omap3_mlc_bch.c

#if 1
// try this one first
#define BCH_POLY 0b001110010101
#else
// other possibility
#define BCH_POLY 0b010100111001
#endif


// we use the top 12 bits since it's more convenient for left-shifting
#if 1
unsigned omap3_mlc_bch_shl1( unsigned x )
{
    if( x >> 31 )
        return ( x << 1 ) ^ ( BCH_POLY << 20 );
    else
        return ( x << 1 );
}
#else
// maybe more efficient version?
unsigned omap3_mlc_bch_shl1( unsigned x )
{
    return ( x << 1 ) ^ ( ( BCH_POLY << 20 ) & -( x >> 31 ) );
}
#endif

unsigned omap3_mlc_bch_word( uint16_t w )
{
    // put 20-bit data word (incl padding) into top 20 bits
    unsigned x = w << 16;

    // shift left and reduce to 12 bits (again in top 12 bits)
    for( int i = 0; i < 20; ++i )
        x = omap3_mlc_bch_shl1( x );

    return w << 16 | x >> 20;
}

void omap3_mlc_bch( unsigned encoded[260], uint16_t const data[256] )
{
    unsigned checksum = 0;
    for( int i = 0; i < 256; ++i ) {
        checksum += data[i];
        encoded[i] = omap3_mlc_bch_word( data[i] );
    }
    encoded[256] = omap3_mlc_bch_word( checksum & 0xffff );
    encoded[257] = 0;
    encoded[258] = 0;
    encoded[259] = 0;
}

void omap3_mlc_bch_verify( unsigned const encoded[260] )
{
    for( int i = 0; i < 160; ++i ) {
        unsigned x = encoded[i];
        for( int i = 0; i < 32; ++i )
            x = omap3_mlc_bch_shl1( x );
        if( x != 0 )
            printf( "encoded[%3d] = 0x%08x  verification failed (0x%03x != 0)\n", i, encoded[i], x >> 20 );
    }
}