Hello people

I have just had a great deal of fun and excitement in moving from P16F84 to
P16F84A devices. Thanks Microchip.

At first when the A version was programmed and plonked in my circuit it
appeared to work ok. But with some testing strange things happened. Writing
a simple test program and circuit will still show this problem.

1)      Firstly though I also have problems programming the A version. Using
a parallel port programmer and the PIX software. The configuration fuse does
not always get programmed. I have too use the 'program fuse' button
separately which works. Weird!

2)      Using an A version placed into a breadboard. Supply +5v. 2 x 10K sil
resistor packs to pull up Port B RB0 to RB3 and RA2 to RA4 and MCLR. The
other pins configured as outputs. RC oscillator mode, R=3K9 C=33pf. And the
code below programmed and run. Linking any of port B RB0 to RB3 repeatably
to 0v will cause RA4 to switch high/low.
        Next. Place 100nf miniature polyester Cap from any of RB0 to RB2 to
0v and repeatably short any of RB0 to RB3 to 0v and RA4 will switch.
        All fine so far.
        Now place the 100nf on RB3 to 0v and short RB3 to 0v repeatably.
Crash! The pic will lockup or all the ports will oscillate (??).
        If you monitor the RA4 output on a scope. You should see some
glitches when you switch RB3 with the cap attached.

I spent a couple of evenings trying to find what was happening. But I yet to
find out why. This weird effect happens with three different A versions but
is fine with the non A versions.

Would some kind soul try the same circuit out with the attached code, Both
with A and non A 16F84. It would be nice to know if it's a problem with all
A versions, or just mine, or just me. The code is in its simplest form ,no
labels defined  or macro's or stuff.


        LIST    p=16F84, r=hex

        __CONFIG        3FF3    ; RC mode, No Protect, No WDT,  PWRTE on

        BCF     3,5             ; Bank 0

        CLRF    05              ; Clear PORT A
        CLRF    06              ; Clear PORT B
        CLRF    0B              ; Clear INTCON register 0000,0000

        BSF     3,5             ; Bank 1
        MOVLW   88              ; No weak pull ups, Prescaler to WDT (Not
used)
        MOVWF   01              ; Set Options register
        MOVLW   0F              ; Set Tri-State settings        RA0 to RA3
as input, RA4 as output
        MOVWF   05              ; Tris  A
        MOVLW   0F              ; Tri-state for Port B          RB0 to RB3
as input, RB4 to RB7 as output
        MOVWF   06              ; Tris B
        BCF     3,5             ; Bank 0

here
        MOVF    6,W             ; Read port B
        ANDLW   0F              ; Keep RB0 to RB3
        XORLW   0F              ; Are all high? (Z=1 if yes)
        BTFSC   3,2             ; Skip if Zero clear
        BSF     5,4             ; Set RA4
        BTFSS   3,2             ; Skip if zero set
        BCF     5,4             ; Clear RA4
        GOTO    here            ; Loop

        END

Please reply both direct to me and the list, as I receive the list in digest
form so will not get your replies until tomorrow otherwise.

Many thanks in advance

Regards

Clive Frederickson
R&D Technician (CECF Group)


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