bit alignment code - LONG

Keith M

Anyone bored enough to look over some code?

I have a stream of bits that arrive from a floppy disk drive and my ISR captures the data. This works fine. I've just added a 24-bit buffer and I'm attempting to byte-sync my datastream to a SYNC value "0x4489." SYNC value does not show up in regular data. I'm attempting this as the data is arriving on the fly. The idea is that unaligned data will simply be written to my external SRAM through the back end of the buffer until a SYNC value shows up. Once that happens, I will adjust the position of the SYNC(and subsequent data) in the buffer by skipping bit writes to the SRAM until I'm properly aligned. Note that while this skipping/aligning is going on, I'm still making regular shifts of data into the buffer.

All writes to the buffer and SRAM are single bit writes. There are no timing/clock/interrupt issues here. Assume one bit arrives via WKPND_B every 2us roughly. Either an edge happened, or RTCC rollover occurred. This is my data.

Am I close or on the right track here?

' -------------------------------------------------------------------------
  INTERRUPT NOCODE
' -------------------------------------------------------------------------

ASM

SETB rb.1            'debug pin(inside interrupt)
CLR RTCC

CLR FSR

ISR_Start:

    MODE $09
    MOV !RB,#%00000000
    'W NOW CONTAINS A %00000001 IF FIRED BY EDGE, 0 IF RTCC

    mov myres, w
    'myres.0 has the actual bit value

    MOVB C, MYRES.0                    'Put myres.0 into C to be shifted in
    'shift the bit into the 24-bit buffer
    RL BYTE1
    RL BYTE2
    RL BYTE3
    MOVB MYRES.0, C                    'this is the output bit if we need to write it

    CJNE BYTE2, #$44, notasync
    CJNE BYTE1, #$89, notasync
    CJE bitcounter, #1, notasync            'this is a sync but its already aligned right

    'we've seen an unaligned sync
    'the number of bits we need to skip = the number of bits we are currently out of alignment
    'so let's subtract the number of bits we've written for the current byte from 8
    'and then skip that number of bits

    MOV skipwrite, #8
    SUB skipwrite, bitcounter

notasync:

    CJE SKIPWRITE, #0, skipdaskip            'if we are not in the process of realigning

    dec skipwrite
    CJA SKIPWRITE, #0, goback

    'skipwrite just went from 1->0, last bit to skip, let's reset bitcounter to 8
    'we are now properly aligned.
    MOV bitcounter, #8

    jmp goback

skipdaskip:

    dec bitcounter
    CJA BITCOUNTER,#0, skipbcreset            'snz wont work because mov is multi-byte instruction

    mov bitcounter, #8                'reset bitcounter

skipbcreset:

    CLRB SCK        'make sure clock is low to start with
    NOP

    MOVB SI, myres.0    'send bit to fram
    NOP            'satisfy 5ns data setup time

    SETB SCK        'raise clock notifying chip to read the data

    'COUNT THE Bit stored
    IJNZ lobyte, goback
    IJNZ hibyte, goback
    inc superhibyte

goback:

CLRB rb.1
MOV W, #-100
retiw

ENDASM

Thanks

Keith