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can not see input

Maybe someone can help me out...????

I am having the following problem....
I am using in- and outputs on P1 of a 87C51R+ micro.
As far as I can tell I have correctly set the inputs of the port by sending a "1" to them.
Even though it all seems to function in dscope, it doesn't on the micro board.
example:

sfr trigger = P1^0;

If not mistaken if I have code reading the bit, shouldn't it react in some way..???
eg:

if(trigger == 1){
    ........
}

this problem doesn't allways exist, only on some projects. Just seem to be able to find out what I am doing wrong. Any help appreciated.

0 Jon Ward over 22 years ago
By default, all lines of P1 are set as inputs. The following code:

if(trigger == 1){ ........ }

checks for all bits except bit 0 to be set to 0. If you are really only interested in bit 0, you should probably use the following:

if(trigger & 0x01){ ........ }

Jon
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0 Richard Collett over 22 years ago
Hi John,

change

sfr trigger = P1^0;
to

sbit trigger = P1^0;
and you should get your expected behaviour.
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0 Richard Collett over 22 years ago in reply to Richard Collett
p.s.
The "^0" syntax only means "bit zero" in an sbit declaration. Anywhere else, it means "XOR 0", so:

sfr trigger = P1^0;
means exactly the same as

sfr trigger = P1;
which is why the compiler did not flag it as an error.
As Jon pointed out, if the other bits of PORT1 are low, your test will work, but if any of them are high, it will fail, because you are testing the value read from the entire port.
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