Can I use the same Data Registry for both input and output pins?

Question

This is for a class in which we are not allowed to use the Arduino Library (Not my choice or preference). I know to use DDRB for the B pins. This is the code that I have so far:

//Global pointers
unsigned char *portDDRB = (unsigned char *) 0x24;
unsigned char *portDataB = (unsigned char *) 0x25;
unsigned char *portPinB = (unsigned char *) 0x23;

void setup () {
  //For the led output I know that I would do this
  *portDDRB |= 0x80;
}

void loop () {
   //And here the code would be like this to light up the light:
   if (condition to turn on light) {
     *portDataB |= 0x80;
   }
   // To Turn off the LED:
   if (condition to turn off the light) {
     *portDataB &= 0x7F
   }
}

I would like for the light to turn on when the input of PB4 is HIGH (from a button press). I just am not sure on what to put in the condition statements or the setup functions.

When I set the DDRB to 0x80, is that just making the PB7 an output and the rest as input?

I am using the Arduino Mega ATmega2560 btw.

From the comments I have this:

//Global pointers
unsigned char *portDDRB = (unsigned char *) 0x24;
unsigned char *portDataB = (unsigned char *) 0x25;
unsigned char *portPinB = (unsigned char *) 0x23;

void setup () {
  //For the led output I know that I would do this
  *portDDRB |= 0x80;
}

void loop () {
   //And here the code would be like this to light up the light:
   if (portPinB & 0x10) {
     *portDataB |= 0x80;
   }
   // To Turn off the LED:
   else {
     *portDataB &= 0x7F
   }
}

Answer 1

Assuming the button is connected between GND and pin 10, without any pull-ups or pull-downs.

void setup () {
    //For the led output I know that I would do this
    DDRB = 0;             // set all pins on port B to INPUT
    DDRB |= _BV(DDB7);   // set PB7 to OUTPUT
    PORTB = 0;            // set all pins on port B to LOW
    PORTB |= _BV(PORTB4); // enable the pull-up resistor on PB4
}

void loop () {
    if( PINB & _BV(PINB4) )
    {
        // if bit PINB4 is set, the button is not pressed
        PORTB &= ~_BV(PORTB7); // turn off led
    }
    else
    {
        PORTB |= _BV(PORTB7); // turn off led
    }
}

Answer 2

When I set the DDRB to 0x80, is that just making the PB7 an output and the rest as input?

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Clarification

I take that question to mean "When I assign the value to 0x80 to DDRB". For example:

DDRB = 0x80;

In the code above that question, you were not assigning, you were "or"ing the value.

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Yes. Typically you would "or" in the value if you want to affect only one pin, eg.

DDRB |= 0x80;

The conventional way to make it an input again would be:

DDRB &= ~0x80;

That is, "and" in the negated bit pattern for that pin.

Remember, you have 3 registers (per port) where "x" is "A", "B", "C" etc.:

• DDRx - data direction - a 1-bit is output, a 0-bit is input

• PORTx - output value - a 1-bit is HIGH, a 0-bit is LOW

  However, if the data direction is input, then a 1-bit is input-pullup, and a 0-bit is not input-pullup.

• PINx - read the input from the port - a 1-bit is HIGH, a 0-bit is LOW

  Note that writing a 1-bit to PINx toggles the output value of that port.

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These remarks apply to the AVR chips, not the SAM ones.