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RE: [avr-gcc-list] Question about gcc preprocessing and port/pin assignm

From: Larry Barello
Subject: RE: [avr-gcc-list] Question about gcc preprocessing and port/pin assignments
Date: Thu, 9 Dec 2004 06:52:34 -0800

Here is an example where an I/O pin is used for both an input and an output.


Although the sample code was written in BASIC, it would be simple to switch
to gcc code.

Question: when GCC optimizes multiple access to an object, is that limited
to the current scope?  Or can that cross curly-brace boundaries?

-----Original Message-----
From:  George Kalovyrnas

Ned Konz wrote:

>There is a useful distinction between registers whose contents are in fact
>nonvolatile (like for instance DDRB) and those whose contents can be
>by hardware events (like PORTB, PINB, and the ADC value registers).

>However, DDRB (say) does not have to be defined as volatile, since hardware
>isn't going to change it.
I think that the safe way is to declare ALL hardware registers as
volatile. You use the example of DDR registers. Consider what will
happen in this case:

DDRB = 0x01;
PORTB = 0x01;
Some code here
DDRB = 0x00;
temp = PINB;

I know that it is rare to use a single port pin as input and output, but
there are cases that this could happen (I know it happened to me). The
optimization would lead to disaster.

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