tabby/avr/main.c

#include <stdio.h>

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

#include <tabby/clock.h>
#include <tabby/command.h>
#include <tabby/printer.h>

#include <tabby/avr/uart.h>

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

static volatile uint8_t body[648];

static volatile uint16_t sum = 0;

static volatile uint16_t i = 0,
                         b = 0;

static inline uint16_t checksum() {
    uint16_t ret = header.data[2]
                 + header.data[3]
                 + header.data[4]
                 + header.data[5];

    size_t i;

    for (i=0; i<header.size; i++) {
        ret += ((uint8_t *)body)[i];
    }

    return ret;
}

/*
 * SPI byte receipt interrupt vector
 */
ISR(SPI_STC_vect) {
    uint8_t value = SPDR;

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

                b = 0;
            }

            SPDR = 0;

            break;
        }

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

            SPDR = 0;

            break;
        }

        case 2: {
            header.type = value;
            i++;
            SPDR = 0;

            break;
        }

        case 3: {
            header.compression = value;
            i++;
            SPDR = 0;

            break;
        }

        case 4: {
            header.size = value;
            i++;
            SPDR = 0;

            break;
        }

        case 5: {
            header.size |= value << 8;
            i++;
            SPDR = 0;

            break;
        }

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

                b++;

                SPDR = 0;
            } else if (b == header.size + 1) {
                sum >>= 8;
                sum  |= value << 8;

                b++;

                SPDR = 0x81;
            } else if (b == header.size + 2) {
                b++;

                SPDR = 0;
            } else if (b == header.size + 3) {
                i = 0;

                SPDR = 0;
            }
        }
    }

    uart_putchar(value, NULL);
}

static void setup_clock_external() {
    /*
     * Configure MISO as output
     */
    DDRB |= (1 << DDB4);
    /*
     * Configure SS as input
     */
    DDRB &= ~(1 << DDB2);

    /*
     * Set SPI slave mode, and shift in/out most significant bit first
     */
    SPCR &= ~((1 << MSTR) | (1 << DORD));

    /*
     * Enable SPI in Mode 3 with interrupts
     */
    SPCR |= (1 << CPOL) | (1 << CPHA) | (1 << SPIE) | (1 << SPE);

    /*
     * Initialize the SPI Data Register and serial buffer
     */
    SPDR = 0;
}

int main() {
    /*
     * Best turn on the serial UART!
     */
    uart_init();

    /*
     * By default, the Game Boy link port is configured to monitor for external
     * clock pulses, so we'll go ahead and do that in this case.
     */
    setup_clock_external();

    /*
     * We do actually want to globally enable interrupts here, so here's that
     * one assembly instruction to do so.
     */
    sei();

    while (1) {
        sleep_mode();
    }

    return 0;
}