One thing I think I see here is return instead of retfie
might be misunderstanding because of the many snip's

Peter

----- Original Message -----
From: "adastra" <foster@ADASTRAN.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, June 23, 2004 10:49 PM
Subject: [PIC]: Mysterious INTCON troubles with PIC16F877 (RP / better font)


> Wise Ones:
>
>
>
> I am a moderately experienced programmer, but I am completely puzzled by
> the behavior of this code fragment.
>
> I've been stuck on this all day. I'm obviously overlooking something.
> Can anybody help?
>
>
>
> This is inside an ISR and is intended to set a flag and clear a register
> after counting down a number of TMR0 overflows.
>
>
>
>   Inter
>
>             (snip)
>
>
>
>             decfsz   ir_cnt     ;counts down 8.9ms
>
>              return             ;return if not yet counted down to 0
>
>   irtst     nop                 ;test point for IR countdown
>
>             movlw   b'01110000' ;clear any latent external interrupt
> (from IR detector)
>
>             movwf   INTCON      ;  and re-enable external interrupt
>
>             nop
>
>             bsf        flags_5,1      ;set this flag denoting end of
> 8.9ms period
>
>             clrf        ir_dat
>
>             return
>
>
>
> ========================================================
>
>
>
> This is what happens, as seen by single-stepping in MPLAB IDE v5.61:
>
>
>
>   irtst     nop                 ;test point for IR countdown
>
>
>
> When the program reaches this point, watch window shows INTCON =
> b'01100110'
>
>
>
>             movlw   b'01110000' ;clear any latent external interrupt
> (from IR detector)
>
>             movwf   INTCON      ;  and re-enable external interrupt
>
>
>
> SURPRISINGLY after the movwf the watch-window shows INTCON = b'11110100'
>
>
> How can b2 (TOIF) still be set ?
>
>
>
>             nop
>
>
>
> SURPRISINGLY after executing this NOP, the watch-window shows INTCON =
> b'11110110'
>
> Now b1, (INTF) is ALSO set, even if I (temporarily) physically strap the
> RB0/INT pin HIGH
>
> in order to be sure that no external interrupt can occur !
>
>
>
>             clrf        ir_dat
>
>             return
>
>
>
> I just don't understand how this can be happening.
>
>
>
> ========================================================
>
>
>
> Here are my INTCON and OPTION_REG setups:
>
>
>
> INTCON setup
>
>         movlw b'11110000'         ; GIE=on TOIE=on (peripheral AND
> timer0 overflow
>
>         movwf INTCON              ;    AND ext-int set up)
>
>
>
>              ;  x-------          ; 7, GIE: Global Internet Enable (1 =
> ints enabled)
>
>              ;  -x------          ; 6, PEIE: Peripheral Interupt Enable
> (1 = p. ints enabled)
>
>              ;  --x-----          ; 5, TOIE: TMR0 Overflow Interrupt
> Enable (1 enables interrupt)
>
>              ;  ---x----          ; 4, INTE: RB0/INT Ext. int (1 enables
> ext ints)
>
>              ;  ----x---          ; 3, RBIE: RB Change Interrupt Enable
> (1 enables port change int)
>
>              ;  -----x--          ; 2, TOIF: TMR0 Overflow Interrupt
> Flag (1 = overflowed)
>
>              ;  ------x-          ; 1, INTF: RB0/INT External Interrupt
> Flag (1=external int.)
>
>              ;  -------x          ; 0, RBIF: RB Port Change Interrupt
> Flag (at least one of RB7:RB4
>
>                                   ;               has changed if this
> flag is set
>
>
>
> OPTION_REG setup
>
>         movlw        b'00001000'
>
>         movwf OPTION_REG          ; load data into OPTION_REG
>
>            ;
>
>                 ;  x-------       ; 7, 0=enable, 1=disable, portb
> pullups
>
>                 ;  -x------       ; 6, 1=/, int edge select bit
> 0=falling
>
>                 ;  --x-----       ; 5, timer0 source, 0=internal clock,
> 1=ext pin.
>
>                 ;  ---x----       ; 4, timer0 ext edge, 1=\
>
>                 ;  ----x---       ; 3, prescaler assign, 1=wdt, 0=timer0
>
>                 ;  -----x--       ; 2,1,0, timer0 prescaler rate select
>
>                 ;  ------x-       ;   000=2, 001=4, 010=8, 011=16, etc.
>
>                 ;  -------x       ;
>
>                                   ; Note! We set the prescaler to the
> wdt, so timer0 overflows
>
>                                                 ; every 256 instructions
> to make an interrupt every 102.4 uS
>
>                                                 ; (for 10 MHz clock)
>
>
>
> TRISB is set for all outputs, except for RBO/INT which is used as an
> external interrupt
>
>
>
> ===========================================================
>
>
>
> Obviously I have to be able to clear the TOIF and INTF flags, but I
> can't seem to do it.
>
> I have tried different '877's on the off chance that the chip was bad,
> but no joy.
>
> Power Supply voltage looks good, with appropriate 0.1uF bypass at chip
> pins
>
> Interrupt context-saving is implemented
>
>
>
> I would be so thankful if someone can tell me what I am missing here.
>
>
>
> Thanks,
>
>
>
>   Foster
>
>
>
>
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