source: avrstuff/V-USB_Dev/firmwares/CrO2/main.c@ 3c17057

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

#include <string.h>
#include <stdbool.h>

#include "usbdrv/usbdrv.h"

static uint8_t ioblock[275]; // Max. Size of an MO5 standard block including sync header
static uint8_t status; // status bits
static uint16_t writePos;
static uint16_t blksz;
static uint8_t cksum;

static uint16_t readPos;
static uint8_t bitpos;
static uint8_t bit; // Currently generated bit

int main() {
        // Init
        status = 0;
        writePos = 0;
        blksz = 0;

        wdt_enable(WDTO_2S);

        // USB
        usbInit();
        sei();

        bit = 1;
        bitpos = 7;
        readPos = 0;

        // Generate sync header (this never changes)
        int i;
        for (i = 0; i< 16; i++)
        {
                ioblock[i] = 1;
        }
        ioblock[16] = 0x3C;
        ioblock[17] = 0x5A;

        // Setup TIMER 1 for MFM pulses generation
        TCCR1A = (1<<COM1A0); // Enable OC1A output
        TCCR1A |= (1<<COM1A1); // Compare match will always SET OC1A instead of toggling it. Thus the output is always high.
        // CTC mode with OCR1A as MAXregister
        TCCR1B = (1<<WGM12);
        OCR1A = 13063; // 800us bit clock
        OCR1B = 6532; // Half-clock for 1 bits
        TIMSK = (1 << OCIE1B) | (1 << OCIE1A); // interrupts on both timer matches.
        TCCR1A |= (bit << FOC1A); // Force toggle of A (make sure output is a logic 1 to allow MO5 to detect tapedrive)

        DDRB |= 2; // OC1A/PB1 as output


        while(1) {
                wdt_reset();
                usbPoll();
                // TODO watch for motor on/off
        }
}


static inline void startGen()
{
        TCCR1A &= ~(1<<COM1A1); // Toggle output on compare match
        TCCR1B |= (1<<CS10);    // Enable timer clock (prescaler = 1)
}


uint8_t usbFunctionSetup(uint8_t data[8]) {
        usbRequest_t *rq = (void *)data;

        usbMsgPtr = ioblock;
        switch(rq->bRequest)
        {
                case 0:
                        {  /* CONFIGURE */
                                status = rq->wValue.bytes[0];
                                return 0;
                        }
                case 1:
                        {  /* RECEIVE/POLL DATA (TAPE > USB) */
                                return 254; 
                        }
                case 2:
                        {
                                /* SEND DATA (USB > TAPE) */
                                /* Get ready, actual data will come in usbFunctionWrite */

                                // store type of MO5 block + size in ioblock
                                writePos = 20; // Leave space for Sync header, block type, and block size.
                                ioblock[18] = rq->wValue.word;
                                ioblock[19] = rq->wLength.word + 2; // Add blktype and cksum to data length
                                blksz = rq->wLength.word + 21;
                                cksum = 0;
                                if (rq->wLength.word == 0)
                                {
                                        // No data in this block - start generating now !
                                        ioblock[20] = 0; // checksum
                                        startGen();
                                } else {
                                        return USB_NO_MSG; // Call usbFunctionWrite to send the data
                                }
                        }
        }
        return 0;
}

uint8_t usbFunctionWrite(uint8_t* data, uint8_t len)
{
        // The data for a single request will be sent as slices of up to 8 bytes.
        for (int i = 0; i < len; i++)
        {
                ioblock[writePos] = data[i];
                cksum -= data[i];
                writePos++;
        }

        ioblock[writePos] = cksum;

        if (writePos >= blksz - 1)
        {
                // start generating
                startGen();
                return 1;
        } else {
                return 0;
        }
        // returns 0: "needs more data"
        // returns 1: "done"
        // returns FF: "error"
}

ISR (TIMER1_COMPA_vect, ISR_NOBLOCK)
{
        // generate next bit
        bit = (ioblock[readPos] >> bitpos) & 1;
        bitpos--;
        if (bitpos > 7) // overflow ?
        {
                bitpos = 7;
                readPos++;
                if (readPos > blksz)
                {
                        // Make sure output is high step 1: match action = force high level
                        TCCR1A |= (1<<COM1A1);

                        // Stop generating (and interrupts)
                        TCCR1B &= ~(1<<CS10);
                        readPos = 0;

                        // make sure output is high step 2 : force a match.
                        TCCR1A |= (1 << FOC1A);
                }
        }
}

ISR (TIMER1_COMPB_vect, ISR_NOBLOCK)
{
        TCCR1A |= (bit << FOC1A); // Force toggle of A on B compare when generating a 1 bit
}