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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
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:
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).
I agree with DrFriedParts answer completely, however, I do want to add that, you can indeed read PORT to find out whether the port/pin is configured as OUTPUT high or low (when DDR = 1, port/pin configured as output) or whether the internal pullup resistor is activated (when DDR = 0, port/pin configured as input). In this way a port port can be used (in a rather silly way) as general storage or to pass information about the state of the output/input pullups, as in the following scenario
Perhaps one part of your program writes something to ports or activates/deactivates pullups, and the other part of the program reads PORT to find out how it is configured. Reading PORT immediately after writing PORT does not make sense: having written to PORT, you know what the value of PORT is.
For your purposes, reading from PORT actually makes sense: The pins in question are output pins, and you simply want to read the last value you wrote, toggle it, and write it back. So you’re using the port register as a byte of externally observable RAM.
On the other hand, for this particular app, you could simply just keep the value in a register and wouldn’t even have to read back the PORT register.
