0x01,        //   LOGICAL_MAXIMUM (1)
    0x75, 0x01,        // REPORT_SIZE (1)
    0x95, 0x08,        // REPORT_COUNT (8)
    0x81, 0x02,        // INPUT (Data,Var,Abs)
    0xc0               // END_COLLECTION

static uchar reportBuffer[3];    /* Buffer for HID reports */
static uchar idleRate;           /* 4 ms units */

uchar usbFunctionSetup(uchar data[8]) {
    usbRequest_t  *rq = (void*)data;
    usbMsgPtr = reportBuffer;
    if ((rq->bmRequestType & USBRQ_TYPE_MASK) == USBRQ_TYPE_CLASS) {
        switch (rq->bRequest) {
            case USBRQ_HID_GET_REPORT:
                return sizeof(reportBuffer);
            case USBRQ_HID_GET_IDLE:
                usbMsgPtr = &idleRate;
                return 1;
            case USBRQ_HID_SET_IDLE:
                idleRate = rq->wValue.bytes[1];
    return 0;

ISR(TIMER0_OVF_vect) {

    /* Fetch input */
    uchar input = ~PINC;
    /* X-axis */
    switch (input & (JOY_LEFT | JOY_RIGHT)) {
        case JOY_LEFT:
            reportBuffer[0] = 0;
        case JOY_RIGHT:
            reportBuffer[0] = 255;
            reportBuffer[0] = 128;

    /* Y-axis */
    switch (input & (JOY_UP | JOY_DOWN)) {
        case JOY_UP:
            reportBuffer[1] = 0;
        case JOY_DOWN:
            reportBuffer[1] = 255;
            reportBuffer[1] = 128;

    /* Buttons */
    reportBuffer[2] = input & (JOY_1 | JOY_2);

    usbSetInterrupt(reportBuffer, sizeof(reportBuffer));

int main(void) {

    usbInit();              /* Initialise USB. */

    PORTC = 0b00111111;     /* Pull high PORTC0..PORTC5 */
    TCCR0B = 0b00000101;    /* CS2..CS0 = 101:  prescaler = /1024 */
    TIMSK0 |= (1 << TOIE0); /* Enable timer 0 overflow interrupt. */
    sei();                  /* Enable global interrupts. */
    for (;;) {
        /* Infinite loop */

I should only really call usbSetInterrupt when a button or axis has changed, rather than every loop, but the above code works as is.

One thing that always bothers me when it comes to electronic projects is the difficulty of providing text output. LCDs are generally quite expensive and low resolution, and typically require a great many pins to drive them. Video display processor chips are difficult to find, and appear to require quite complex external circuitry (the best thing I've found thus far are some TMS9918 chips being sold as spares for MSX computers). Having briefly experimented with generating PAL video signals in software before, I thought I'd try the two-resistor approach to getting PAL video output on an ATmega168.

I had a hunt around and found AVGA, which is close to what I wanted - video output from an AVR using cheap hardware. However, it outputs RGB directly, and I don't own a TV or RGB converter so couldn't use that - all I have is a VGA box (accepting composite or S-Video input) and a TV capture card (also only accepting composite or S-Video input). AVGA does work with VGA monitors, but I'd like to keep the hardware interface simple - just two resistors, ideally.

tvText demo screen

In the end, I ended up writing my own library. It currently has the following specifications:

  • 32×16 characters: 512 bytes (half of the total SRAM on the ATmega168) are used to store the text buffer.
  • Full 256 characters at a resolution of 6×8 pixels each.
  • Total screen resolution: 192×128.

The library is interrupt-driven, and uses the sixteen-bit TIMER1 to schedule events. This means that the AVR is only busy generating video signals when it absolutely has to, leaving some CPU time to the user program. When outputting at full quality, the AVR appears to be capable of running user code at 3.3 MIPS, but by skipping alternate scanlines (each scanline is scanned twice anyway, so this mainly just makes the display appear darker) the AVR appears to be running user code at 9.9 MIPS. (I say "appears" as my calculation has been to execute a busy loop that would normally take one second on the AVR running at its normal 20 MIPS then seeing how long it takes with the video output driver enabled).

The above video demonstrates some of the currently rather limited features of the library. The text console handles a subset of the BBC Micro VDU commands - I'd like to support as many of its features as possible. The code behind the BASIC-like part of the demo is simply written like this:

#include "tvtext/tvtext.h"

void type_string_P(const char* s) {
    char c;
    while ((c = pgm_read_byte(s++))) {

int main(void) {


    tvtext_puts_P(PSTR("AVR Computer 1K\r\n\nATmega 168\r\n\nBASIC\r\n\n>"));
    type_string_P(PSTR("10 PRINT \"AVR Rules! \";\r\n"));
    type_string_P(PSTR("20 GOTO 10\r\n"));

    for (int i = 0; i <= 200; ++i) {
        tvtext_puts_P(PSTR("AVR Rules! "));

    tvtext_puts_P(PSTR("\r\nEscape at line 10\r\n>"));
    type_string_P(PSTR("CHAIN \"DEMO\""));
    // ...


All of the high-level console code - text output, viewport scrolling, cursor positioning &c - has been written in C, so should be relatively easy to be customised. The output driver itself has been written in assembly as timing is critically important.

With a few more features and a bit of tidying up I hope that people would find this a useful library. I'd certainly like to get a blinking cursor working within the driver, and if I add support for a reduced 128-character version I could save quite a bit of ROM space and add support for "coloured" - inverted, that is - text. NTSC support would also be quite useful.

Subscribe to an RSS feed that only contains items with the tvText tag.

FirstLast RSSSearchBrowse by dateIndexTags