microcontroller - Difference between reading PORT and PIN

What is the difference between reading PORTA and PINA? What is the relation between a port and its pins?I'm really confused!!! sample of reading PORT:(reading pin is not useful here)

Assembly f=1MHZ ATmega8 I need to complement PORTB every 1ms.(Dutycycle=50%)

    LDI R16,$FF

    OUT DDRB,R16
    MAIN: LDI R16,00
    DELAY:CPI R16,200
    BREQ LOOP
    INC R16
    RJMP DELAY
    loop:IN R17,PORTB
    COM R17
    OUT PORTB,R17
    OUT R17,PORTB

    RJMP MAIN

Answer

PORT is the OUTPUT buffer, PIN is the INPUT buffer.

When you want to set the pin to a "high" or "low" voltage, write to the PORT register.

When you want to know what voltage is currently presented to a pin, read the PIN register.

The bits of these registers represent the corresponding pins of the general-purpose input/output port.

Here is a simplified schematic of the electronics inside the AVR connected to a single pin (go here for complete datasheets).

This circuit block is repeated for each pin. Eight of these form a port (port A, for example).

Starting at the left-most square (which represents the physical connection to the outside world), you can see three paths:

1. The upper-most path is the software selectable pull-up resistor


2. The middle path is used when the pin is configured as an output


3. The lowest path is used when the pin is configured as an input

It should be noted that some or all of this circuitry can be bypassed when the pin is shared with an internal peripheral. For example, the Analog-to-Digital Converter (ADC).