Лично я делаю так:
/******************************************************
*   Преобразует в строку length младших битов         *
*   слова word. Строка dest должна иметь размер       *
*   по меньшей мере length + 1                        *
******************************************************/
char* bitwise(uint32_t word, char* dest, uint8_t length)
{
    register uint8_t i;
    for (i = 0; i < length; i++)
        dest[length - i - 1] = word & (1UL << i) ? '1' : '0';          
    dest[length] = '\0';
    return dest;
}

char* bitwise(uint32_t word, char* dest, uint8_t length)
{
    dest += length;
    *dest = '\0';
    while( length-- )
    {    *--dest = (word & 1) + '0';
        word >>= 1;
    }
    return dest;
}

    132          char* bitwise(unsigned long word, char* dest, unsigned char length)
   \                     bitwise:
    133          {
   \   00000000   93BA               ST      -Y, R27
   \   00000002   2E3A               MOV     R3, R26
   \   00000004   2F79               MOV     R23, R25
   \   00000006   2E28               MOV     R2, R24
   \   00000008   01C8               MOVW    R25:R24, R17:R16
   \   0000000A   01D9               MOVW    R27:R26, R19:R18
   \   0000000C   010A               MOVW    R1:R0, R21:R20
    134              register unsigned char i;
    135              for (i = 0; i < length; i++)
   \   0000000E   E050               LDI     R21, 0
   \   00000010   2366               TST     R22
   \   00000012   F0D9               BREQ    ??bitwise_0
   \   00000014   01F0               MOVW    R31:R30, R1:R0
   \   00000016   0FE6               ADD     R30, R22
   \   00000018   1FF5               ADC     R31, R21
   \   0000001A   9731               SBIW    R31:R30, 1
    136                  dest[length - i - 1] = word & (1UL << i) ? '1' : '0';
   \                     ??bitwise_1:
   \   0000001C   E001               LDI     R16, 1
   \   0000001E   E010               LDI     R17, 0
   \   00000020   E020               LDI     R18, 0
   \   00000022   E030               LDI     R19, 0
   \   00000024   2F45               MOV     R20, R21
   \   00000026   ........           CALL    ?L_SHL_L03
   \   0000002A   2308               AND     R16, R24
   \   0000002C   2319               AND     R17, R25
   \   0000002E   232A               AND     R18, R26
   \   00000030   233B               AND     R19, R27
   \   00000032   2B01               OR      R16, R17
   \   00000034   2B02               OR      R16, R18
   \   00000036   2B03               OR      R16, R19
   \   00000038   F011               BREQ    ??bitwise_2
   \   0000003A   E301               LDI     R16, 49
   \   0000003C   C001               RJMP    ??bitwise_3
   \                     ??bitwise_2:
   \   0000003E   E300               LDI     R16, 48
   \                     ??bitwise_3:
   \   00000040   8300               ST      Z, R16
   \   00000042   9553               INC     R21
   \   00000044   9731               SBIW    R31:R30, 1
   \   00000046   1756               CP      R21, R22
   \   00000048   F348               BRCS    ??bitwise_1
    137              dest[length] = '\0';
   \                     ??bitwise_0:
   \   0000004A   E000               LDI     R16, 0
   \   0000004C   01F0               MOVW    R31:R30, R1:R0
   \   0000004E   0FE6               ADD     R30, R22
   \   00000050   1FF0               ADC     R31, R16
   \   00000052   8300               ST      Z, R16
    138              return dest;
   \   00000054   0180               MOVW    R17:R16, R1:R0
   \   00000056   2D82               MOV     R24, R2
   \   00000058   2F97               MOV     R25, R23
   \   0000005A   2DA3               MOV     R26, R3
   \   0000005C   91B9               LD      R27, Y+
   \   0000005E   9508               RET
    139          }

    143          char* bw(unsigned long word, char* dest, char length)
   \                     bw:
    144          {
    145              dest += length;
   \   00000000   E070               LDI     R23, 0
   \   00000002   0F46               ADD     R20, R22
   \   00000004   1F57               ADC     R21, R23
    146              *dest = '\0';
   \   00000006   01FA               MOVW    R31:R30, R21:R20
   \   00000008   8370               ST      Z, R23
   \   0000000A   2366               TST     R22
   \   0000000C   F069               BREQ    ??bw_0
   \                     ??bw_1:
   \   0000000E   956A               DEC     R22
    147              while( length-- )
    148              {    *--dest = (word & 1) + '0';
   \   00000010   2F70               MOV     R23, R16
   \   00000012   7071               ANDI    R23, 0x01
   \   00000014   5D70               SUBI    R23, 208
   \   00000016   01FA               MOVW    R31:R30, R21:R20
   \   00000018   9372               ST      -Z, R23
   \   0000001A   01AF               MOVW    R21:R20, R31:R30
    149                  word >>= 1;
   \   0000001C   9536               LSR     R19
   \   0000001E   9527               ROR     R18
   \   00000020   9517               ROR     R17
   \   00000022   9507               ROR     R16
    150              }
   \   00000024   2366               TST     R22
   \   00000026   F799               BRNE    ??bw_1
    151              return( dest );
   \                     ??bw_0:
   \   00000028   018A               MOVW    R17:R16, R21:R20
   \   0000002A   9508               RET
    152          }

#include <stdio.h>

main(argc, argv)
int argc;
char **argv;
{
    while (*argv != argv[1] && (*argv = argv[1]) && (argc = 0) || (*++argv
    && (**argv && ((++argc)[*argv] && (**argv <= argc[*argv] ||
    (**argv += argc[*argv] -= **argv = argc[*argv] - **argv)) &&
    --argv || putchar(**argv) && ++*argv--) || putchar(10))));
}