Re: Minimal USB implementation

Very interesting. Your effort should also be useful to someone learning how USB works hands-on, as it clears away all but the essentials.

I've noticed that calling assembly routines frustrates the C optimizer, because it must assume the worst about registers preserved. Here was an attempt I made at inlining the CRC routine and communicating what registers it trashed (though it may do it wrong, as I still find the asm specification syntax confusing):

static __attribute__((naked)) inline void usbCrc16Append( volatile unsigned char* data, unsigned char len )
{
    asm volatile (
"\n    ldi     r20, 0xFF"
"\n    ldi     r21, 0xFF"
"\n    rjmp    usbCrc16LoopTest"
"\nusbCrc16r18Loop:"
"\n    ld      r18, Z+"
"\n    eor     r18, r20      ; r19 is now 'x' in table()"
"\n    mov     r19, r18      ; compute parity of 'x'"
"\n    swap    r18"
"\n    eor     r18, r19"
"\n    mov     r20, r18"
"\n    lsr     r18"
"\n    lsr     r18"
"\n    eor     r18, r20"
"\n    inc     r18"
"\n    andi    r18, 2        ; r18 is now parity(x) << 1"
"\n    cp      r1, r18       ; c = (r18 != 0), then put in high bit"
"\n    ror     r19           ; so that after xoring, shifting, and xoring, it gives"
"\n    ror     r18           ; the desired 0xC0 with r21"
"\n    mov     r20, r18"
"\n    eor     r20, r21"
"\n    mov     r21, r19"
"\n    lsr     r19"
"\n    ror     r18"
"\n    eor     r21, r19"
"\n    eor     r20, r18"
"\nusbCrc16LoopTest:"
"\n    subi    %1, 1"
"\n    brsh    usbCrc16r18Loop"
"\n    com     r20"
"\n    com     r21"
"\n    st      Z+, r20"
"\n    st      Z, r21"
"\n"
    : "=z" (data), "=r" (len)
    : "0"  (data), "1"  (len)
    : "memory", "r18", "r19", "r20", "r21" );
}

Also, that's for the optimized CRC routine, so you'll want to convert the slower, shorter one.