I/O Ports

I/O Ports

The ATmega32 has four I/O ports:
- PORTA
- PORTB
- PORTC
- PORTD
Each port is 8 bits wide.
Each port has a data direction register associated with it (DDR? where ? is A – D).
The data direction register configures each bit of the corresponding port as either an input or output bit.
A zero bit in the DDR makes the corresponding bit on the port act as an input bit.
A one bit in the DDR makes the corresponding bit on the port act as an output bit.
We write to DDR? and PORT? using the OUT instruction.
Each port has two labels:
- PORT? – address where the output port is located in memory.
- PIN? – address where the input port is located in memory.
All ports default to be input ports.
To make all bits of a PORTB be output bits, we can do the following:

       ser  r16        ; Sets all the bits in r16 to 1 (r16 = 255)
       out  DDRB, r16

Output

We set up a port to be an output port like this:

       ldi  r16, 0xff
       out  DDRB, r16

We can send values to the port using the OUT instruction:

       out PORTB, r17   ; sends value of r17 to PORTB

Input

We set up a port to be an input port like this:

       ldi  r16, 0x00
       out  DDRC, r16

We can send values to the port using the IN instruction:

       in  PINC, r17   ; sends value of PINC to r17

When using the port as an input port we must use the PIN? label instead of the PORT? label.

Mixed I/O and Masking

Each bit of a port can be configured independently to be an input or output pin.
We can make the most significant four bits of PORTA to be input bits and the least significant four bits of PORTA to be output bits:

       ldi  r16, 0b00001111
       out  DDRA, r16

When we send output to PORTA we only really want to send four bits.
We can using masking operations to clear the most significant four bits.
Suppose R16 contains the value we wish to send out to PORTA

       andi r16, 0x0f  ; masking operation that clears 4 most sign. bits
       out  PORTA, r16

Enabling Pull-up Resistors for Input Ports

The ATmega32 has built-in pull-up resistors.
The pull-up resistors make the input pins high (instead of floating) if they are not hooked up to anything.
To enable the pull-up resistors for a port, do the following:

       clr   r16
       out   DDRA, r16  ; Configure PORTA as input on all bits
       ser   r16
       out   PORTA, r16 ; Enable pull-up resistors for all input bits on PORTA