I would appreciate any views on the following question, since I am about
to freeze my code!  I have e-mailed Microchip directly but have not yet
received a response.  Perhaps someone here has practical experience?

In a nutshell, my question is whether it is possible for an interrupt to
occur during a "BCF INTCON,GIE" instruction on any of the 16F87X family.

I started working with the PIC16C84 back in 1993, at which time section
3.12 (Interrupts) of the data sheet stated in Note 2: "If an interrupt
occurs while the Global Interrupt Enable (GIE) bit is being cleared, the
GIE bit may unintentionally be re-enabled by the user's Interrupt
Service Routine (the RETFIE instruction) ..."

As a result, I have got into the habit on all PICs of clearing GIE using
a sequence such as:

        LOOP    BCF     INTCON,GIE
                BTFSC   INTCON,GIE
                 GOTO   LOOP

However, whilst converting some code to run in a PIC16F627, I discovered
the following new Note 2 in section 14.6 of the PIC16F62X data book:
"When an instruction that clears the GIE bit is executed, any interrupts
that were pending for execution in the next cycle are ignored ..."

I assume that this provision prevents the problem which the above loop
was intended to cure, and that in the 16F62X it is adequate to use just:

                BCF     INTCON,GIE

Furthermore, I have found a note in Table B-1 of the PIC16F84A data
sheet which suggests that the above problem also does not apply to the
PIC16F83/84/84A or the PIC16CR83/84.

So far, so good...

However, I am also in the process of adapting some PIC16C84 code to run
in a PIC16F870, and I have not been able to find any indication in the
latter data book as to whether this problem exists or has been cured
(section 11.10 "Interrupts" does not contain a corresponding Note 2).

I have been unable to recreate this problem scenario on the PIC16F870
during a brief simulation using MPLAB, and so I imagine that it probably
does not apply.  However, I do not wish to remove the safeguard of the
extra loop unless I am 100% sure of this, as I do not wish to run the
risk of intermittent failures due to interrupts occasionally remaining
active during critical code sections in which they should be disabled.

Thanks in advance for any firm pointers.
--
Ian Chapman
Chapmip Technology, UK

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