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From: r.stolker at student.avans.nl<r.stolker@s...>
Date: Wed Dec 6 00:07:31 CET 2006
Subject: [oc] PPX16 core bug report
Hi all,

I was experimenting with the PPX16 core and found a bug. I've also
contacted the author of this core, Daniel Wallner.

The problem I&#8217;ve came across has to do with the Carry flag and the
SUBLW command. It seems that the carry flag is not set correctly, i&#8217;ll
explain:

The instruction description for SUBLW from the PIC16F84 datasheet
says; When the result is positive or zero the carry flag should be &#8216;1&#8217;.
When the result is negative the carry flag should be &#8216;0&#8217;.

The PPX16 core is doing the opposite; When the result is positive the
carry flag is &#8216;0&#8217;, when the result is negative or zero the carry flag is &#8216;1&#8217;.


Here&#8217;s a code snippet to give an example:

MOVLW 6
MOVWF main_w_L0
MOVF main_w_L0, 0
SUBLW 5
BTFSS STATUS, C ; Carry flag is set; not correct

MOVLW 5
MOVWF main_w_L0
MOVF main_w_L0, 0
SUBLW 5
BTFSS STATUS, C ; Carry flag is set; correct

MOVLW 4
MOVWF main_w_L0
MOVF main_w_L0, 0
SUBLW 5
BTFSS STATUS, C ; Carry flag is not set; not correct


I&#8217;ve tested this by outputting the current opcode on the seven segment
display and used a switch to manually create the clock for the core so I
could single step through the opcodes.


Workaround: Only use the SUBWF command which works correct.