Ruben J=F6nsson wrote:
>> Ruben J=F6nsson wrote:
>>>>>> MOVF PORTB,W
>>>>>> MOVWF SHADOWB
>>>>>> MOVLW B'100'
>>>>>> IORWF SHADOWB,W
>>>>>> MOVWF PORTB
>>> Yes, there still could be problems if the interrupt is just after IORWF =

>>> SHADOWB,W. Then changes done in the interrupt routine will be undone wh=
en
>> the =

>>> non interrupt code does MOVWF PORTB. The key is, appart from using a sh=
adow
>>> register, is to never read back from the port to the shadow register.
>> Hmmm you are right. But I don't understand your solution?
>>
>>     BSF shadow, 0
>>     MOVFW shadow
>>     =3D=3D=3D> interrupt here
>>     MOWF port
>>
>> This sequence does not read the port, but changes made by the interrupt =

>> would still be lost.
>>
>> -- =

> =

> Yes, you are right. I only thought about manipulating the shadow register =

> itself. Not actually copying it out to the port. The copying will require=
 that =

> interrupts are disabled if the copying can be done from both non isr and =
isr =

> code.
> =

> It will work if you don't need to copy, that is if you don't need a shado=
w =

> register. But then you have to be aware of the rmw issue. Unless you have=
 a =

> latch register in the hardware, that is.
> =

> However, the initial point was that even with a latch register (or shadow =

> register) you have to be aware of interrupts if some bits in the register=
 can =

> be manipulated by the isr code while other bits are manipulated by non is=
r code =

> at the same time.
> =

> /Ruben  =

>

A nice way to solve this (and if the I/O pins do not have to
e switched fast, such as e.g. indicator LEDs), is to only
BSF/BCF the "shadow" bits in the main code, and only copy
to the port in the ISR. That way you can manipulate the
indicator-LED bits wherever there is a need, and have a
safe update of the PORTx register in a single place in the
ISR.

Jan-Erik.


> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D
> Ruben J=F6nsson
> AB Liros Electronic
> Box 9124, 200 39 Malm=F6, Sweden
> TEL INT +46 40142078
> FAX INT +46 40947388
> ruben@pp.sbbs.se
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D
-- =

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