DS2423_COUNTER_BUFSZ
buf = malloc(DS2423_COUNTER_BUFSZ, M_DEVBUF, M_NOWAIT);
onewire_read_block(sc->sc_onewire, &buf[3], DS2423_COUNTER_BUFSZ-3);
crc = onewire_crc16(buf, DS2423_COUNTER_BUFSZ-2);
free(buf, M_DEVBUF, DS2423_COUNTER_BUFSZ);