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
 }
}

• Gerben: You check if the PB4 bit is set in the PINB register, by bitwise and-ing it with all zeros except for the bit you are interrested in. So if( PINB & _BV(PINB4) ) or if( PINB & (1<<PINB4) ). PS by using setup and loop you are still using the Arduino library. PS2 you need to change DATAB to turn the led on, not DDRB
• kingcobra1986: I guess those are the only two we get to use. What do I initialize PINB and PINB4 to? As for the DataB, thats what I have, just tired.
• Gerben: PINB is (kind of) an output register. You can only read from it. It reports the HIGH and LOW voltages on the B-register's pins.
• kingcobra1986: I added to my Question does that look right? Is PINB included in the Arduino Library? If you make it an answer, you'll get the upvote btw.
• Gerben: yes, except you need to change the |= to =, or the bit will only be set, and never cleared.

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
 }
}

• kingcobra1986: Wait which |= were you telling me to change to =. Also I am good to use my portPinB in place of your PINB right?
• Gerben: You can use portPinB, but why would you?
• kingcobra1986: The professor's "Blinky" example used it. So I figured that I would need to use it too. I'd rather be safe. He stated that if we use anything from the arduino library we would get a 0 for the entire assignment. I would rather be safe then sorry and declare/initialize everything myself.
• Gerben: It's part of libc. You could just ask him. He'd be more of an asshole if he didn't allow this. I find it quite odd of him to force you to use the Arduino IDE, but not use any other part of Arduino; well; except for the loop and setup; oh; and the initialization that Arduino does.
• Majenko: @kingcobra1986 PORTB etc come with the compiler, and are nothing to do with the Arduino API. If your "professor" thinks they are anything to do with Arduino then he is a fool.

Answer 2

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

---

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.

---

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.

---

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

• Nick Gammon: I was responding to the sentence, not the code. If you set DDRB to 0x80 then it has that effect. I think the words "set a variable" are generally understood to mean "assign a value to a variable". That is, replace the old contents with the new ones.
• Nick Gammon: I clarified my answer to show exactly what I was responding to.