I have experienced a problem with the prescaler on a 16C84-04. It seems
to work as expected except when the ratio is set to 1:2. In this case I
get an actual ratio of 1:4.
I have assigned the prescaler to TMR0 in the OPTION register as per the
recommended code sequence in the data book (since the power up
assignment is to the watchdog). The code I used is included below - it
just sets up the timer and then outputs a pulse on bit 0 of port B on
every interrupt (from TMR0).

With a 2MHz Xtal clock I get pulses spacings as follows:

prescaler setting       interrupt rate (pulse spacing)
-----------------       ------------------------------
000                     2msecs (should be 1msec)
001                     2msecs
010                     4msecs
011                     8msecs
100                     16msecs

without the prescaler there should be a TMR0 interrupt every 0.5 msec
(i.e. 256 times instruction period of 500nsec)
The watchdog is disabled in the config word and the osc is set to XS.


Main program - Initialisation and wait loop
**************************************************************
MAIN    CLRWDT
        BSF     STATUS,RP0
        MOVLW   B'11000100'     ;Prescaler set to 100
        MOVWF   OPTREG^80H      ;SET TMR0 increment on clock
        BCF     STATUS,RP0      ;SELECT REGISTER BANK 0

        CLRF    PORTB
        BSF     STATUS,RP0      ;SELECT REGISTER BANK 1
        CLRF    TRISB^80H       ;SET PORTB TO ALL OUTPUTS
        BCF     STATUS,RP0      ;SELECT REGISTER BANK 0
        MOVLW   B'10100000'
        MOVWF   INTCON          ;SET GIE and T0IE

;wait for TMR0 interrupts
LOOP    NOP
        GOTO    LOOP
***********************************************************

Interrupt routine
***********************************************************
INT_RTN BSF     PORTB, BIT      ;SET SELECTED BIT TO 1
        BCF     INTCON, T0IE    ;clear timer flags
        BCF     INTCON, T0IF    ;
        BSF     INTCON, T0IE    ;
        NOP     ;just some time padding
        NOP     ;
        NOP     ;
        NOP     ;
        NOP     ;
        NOP     ;
SETLO   BCF     PORTB, BIT      ;SET SELECTED BIT TO 1
        RETFI
****************************************************************

I also see a similar effect if I try to get a ratio of 1:1 by assigning
the prescaler to the Watchdog Timer as per the data book using the
options reg.

I would wlecome any comments from anybody as I feel I must be doing
something wrong somewhere - but I cannot see what or where.

I hope my asking this type of question falls within the acceptable use
of this mailing list - I am new to the list and to PICS.
--
regards
martyn fletcher

martyn@fletcher.demon.co.uk