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I'm struggling to see why this code occassionally misses the first
byte of a data stream, and would appreciate any insights and/or
improvements to make the problem go away

Master is an 18F452 (39.3216MHz), receiver is a 16F88 (4MHz).
I2C speed is 100kHz, SCL and SDA have 1k pull-ups

Attached is a typical occurence

[address] [T or D] [data bytes] is sent from the Master. What
happens is that the F88 recognises its address and Acks. It is then
supposed to detect whether the character after the address is "T"
(data following is for 3 time registers) or "D" (data following is for
another use). The address is always Acked, but the next byte is
sometimes ignored. It's as though the F88 has recognised its address,
and therefore Acked, but then decides it didn't recognise it after all

The double-blip on "marker" is at "header" is quite telling as to what
the F88 is doing after the address byte

As can be seen from the "marker" line in the top half ("Time not set"),
program flow appears to go back to the loop around w_adata. The
result of this is that when the program does get its drawers on and
picks up the data, it's meaningless because one byte has been lost. For
some reason the first of three time data bytes has been interpreted as
the "T"

In the lower half ("Time set"), the T character IS detected successfully
and the data does end up in the time registers. Same as if it was D -
data is stored correctly in the r_byte array and is usable. 

The "tick" is from an external 0.5Hz clock, and is timed to be well
away from any I2C or RS232 comms, so I'm confident that is not
the problem. The problem occurs at any stream length from 3 to 40
characters. The first byte after the T or D is missed, all others are
picked up. Before and after reception, whether successful or not,
SCL/SDA are high. I've looked at them with a high-speed analyser
and see no noise or ringing. I have had trouble with I2C on the F88
before and would not be surprised if this is silicon cooties again

Any ideas ?

TIA

;================================================
;        Wait for I2C
;================================================

w_adata  bank1
         btfsc   b_full       ;address byte before data
         goto    chk_addr     ;something received

         bank0
         bsf     marker
         btfsc   clock        ;update time if a tick happened
         call    update_time
         bcf     marker
         goto    w_adata      ;else loop

;-------------

chk_addr bank0
         bcf     led
         btfsc   pir1,sspif   ;test for address match
         goto    header       ;yes
         movfw   sspbuf       ;no
         movfw   sspbuf
         bcf     sspcon,sspov
         goto    w_adata

;look for T or D as first character

header   bsf     marker
         nop
         bcf     marker
         nop
         bsf     marker
         nop
         bcf     marker

         bcf     sspcon,sspov
         bcf     pir1,sspif
         movfw   sspbuf
         movfw   sspbuf
         bank1
         btfss   b_full       ;byte01 = D or T
         goto    $-1
         bank0
         bsf     marker
         bcf     sspcon,sspov
         bcf     pir1,sspif
         movfw   sspbuf
         movwf   temp         ;hold D or T
         bcf     marker

         bcf     t_d          ;default = data
         xorlw   "T"          ;test if "T", set time
         bnz     set_dev      ;not "T", is other data
         bsf     t_d          ;note as time set

set_dt   movlw   tcntl        ;store time in time registers
         movwf   fsr
         goto    data_pu

set_dev  movlw   r_byte       ;store data in buffer
         movwf   fsr

data_pu ...... pick up rest of data and rest of program

===============================================
If you aren't part of the solution, you're part of the precipitate


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