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Re: [avr-gcc-list] AVR GCC compiler for win98

From: Nils Kristian Strom
Subject: Re: [avr-gcc-list] AVR GCC compiler for win98
Date: 18 Apr 2002 23:23:38 -0400

On Thu, 2002-04-18 at 16:19, Rob Ward wrote:
>Hi. I am using GCC under w2k. I tried using this code from avrfreaks...
>#define sw4_pin (1<<4) //this could be (1<<PD4)
>#define sw4_port PORTD
>#define sw4_dir DDRD
>#define sw4_in PIND
>Then the following to toggle the pin / i/o direction...
>sw4_dir &= ~sw4_pin;//set port as input
>sw4_port |= sw4_pin; //switch on pull up
>however, I get "Invalid lvalue in assignment" for both the last two
>lines when compiling. Could anyone tell my why this is?

If you look deeper into the header files, which declare PORTD and DDRD,
you will se that they look like this:
#define PORTD      0x12
#define PIND      0x10

So what you are writing, is in fact
0x12 |= 0x10;

So the compiler complainst Invalid lvalue (lvalue = left side value).
The compiler error makes prefect sense ;-)

The reason for this, is that IO registers (PORTB etc) are different from
normal registers (r0 through r31).  You need to use in/out instructions
to move the contents of the IO registers to normal registers, then do
your arithmetic on these.  Most people use the macros inp(), outp(),
sbi() and cbi() for this.

You must use these macros.

outp( sw4_pin, sw4_dir ); /* output */
foo = ~(inp( sw4_in ) );  /* read and invert */
outp( foo, sw4_port );    /* output */


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