The PINx Register

• When a pin is set to input, the PINx register contains the value applied to the pin. The pins have an electrical threshold of around 2.5 volts. If a voltage above this level is applied to an input pin, the corresponding bit of the PINx register will be a 1. Below this voltage, the bit will be a zero. See the ATMega16 schematic for the specific threshold voltages. To read the value of an input port, you can use the inb(reg) function. It returns an 8-bit number that is the value of the 8 bits in the specified register.

  u08 foo;  // declare an 8-bit variable
  
  outb(DDRD, 0x00);   // set port D pins to input
  outb(PORTD, 0xFF);  // set pull-ups on port D
  
  foo = inb(PIND);    // read the value of the port D pins
                      // and store it in the variable foo
  

• There is also a function available for checking the state of an individual bit, without reading the entire PINx register. The function bit_is_set(reg,bit) returns a 1 if the given bit in the given register is set, and a 0 if the bit is cleared.

  u08 bar;  // declare an 8-bit variable
  
  outb(DDRD, 0x00);    // set port D pins to input
  outb(PORTD, 0xFF);   // set pull-ups on port D
  
  bar = bit_is_set(PIND,1);
                       // bar now contains the logic value at Port D pin 2