Many of you will recall a couple of recent threads involving my BIN2BCD
acro and the DAW instruction. These threads revolved around the details of
how the DAW instruction handled carrry.

I had assumed, when writing the macro, the DAW would 'do the right thing'.
This means that if carry was set before the DAW instruction it would not be
cleared by the DAW, but that if carry was clear, then DAW could set it in
appropriate circumstances.

Someone posted that their experience was that the DAW instruction would
clear carry. This meant that the very simple sequence my macro used:

    movf    VAL_A,W
    addwf  VAL_B,W
    daw
    movwf  SUM
    movf    VAL_A+1,W
    addwfc VAL_B+1,W
    daw
    movwf   SUM+1
    etc.

had to be complicated by code to save the carry before the DAW instructions,
and then 'or' it in to the carry out of the DAW.

I have recently encounted an errata document on the MChip site:
DS80152A - PIC18F4320 Rev. B2 "ES" Silicon Errata Sheet

This document seems to indicate that my interpretation of the operation of
the DAW was correct, but that at least some PICs have a bug in their
implementation.

Section 3 of that document reads as follows:

3. Module: DAW Instruction
The DAW instruction may improperly clear the CARRY bit (STATUS<0>) when
executed.

Work around

Test the CARRY bit state before executing the DAW instruction. If the CARRY
bit is set, increment the next higher byte to be added, using an instruction
such as INCFSZ (this instruction does not affect any STATUS flags, and will
not overflow a BCD nibble). After the DAW instruction has been executed,
process the CARRY bit normally (see Figure 2).

FIGURE 2: PROCESSING THE CARRY BIT DURING BCD ADDITIONS

MOVLW 0x80 ; .80 (BCD)
ADDLW 0x80 ; .80 (BCD)
BTFSC STATUS,C ; test C
INCFSZ byte2 ; inc next higher LSB
DAW
BTFSC STATUS,C ; test C
INCFSZ byte2 ; inc next higher LSB
This is repeated for each DAW instruction.


Interesting indeed!

Bob Ammerman
RAm Systems

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