Accessing I/O in ATmega328P (Arduino) using Assembly and C

Prerequisites:

This is a continuation of the following tutorials:

ATmega328P and its Architecture

The Pinout (with ports):

Zoom-In to see the Ports and the bit numbers.

Note: This project is a continuation of this content. Please take a look at it before starting this project.

Three ports, Port B, Port C, and Port D can be used as general I/O pins. Port B and Port D are used as Digital I/O while Port C is used for Analog I/O. However, Port C can also be used as Digital I/O in case you need more I/O,

Now the digital I/O can be made high or low by writing the respective bits into the port registers.

There are 3 states of an I/O pin, HIGH, LOW, and HIGH IMPEDANCE.

HIGH refers to logic 1 and LOW refers to logic 0. While HIGH IMPEDANCE refers to the "NC (No Connection)" state. It's always recommended not to leave any pins to be in a floating / high impedance state as it may sometimes lead to failure of design.

Thus the other pins can be pulled down.

Wait, what's pull-down?

The ATmega328P is equipped with pull-up and pull-down resistors. This means that, if they are not in use or if they're connected to a switch they can be tied to VCC or Ground to avoid the floating state.

The diagram below depicts the arrangement of how the pull-up or pull-down resistor can be used while connecting a switch.

The assembly language:

In the assembly language, certain keywords or mnemonics are used to specify the instructions that the processor needs to execute.

Few Basic Instructions of the Assembly language are listed below with the Syntax for a better understanding of the usage.

LDI – Load Immediate

Loads an 8 bit constant directly to register 16 to 31.

LDS – Load Direct from Data Space

Loads one byte from the data space to a register. For parts with SRAM, the data space consists of the Register File, I/O memory, and internal SRAM (and external SRAM if applicable). For parts without SRAM, the data space consists of the register file only. The EEPROM has a separate address space.

STS – Store Direct to Data Space

Stores one byte from a Register to the data space. For parts with SRAM, the data space consists of the Register File, I/O memory, and internal SRAM (and external SRAM if applicable). For parts without SRAM, the data space consists of the Register File only. The EEPROM has a separate address space.

IN - Load an I/O Location to Register

Loads data from the I/O Space (Ports, Timers, Configuration Registers, etc.) into
register Rd in the Register File.

OUT – Store Register to I/O Location

Stores data from register Rr in the Register File to I/O Space
(Ports, Timers, Configuration Registers, etc.).

CBI – Clear Bit in I/O Register

Clears a specified bit in an I/O Register. This instruction operates on the lower 32
I/O Registers – addresses 0-31.

CBR – Clear Bits in Register

Clears the specified bits in register Rd. Performs the logical AND between the
contents of register Rd and the complement of the constant mask K.The result will be placed in register Rd.

SBI – Set Bit in I/O Register

Sets a specified bit in an I/O Register. This instruction operates on the lower 32 I/O Registers – addresses 0-31.

SBR – Set Bits in Register

Sets specified bits in register Rd. Performs the logical ORI between the contents of register Rd and a constant mask K and places the result in the destination register Rd.

SBIC – Skip if Bit in I/O Register is Cleared

This instruction tests a single bit in an I/O Register and skips the next instruction
if the bit is cleared. This instruction operates on the lower 32
I/O Registers – addresses 0-31.

SBRC – Skip if Bit in Register is Cleared

This instruction tests a single bit in a register and skips the next instruction if
the bit is cleared.

SBIS – Skip if Bit in I/O Register is Set

This instruction tests a single bit in an I/O Register and skips the next instruction
if the bit is set. This instruction operates on the lower 32
I/O Registers – addresses 0-31.

SBRS – Skip if Bit in Register is Set

This instruction tests a single bit in a register and skips the next instruction if the bit is set.

RJMP – Relative Jump

Relative jump to an address within PC - 2K +1 and PC + 2K (words). For AVR microcontrollers with Program memory not exceeding 4K words (8K bytes), this instruction can address the entire memory from every address location.

RET – Return from Subroutine

Returns from the subroutine. The return address is loaded from the STACK. The Stack Pointer uses a pre-increment scheme during RET.

; - A semicolon is used to comment in the assembly language however Arduino IDE supports "//" too.

So that's sufficient for now, we will explore more in the upcoming projects.

So let's get started.

Assembly Programming:

Problem statement: "There are two LEDs, one connected to PB2 and the other to PB3. A push-button is connected to PB4. The task is to write an assembly code such that when the push button is not pressed, the LED connected to PB2 glows and the LED connected to PB3 does not glow. When the push button is pressed, the LED connected to PB3 glows and the LED connected to PB2 does not glow."

I used a common cathode RGB LED for ease. The pinout is as follows:

First of all, we need to assign the direction of the ports, i.e, whether they are going to be used as INPUT or OUTPUT. So, PB2 and PB3 have to be configured as O/P and PB4 has to be configured as I/P.

The Data Direction Register (DDRx) is used to specify whether that pin is going to be an I/P or an O/P.

This can be done in two ways.

Method 1:

LDI R16, 0xEF    ; 0x is to indicate that the following number is in Hexa
OUT DDRB, R16    ; Set the DDR Register

In AVR Assembly, 0 in a data direction register specifies that it's an I/P and 1 specifies that it's an O/P.

EF in hexadecimal is 1110 1111 in binary. Which corresponds to assigning

So now PB4 is assigned as I/P and all other pins as O/P

The statement LDI R16, $EF -> means to load the value EF into the register R16 and the statement OUT DDRB, R16 -> means to assign the respective direction bits that specify the I/P and O/P.

Method 2:

SBI DDRB, 2    ; Set PB2 as Output
SBI DDRB, 3    ; Set PB3 as Output
CBI DDRB, 4    ; Set PB4 as Input

The above code does the following changes in the Data Direction Register:

The only change is that the bits other than 2, 3, and 4 are not defined

Enable Pull-Up Resistor:

SBI PORTB, 4    ; Enable pull-up resistor

It's important to do so because when the push-button is not pressed, the input pin of the MCU will be left hanging which means it's in the high-impedance state.

Now, let's check the state of the button and determine the state of the LEDs. Since we have pulled-up the input, we need to connect the other end of the switch to a logic HIGH (VCC)

L2: SBIS PINB, 4    ; Skips below statement if Push-button is not pressed
    RJMP L1
    SBI PORTB, 2    ; Turn ON LED -> PB2 if not pressed
    CBI PORTB, 3    ; Turn OFF LED -> PB3 if not pressed
    SBIC PINB, 4    ; Skips below statement if Push-button is pressed
    RJMP L2
L1: SBI PORTB, 3    ; Turn ON LED -> PB3 if pressed
    CBI PORTB, 2    ; Turn OFF LED -> PB2 if pressed
    RET

Working:

After jumping to SBI PORTB, 2 the consecutive statements are executed until the next jump at RJMP L2.

RET returns to the void loop ( ) of the main program and hence this code will start from the beginning.

The e-C way: (C code)

void setup() 
{
  // put your setup code here, to run once:
  pinMode (10, OUTPUT);         // GPIO 10 as Output
  pinMode (11, OUTPUT);         // GPIO 11 as Output
  pinMode (12, INPUT);          // GPIO 12 as Input
  pinMode (12, INPUT_PULLUP);   // Enable pull-up 
}

void loop() 
{
  // put your main code here, to run repeatedly:
  if ( digitalRead(12) == LOW ) // Read the state of button
  {                             // Execute this if button-pressed
    digitalWrite (10, LOW);     // LED -> PB2 OFF
    digitalWrite (11, HIGH);    // LED -> PB3 ON
  }
  else
  {                             // Execute this if button not pressed
    digitalWrite (10, HIGH);    // LED -> PB2 ON
    digitalWrite (11, LOW);     // LED -> PB3 OFF
  }
}

Upload your code:

There have to be two files to code in assembly language in the Arduino IDE. One.ino file to provide the basic info to the IDE to use the assembler and one.s file which contains the assembly code.

Please note that the .ino file and the .s file have to be inside the same folder with the same name.

Hit compile and then hit upload.

Congratulations! 🎉

You have successfully uploaded the assembly code into your Arduino Uno!

Final Result: