3.2.3 ``Registers'' for the ports

For port A, there are three ``variables'' defined. Bit $n$ of each of these variables corresponds to a pin $n$ of port A.

• DDRA (Data Direction Register A): this ``register'' (not to be confused with registers in a processor) controls the directions of the 8 pins of port A. A bit value of 0 means the corresponding pin is configured as input, whereas a bit value of 1 means the corresponding pin is configured as output.

  DDRA is a read/write variable.

• PINA (Port INput A): this is a read-only variable (writing to it doesn't do anything) that reports the sensed state of the pin. A bit value of 0 means the pin is sensing a low voltage, whereas a bit value of 1 means the pin is sensing a high voltage.

• PORTA (data register of PORT A): this register is a read-write register, and its bits has different meanings depending on whether the corresponding pin is configured for input or output.

  For an output pin, a bit value of 0 means the pin drives low (tries to lower the voltage), whereas a bit value of 1 means the pin drives high (tries to raise the voltage).

  For an input pin, a bit value of 0 means the pin has no ``pull-up''. A bit value of 1 means the pin is ``pulled up'' by a large resistor. We'll discuss the concept of ``pull up'' later.