DrStein99 
I want to run 2 pins input and 4 pins output on port-D of my mega8 AVR chip. Normally, when the port is all output I just assign - like;
portd=0xFF; portd=0x001;
Now if the first two pins are input, and I assign a value to port-d, what effects will it have? Will it just ignore my code and report what the hardware inputs read? Will it tangle with interrupts if I am using them?
Please advise.
mthulasibabu hi!
u r going to use two pins are input pins and 4 pins r going to use as an output pins. ok
u must know about DDR register. This one explains Data Direction Register.
if in this register u will set perticular bit as 1, it will act as output pin or u will set as 0 i will act as input pin.ok. r u accept!
u r going to use first two pins r input pins such that pin 0&1
eg:
DDRD=0XFC;//0X1111 1100
i hope now u cleared about this input and output port declaration.
M.Thulasibabu India
Setting PORTx bits corresponding to input pins turns on/off the internal
pullups, so you probably want to leave them alone. More generally, you
should avoid altering bits you're not interested in; this notion will become
more relevant as your program gets larger and you start apportioning bits
to different sections of your program.
To set an output pin in PORTD, say PD2:
PORTD |= (1 << PD2); // Set pin PD2 high PORTD &= ~(1 << PD2); // Set pin PD2 low
which don't affect the other bits.
Will it tangle with interrupts if I am using them?
DrStein99 Ok, I understand setting put the port as input pins and output using ddrd. And I know how to change individual pins, one at a time.
What I need to do is modify FOUR pins at ONE time, and leave the two inputs alone. Assign a char value, because all four must be modified; and not all set on or off; like "0011" or change to "0110" at times, and also randomly "1001"
Assuming PD0/1 input, PD2/3/4/5 output, PD6/7 unknown.
#define OUTPUT_LOWBIT PD2
#define OUTPUT_ALLBIT ((1 << PD5)|(1 << PD4)|(1 << PD3)|(1 << PD2))
unsigned char outputvalue = 3; // Set PD5:2 to 0b0011, e.g.
PORTD = (PORTD & ~OUTPUT_ALLBIT) // Mask the output bits
| (outputvalue << OUTPUT_LOWBIT); // set the output PD5:2
mthulasibabu ok. u can set like below!
code:
PORTD=(0<<PD3)|(0<<PD2)|(1<<PD1)|(1<<PD0);//0011
PORTD=(0<<PD3)|(1<<PD2)|(1<<PD1)|(0<<PD0);//0110
PORTD=(1<<PD3)|(0<<PD2)|(0<<PD1)|(1<<PD0);//1001
ok. remaining pins are not affect.
u must know about bitfield (&,~,|,^).
i hope after u understand about this bitfields u can handle any type of program.
ok.
M.Thulasibabu India
awneil 
u must know about bitfield (&,~,|,^).
Look up the terms "bitwise" and "bitfield" in a 'C' textbook...