04 - AVR Input Output Port Programming

Updated 4 Oct 2026

I/O Pins and I/O Port Registers

I/O Pins of ATmega328

The ATmega328 has 28 pins that connect to external devices. These pins function as:

Input pins → Read values from external devices.
Output pins → Write values to external devices.

I/O Pin Groups:

Port B (PB0 – PB7) → 8 pins
Port C (PC0 – PC5) → 6 pins
Port D (PD0 – PD7) → 8 pins
The remaining 6 pins: VCC, GND, RESET, AREF, AGND, AVCC.
Pins must be programmed to function as input or output.

I/O Pins of Arduino UNO Board

I/O Port Registers

Each port (B, C, D) has 3 key registers to control pin behavior:

DDRx → Sets the direction (input/output).
PORTx → Sends data to output pins.
PINx → Reads data from input pins.

Register Functions:

PINx Register: Stores values received from input pins.
PORTx Register: Stores values sent to output pins.
DDRx Register: Determines whether pins act as input (0) or output (1).

Setting Input/Output:

Output Mode (1 in DDRx) → Value in PORTx is sent to the pin.
Input Mode (0 in DDRx) → Value from pin is stored in PINx register.

Output Port Programming

To allow the AVR to write a value to a pin (i.e., output pin) via Px0 – Px7, there are two steps:

Declare an output pin by setting the bit in the ==DDRx register corresponding to this pin to 1. ==
Write the value to this pin by setting the bit in the PORTx register corresponding to this pin.

Example

Set the pin PB2 and PB4 as output pins. Set the output of PB2 to 0 and the output of PB4 to 1.
```
LDI R16, 0x14
OUT DDRB, R16
LDI R17, 0x10
OUT PORTB, R17
```
Set all pins in Port D as output pins. Set PD0-PD2 to 0 and PD3-PD7 to 1.
```
LDI R16, 0xFF
OUT DDRD, R16
LDI R17, 0xF8
OUT PORTD, R17
```

Input Port Programming

To allow the AVR to read a value from a pin (i.e., input pin) via Px0 – Px7, follow these three steps:

Declare an input pin by setting the bit in the ==DDRx register corresponding to this pin to 0.==
Set the bit in the PORTx register corresponding to this pin to 1.
Read the value from this pin by reading the value from the bit in the PINx register corresponding to this pin.

Example: Set up Input Pins

Set all pins in Port B as input pins. Read the values of the pins in Port B and store them in R18.
```
LDI R16, 0x00
OUT DDRB, R16
LDI R17, 0xFF
OUT PORTB, R17
IN R18, PINB
```
LDI R16, 0x00, OUT DDRB, R16 → Set all pins as input.
LDI R17, 0xFF, OUT PORTB, R17 → Initialize Port B to 1.

I/O Bit Manipulation Instructions

I/O Bit Access Instructions

ATmega328 ports allow access to either all 8 bits or a single bit. The following instructions enable single-bit manipulation in any of the 64 I/O registers (not just port registers).

1. SBI (Set Bit in I/O Register)

Sets a specific bit in an I/O register to HIGH (1).

Syntax: SBI ioReg, bit_num
Parameters:
- ioReg: I/O register name or address ( $00 -$ 3F$).
- bit_num: Bit position to set to 1.
Example: Sets bit 5 in PORTB high.
```
SBI PORTB, 5
```

SBI PORTB, 5 means you're specifically turning pin PB5 HIGH. ไม่ได้หมายความว่าตั้งตัวแปรอะไรให้มีค่าเป็น 5 นะ ทีแรกก็งง ๆ 5555 ==The bit number represents the pin index==

2. CBI (Clear Bit in I/O Register)

Clears a specific bit in an I/O register (sets it to LOW, 0).

Syntax: CBI ioReg, bit_num
Parameters:
- ioReg: I/O register name or address ( $00 -$ 3F$).
- bit_num: Bit position to clear to 0.
Example: Clears bit 2 in PORTB.
```
CBI PORTB, 2
```

3. SBIS (Skip if Bit in I/O Register is Set)

Skips the next instruction if the specified bit in the I/O register is HIGH (1).

Syntax: SBIS a, b
Parameters:
- a: I/O register name or address ( $00 -$ 3F$).
- b: Bit position to check.

Example: If bit 2 in PINB is HIGH, it skips RJMP AGAIN.

AGAIN:  SBIS PINB, 2 ; Skip if PB2 is HIGH
        RJMP AGAIN  ; Keep checking if LOW

4. SBIC (Skip if Bit in I/O Register is Cleared)

Skips the next instruction if the specified bit in the I/O register is LOW (0).

Syntax: SBIC a, b
Parameters:
- a: I/O register name or address ( $00 -$ 3F$).
- b: Bit position to check.

Example: If bit 3 in PINB is LOW, it skips RJMP HERE.

HERE:   SBIC PINB, 3 ; Monitor PB3 for HIGH
        RJMP HERE    ; Stay in the loop

SBIS does not check for a specific number in the whole register. It only checks whether a single bit is 1 or 0.

Example 1

We connect a switch, an LED, and Rs to the Arduino UNO board according to the diagram below. We write and upload the following assembly program.

What will happen to the LED if we press the switch?
What will happen to the LED if we do not press the switch?

Default แล้ว แต่ละรูจะมีไฟ 5 V หมายถึง 1 (Digital) เวลา Set ให้เป็น 0 มันก็จะต่อเข้า ground

Example 2

Assume that we connect a switch (similar to the previous example) to the pin PB2 of the Arduino UNO board. We write and upload the following assembly program.

What will happen if we press the switch?
What will happen if we do not press the switch?

    CBI     DDRB, 2       ; Clear bit 2 of DDRB (set PB2 as input)
    SBI     PORTB, 2      ; Set bit 2 of PORTB (enable pull-up resistor on PB2) (This line can be neglected)
    LDI     R16, 0xFF     ; Load immediate value 0xFF into register R16
    OUT     DDRD, R16     ; Set all bits of DDRD to 1 (configure PORTD as output)
 
AGAIN:  
    SBIS    PINB, 2       ; Skip next instruction if bit 2 of PINB is set (button not pressed)
    RJMP    OVER          ; If button is pressed (bit 2 is 0), jump to OVER
 
    LDI     R16, 'Y'      ; Load ASCII value of 'Y' into R16
    OUT     PORTD, R16    ; Output 'Y' to PORTD
    RJMP    AGAIN         ; Repeat the loop
 
OVER:  
    LDI     R16, 'N'      ; Load ASCII value of 'N' into R16
    OUT     PORTD, R16    ; Output 'N' to PORTD
    RJMP    AGAIN         ; Repeat the loop