USB-I2C/firmware/main.c

/*
 * USB (CDC-ACM) <-> I2C bridge, ATmega8U2 @ 16 MHz.
 *
 * Request frame (host -> device), several frames may be concatenated freely:
 *   A5 01 addr wlen rlen wdata[wlen]   I2C transaction (7-bit addr, wlen/rlen <= 32)
 *   A5 02 n                            set SCL delay (0 = fastest, default 4 ~ 400 kHz)
 *   A5 03                              ping
 *   A5 04                              bus recovery
 * Reply (device -> host), one per request, in order:
 *   status [payload]
 *   status: 0 OK, 1 NACK on address, 2 NACK on data, 3 bus error/timeout,
 *           0x10 unknown op, 0x11 bad length
 *   payload: XFER -> rlen bytes (only if status==0); PING -> proto, maxlen
 * Bytes outside a frame are ignored (so ModemManager "AT" probes are harmless).
 */
#include <avr/io.h>
#include <avr/wdt.h>
#include <string.h>
#include "usb.h"
#include "i2c.h"

#define MAGIC   0xA5
#define MAXLEN  32
#define PROTO   1

enum { OP_XFER = 1, OP_SPEED = 2, OP_PING = 3, OP_RECOVER = 4, OP_LINES = 5 };
enum { ST_BAD_OP = 0x10, ST_BAD_LEN = 0x11 };

static uint8_t txb[USB_PACKET];
static uint8_t txn;

static void flush(void)
{
    if (txn) { usb_tx(txb, txn); txn = 0; }
}

static uint8_t *reply_begin(uint8_t len)       /* reserve len bytes in the tx buffer */
{
    if (txn + len > USB_PACKET) flush();
    uint8_t *p = &txb[txn];
    txn += len;
    return p;
}

static void reply1(uint8_t st) { *reply_begin(1) = st; }

/* ---- frame parser ---- */
static uint8_t fbuf[4 + MAXLEN];
static uint8_t fn;
static bool    in_frame;

static void frame_done(void) { in_frame = false; fn = 0; }

static void feed(uint8_t b)
{
    if (!in_frame) {
        if (b == MAGIC) { in_frame = true; fn = 0; }
        return;
    }
    fbuf[fn++] = b;

    uint8_t need;
    switch (fbuf[0]) {
    case OP_XFER:
        if (fn < 4) return;
        if (fbuf[2] > MAXLEN || fbuf[3] > MAXLEN) { reply1(ST_BAD_LEN); frame_done(); return; }
        need = 4 + fbuf[2];
        break;
    case OP_SPEED:
    case OP_LINES:   need = 2; break;
    case OP_PING:
    case OP_RECOVER: need = 1; break;
    default:         reply1(ST_BAD_OP); frame_done(); return;
    }
    if (fn < need) return;

    switch (fbuf[0]) {
    case OP_XFER: {
        uint8_t rlen = fbuf[3];
        uint8_t *p = reply_begin(1 + rlen);    /* reserve worst case, trim on error */
        uint8_t st = i2c_xfer(fbuf[1] & 0x7F, &fbuf[4], fbuf[2], p + 1, rlen);
        p[0] = st;
        if (st != I2C_OK) txn -= rlen;
        break; }
    case OP_SPEED:   i2c_set_delay(fbuf[1]); reply1(0); break;
    case OP_PING:    { uint8_t *p = reply_begin(3); p[0] = 0; p[1] = PROTO; p[2] = MAXLEN; break; }
    case OP_RECOVER: reply1(i2c_recover()); break;
    case OP_LINES:   { uint8_t *p = reply_begin(2); p[0] = 0; p[1] = i2c_lines(fbuf[1]); break; }
    }
    frame_done();
}

int main(void)
{
    MCUSR = 0;
    wdt_disable();
    CLKPR = 0x80; CLKPR = 0;                    /* full 16 MHz, no prescaler */

    i2c_init();
    usb_init();

    TCCR0B = (1 << CS02) | (1 << CS00);         /* clk/1024: overflow every ~16.4 ms */
    uint8_t idle = 0;
    uint8_t rx[USB_PACKET];

    for (;;) {
        usb_task();

        uint8_t n = usb_rx(rx);
        if (n) {
            idle = 0;
            for (uint8_t i = 0; i < n; i++) feed(rx[i]);
            flush();                            /* one IN packet for the whole OUT packet */
        }
        if (TIFR0 & (1 << TOV0)) {              /* resync if a frame stalls ~100 ms */
            TIFR0 = (1 << TOV0);
            if (in_frame && ++idle > 6) frame_done();
        }
    }
}