AVR Assembly Code Subroutines in Arduino Programming

You can't just import an .asm assembly program directly into the Arduino IDE. It does have the ability to handle blocks of assembly language. Let's see how.

Open a new sketch

Save as ASMcount. Replace the default code with this C language program.

// C program sketch declares 2 functions - start and forever
// the functions are defined externally in S subroutine file
extern "C" {
  // function prototypes
  void start();
  void forever();
}
void setup(){ start(); }
void loop() { forever(); }

Arduino IDE will know what to do.

Notepad assembly.S

Use notepad text editor to create and save a file in the same folder as our Arduino sketch. The filename extension needs to be capital .S. Text written in this file will be compiled as assembler subroutine code.

; Arduino sketch .S subroutine written in AVR assembly code
#define __SFR_OFFSET 0x20
#include "avr/io.h"
.global start
.global forever

start:
  LDI R16, 0xFF     ; Setting all PORTB as output
  STS DDRB, R16
  LDI R17, 0x00
  STS PORTB, R17    ; Writing initial value 0 to PORTB
  LDI R16, 0x00
  STS TCCR1A, R16   ; Setting all bits of TCCR1A as 0
  RET

forever: 
  LDI R16, 0xF8
  STS TCNT1H, R16   ; Writing loop delay 0xF8 to TCNT1H (8bit)
  LDI R16, 0xFB
  STS TCNT1L, R16   ; Writing loop delay 0xFB to TCNT1L (8bit)
  LDI R16, 0x05
  STS TCCR1B, R16   ; Writing 0x05 into TCCR1B
L:LDS R0, TIFR1     ; Load the value of TIFR1 into R0
  SBRS R0, 0        ; Skip the next statement if overflow 
  RJMP L            ; Loop until overflow occurs.
  LDI R16, 0x00
  STS TCCR1B, R16   ; Stop the Timer/Counter1
  LDI R16, 0x01
  STS TIFR1, R16    ; Clear the overflow flag by writing 1 
  
  ; increment PORTB blinks led
  INC R17           ; Increment R17 register
  STS PORTB, R17    ; Toggle the LED output
  RET               ; return to label forever:

Windows file explorer should look like this. We will find the subroutine file easily. These are small files.

Menu Sketch->Add File

Click on the Sketch menu at the top and Add File. Locate our assembly.S file.

Select the file to add to our sketch. Now we can edit the file using Arduino IDE.

Compile

Click the checkmark, or use the menu. You can be viewing either file. Verbose preferences allow you to see more detailed build and upload messages. Just like a real IDE! See how compact the code is?

Assembly Code?

Beginning with the label start: we see assembly instructions that will be interpreted by our AVR-GCC toolchain. The Arduino IDE handles whether we compile for an ATmega328P chip in a Uno or some other AVR processor.

start: and forever: just like setup( ) and loop( ) in Arduino language. We setup the IO of PORTB, set to 0 and set a timer to 0.

Alphabet Soup

Variables like DDRB and PORTB are known by the Arduino IDE compiler because the line #include <avr/io.h> adds C/C++ header files with lines such as #define to provide numeric values. Assembly programs use definition files filled with .equ statements to do the same.

Either approach provides a map of the processor chip and where to find resources such as registers. If you are porting your assembly code from Atmel Studio then .equ and .def statements may conflict. Try commenting them out to see if it reduces error messages.

Next we give values to two timing controller circuits TCNT1H and TCNT1L. Like PORTB they are SFR special function registers. 8bit latching circuits that will retain a binary number.

All of these details are documented on the website for the ATmega328P processor chip.

forever:

Sounds impressive. It is just a label that allows our loop to come back to the beginning. Using 0xF8 for timer controller TCNT1H and 0xFB for TCNT1L gives us a blink rate of about two seconds. Try other values and see the speed change.

Upload

Arduino IDE will upload this code to a Uno board. Add more LEDs to the other pins and watch a binary count.

Blink Lights

A binary count toggles the pins of PORTB.

More Info

I am using code borrowed from Yeshvant Muniraj's Arduino Blink Example in Assembly Using Timer an excellent project! Perform Accessing IO in ATmega328 (Arduino) for more details.