tabby/avr/main.c

#include <stdio.h>

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

#include <tabby/clock.h>
#include <tabby/packet.h>

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

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 const uint16_t timer_counter_intervals[4] = {
    1953, 977, 61, 31
};

static volatile tabby_avr_buffer buffer = {
    .len = 0,
    .cur = 0
};

/*
 * Internal clock source interrupt vector
 */
ISR(TIM0_COMPB_vect) {
    value_in >>= 1;

    if (PORTB & (1 << PORTB2)) {
        value_in |= 0x80;
    }

    if (value_out & 0x80) {
        PORTB |=  (1 << PORTB3);
    } else {
        PORTB &= ~(1 << PORTB3);
    }

    value_out <<= 1;

    if (--bits == 0) {
        uart_putchar(value_in, NULL);

        if (buffer.cur < buffer.len) {
            value_out = buffer.data[++buffer.cur];
        } else {
            buffer.len = 0;
            buffer.cur = 0;
        }

        bits = 8;
    }

    /*
     * Strobe the SCK pin
     */
    PORTB |=  (1 << PORTB1);
    PORTB &= ~(1 << PORTB1);
}

/*
 * SPI byte receipt interrupt vector
 */
ISR(SPI_STC_vect) {
    uart_putchar(SPDR);

    if (buffer.cur < buffer.len) {
        SPDR = buffer.data[++buffer.cur];
    } else {
        buffer.len = 0;
        buffer.cur = 0;
    }
}

static void setup_clock_internal(tabby_clock_speed speed) {
    /*
     * Configure MISO as output
     */
    DDB3 |= (1 << PORTB3);

    /*
     * Configure MOSI as input
     */
    DDB2 &= ~(1 << PORTB2);

    /*
     * Configure SCK pin as output
     */
    DDB1 |= (1 << PORTB1);

    /*
     * Enable timer interrupt vector
     */
    TIMSK = (1 << TOIE1);

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

    /*
     * Set timer interval
     */
    OCR1A = timer_counter_intervals[speed];

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

static void setup_clock_external() {
    /*
     * Disable internal timer interrupts
     */
    TCCR1B = 0;
    OCR1A  = 0;
    TIMSK  = 0;

    /*
     * Configure MISO as output
     */
    DDB3 |= (1 << PORTB3);

    /*
     * Configure MOSI as input
     */
    DDB2 &= ~(1 << PORTB2);

    /*
     * 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;

    buf.len = 0;
    buf.cur = 0;
}

static void snooze() {
    set_sleep_mode(mode);
    sleep_enable();
    sleep_mode();
    sleep_disable();
}

int main() {
    tabby_command state = TABBY_COMMAND_NONE;

    uint8_t last = 0x00;

    int received = 0;

    /*
     * 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) {
        uint8_t c = uart_getchar(NULL);

        received++;

        switch (state) {
            case TABBY_COMMAND_NONE: {
                if (last == 0 && c == TABBY_PACKET_START) {
                    continue;
                } else if (last == TABBY_PACKET_START) {
                    state = c;
                }

                break;
            }

            case TABBY_COMMAND_SEND: {
                if (received == 3 || received == 4) {
                    buffer.len = (buffer.len >> 8) | (c << 8);
                } else {
                    buffer.data[buffer.cur++] = c;
                }

                break;
            }

            case TABBY_COMMAND_CLOCK_SOURCE:
            case TABBY_COMMAND_CLOCK_SPEED:

            default:
                goto error_invalid_packet;
        }

        last = c;

        received++;

        continue;

error_invalid_packet:
        state    = TABBY_COMMAND_NONE;
        last     = 0;
        received = 0;

        buffer.cur = 0;
        buffer.len = 0;
    }

    return 0;
}