I/O Ports

• The ATmega32 has four I/O ports:
  • PORTA – This port is also hard-wired to the A/D converter subsystem.
  • PORTB – Typically used for output.
  • PORTC
  • PORTD – Pins 0 and 1 (PD0 and PD1) are hard-wired for serial communication on the SunRom boards.
• 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 are configured as input ports by default.
• 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

• We can configure all pins of PORTB for output by doing 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

• 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.

• 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

• 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