tabby/avr/send.c

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

#include <tabby/printer.h>
#include <tabby/avr/uart.h>

#define TIMER1_INTERVAL 1953

enum {
    TABBY_SEND_READING  =  0,
    TABBY_SEND_BUFFERED = (1 << 0),
    TABBY_SEND_COMPLETE = (1 << 1)
};

static volatile uint8_t bits      = 0;
static volatile uint8_t value_in  = 0x00; /* Data coming in from Game Boy */
static volatile uint8_t value_out = 0x00; /* Data going out to Game Boy */

static volatile tabby_printer_packet header = {
    .type        = 0,
    .compression = 0,
    .size        = 0
};

static volatile uint8_t body[TABBY_PRINTER_MAX_PACKET_SIZE];

static volatile uint16_t sum = 0x0000;

static volatile uint8_t device = 0x00,
                        status = 0x00,
                        flags  = TABBY_SEND_READING;

static volatile uint16_t i = 0,
                         b = 0;

/*
 * So like, we're abusing pins here.  So badly, it's beyond.  So here's what's
 * going down: We want to retain the same physical I/O connections between
 * both the sending and receiving flavors of the whole Game Boy Printer
 * endeavor, and that means that we must absolutely bit bang MOSI and MISO
 * opposite from their ordinary roles.  The reason being: There is no notion
 * of MISO or MOSI from the Game Boy's (nor its peripherals') point of view.
 * The SI and SO pins over a link cable are crossed over, so SI on one end
 * always connects to SO on the other, and vice-versa.
 */
static void spi_start() {
    /*
     * Configure MISO as output...Oddly
     */
    DDRB |=  (1 << DDB4);

    /*
     * Configure MOSI as input...Also oddly
     */
    DDRB &= ~(1 << DDB3);

    /*
     * Configure SCK pin as output
     */
    DDRB |= (1 << DDB5);

    /*
     * Set output pins clear by default
     */
    PORTB &= ~((1 << PORTB5) | (1 << PORTB4));

    /*
     * Set timer clock divider to 1/1
     */
    TCCR1B = (1 << CS10);

    /*
     * Set timer interval
     */
    OCR1A = TIMER1_INTERVAL;

    /*
     * Reset timer counter to zero
     */
    TCNT1 = 0;

    /*
     * Enable timer interrupt vector for match on OCR1A
     */
    TIMSK1 = (1 << OCIE1A);
}

static void spi_end() {
    PORTB  &= ~((1 << PORTB5) | (1 << PORTB4));
    DDRB   &= ~((1 << DDB5)   | (1 << DDB4) | (1 << DDB3));
    TIMSK1 &= ~ (1 << OCIE1A);
    TCNT1   =   0;
    OCR1A   = TIMER1_INTERVAL;
    TCCR1B  =  (1 << CS10);
}

ISR(TIMER1_COMPA_vect) {
    value_in >>= 1;

    PORTB |= (1 << PORTB5);

    if (PINB & (1 << PINB3)) {
        value_in |= 0x80;
    }

    if (value_out & 0x80) {
        PORTB |=  (1 << PORTB4);
    } else {
        PORTB &= ~(1 << PORTB4);
    }

    value_out <<= 1;

    if (--bits == 0) {
        if (i < sizeof(header)) {
            value_out = header.data[i++];
        } else if (b < header.size) {
            value_out = body[b++];
        } else if (b == header.size) {
            value_out = sum & 0x00ff;
            b++;
        } else if (b == header.size + 1) {
            value_out = (sum & 0xff00) >> 8;
            b++;
        } else if (b == header.size + 2) {
            device    = value_in;
            value_out = 0x00;
            b++;
        } else if (b == header.size + 3) {
            status    = value_in;
            value_out = 0x00;

            i      = 0;
            b      = 0;
            flags &= ~TABBY_SEND_BUFFERED;
            flags |=  TABBY_SEND_COMPLETE;

            spi_end();
        }

        bits = 8;
    }

    /*
     * Chill out for 60µsec, then ride out the rest of the time until the next
     * interrupt with a high as fuck SCK
     */
    _delay_us(60);

    PORTB &= ~(1 << PORTB5);
}

static uint16_t checksum() {
    uint16_t sum = 0;
    size_t i;

    for (i=2; i<6; i++) {
        sum += header.data[i];
    }

    for (i=0; i<header.size; i++) {
        sum += body[i];
    }

    return sum;
}

int main() {
    uint16_t sum = 0,
             c;

    uart_init();

    sei();

    while (1) {
        uart_putchar('O', NULL);

        c = uart_getchar(NULL);

        if (c == 'K') {
            break;
        }
    }

    while (1) {
        if (flags & TABBY_SEND_COMPLETE) {
            uart_putchar(device, NULL);
            uart_putchar(status, NULL);

            flags = TABBY_SEND_READING;
        }

        while (flags != TABBY_SEND_READING);

        c = uart_getchar(NULL);

        switch (i) {
            case 0: {
                if (c == 0x88) {
                    header.data[0] = c;
                    i++;

                    b = 0;
                }

                break;
            }

            case 1: {
                if (c == 0x33) {
                    header.data[1] = c;
                    i++;
                } else {
                    i = 0;
                }

                break;
            }

            case 2: {
                switch (c) {
                    case 0x01:
                    case 0x02:
                    case 0x04:
                    case 0x08:
                    case 0x0f: {
                        header.type = c;
                        i++;

                        break;
                    }

                    default: {
                        i = 0;

                        break;
                    }
                }

                break;
            }

            case 3: {
                header.compression = c;
                i++;

                break;
            }

            case 4: {
                header.size = c;
                i++;

                break;
            }

            case 5: {
                header.size |= c << 8;
                i++;

                break;
            }

            default: {
                if (b < header.size) {
                    body[b++];
                } else if (b == header.size) {
                    sum = c;
                    b++;
                } else if (b == header.size + 1) {
                    sum |= c << 8;

                    i = 0;
                    b = 0;

                    if (sum == checksum()) {
                        flags |= TABBY_SEND_BUFFERED;

                        spi_start();
                    }
                }
            }
        }
    }

    return 0;
}