At 03:35 PM 7/8/97 +0100, you wrote:
>HI guys...
>
>I'm working on a project with a 16C554, using the RB0 external interrupt to

where did you get on hold of 16C554 with Interrupts?

>receive serial PWM data, and the RB4..7 interrupt-on-change to do mains
>zero-crossing detect.
>
>It's been mentioned that this RB4..7 interrupt is pretty useless because
>any read of PORTB simultaneous with a level-change reads the new value into
>the latches and doesn't generate an interrupt.

we have made projects using 'on change interrupt' and all work fine,
still we wouldnt not use it in critical places where no fron can be missed.

>I  explicity read/write this port only to clear the interrupt. Are there
>other implicit PORTB-reads that will make me drop an interrupt? The only

there was some discussion about it, it looks like all commands that
have the PORTB adress inside the command word do a portb read (even
if command shouldnt) some RETLW could access portb and something like.

It maybe that this feature of 'ghost accesses' to register file has
been fixed by Microchip in some newer chips. PIC16C84 I think still
has some commands that do a "ghost read".

>other thing I do with PORTB is toggle the INTEDG bit of the OPTION register
>for the PWM detect.
>
>Any comments appreciated
>
>
>Alec
>
>
>ps. I note that the Atmel AT90S8515 RISC processor has "120 powerful
>instructions".
>In what novel manner can this be described as a Reduced Instruction Set?
>Reduced from what?

Same way as PICmicro is RISC.

its the way of counting. and marketing. If RISC is hot, we call it RISC.
ST62 has real few commands but nobody calls it RISC.

AVR "120" instructions is "high count" it is possible to count much
less instructions than 120. just the way of counting.

in 120 are counted 20 bit_branches on conditon,
I would count then as one, and so on.

if you count SKNC SKPC and... for PIC then the amount of
PICmicro Instructions will also be much higher that what it
is written in databooks

antti












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