How can I detect HIGH or LOW input on a NXP microcontroller?

Question

I am using NXP micro-controller P89V51RD2. I need to detect HIGH or LOW from 2 single bits. How do I store input state to a pre-defined address so that my functions can detect it as data?

I need to use 2 transistors coupled emitter to base to detect change in resistance(indirectly by measuring current and voltage) in a load cell. The coupling is an attempt for a rudimentary type of switching. This I need to give to a bit in e.g. port P1 for detection. I have trouble getting the right code ( H/L bit identification) for this AND connection configuration (buffer or another signal conditioner needed?) for this.

The code compiling is done in Keil μVision v4.

The official website for Keil says it can be done by code : sbit b1 = P1^0. However, I don't know how to proceed afterwards as this code doesn't give required results.

Answer 1

I looked at the guide you linked to, and your problem is that you have confused a declaration with a program statement. When you write sbit b1 = P1^0;, you have told the compiler how to get the bit you want, but you haven't created any code yet. The way you would use it, then, is to treat this declared "b1" as if it were a variable representing the port bit you wanted. So,

if(b1) {
  led_on();
}
else {
  led_off();
}

When the compiler encounters b1, it will go read the port and extract the bit, based on the earlier declaration.

That particular way of declaring port bits is particular to that compiler, so it confuses anyone who hasn't seen it before (me included).

Answer 2

Your code is not correct. b1 = P1^0 means P1 XOR 0. Anything XOR 0 gives the original value.

To assign bit 0 to b1, you need to write b1 = P1 & 1, or preferably:

b1 = P1 & (1<<0)

The latter form is universal, to get bit 1 you'd write 1<<1. For bit 2, you'd write 1<<2, and so on.