AVR1: Hello World on AVR

In this scenario, you will implement a classical “Hello World” application. For microcontrollers, the “Hello World” application is a blinking LED. Because we use remote observation via an integrated video camera stream, the proper LED frequency is at most 1 or 2Hz, depending on the connection quality. In this scenario, you will use blocking nested loops and manual calculation of the ticks needed to run them, to obtain precise delay, as presented in the chapter Introduction to the Arduino Uno programming in Assembler.\\

Prerequisites
You need to book one of the AVR laboratory nodes and ensure the video stream is live.

Scenario
Create an application that toggles the built-in LED on and off 1 time per second (the full cycle is then 2s). There are 4 LEDs on the shield you can observe; use GPIO 13 (the built-in LED on the Arduino Uno).

Result
Flashing LED, observable via video stream.

Start
Mind to use AVR GCC syntax (as in the instruction): node compilation facilities are preconfigured, and you do not need to build a Makefile; still, it is necessary to follow the exact AVR GCC syntax, e.g., in the case of .equ.

Step 1
Compose an empty application with an infinite loop:

.org 0x0000
    rjmp RESET
 
RESET:
 
LOOP:
   rjmp LOOP

Compile and check for any errors. Correct errors (if any), re-compile, and upload to the device. Naturally, nothing will happen now: your code is dummy, but you will ensure proper toolkit operation.

Step 2
Add to your code declarations regarding GPIO control ports for GPIO 13:

.equ DDRB,  0x04
.equ PORTB, 0x05
.equ PB5, 5                 ; PB5 is GPIO 13, and it is a built-in LED
...

Step 3
Configure GPIO 13 for output:

...
RESET:
    ldi r16, 1 << PB5        ; Set bit 5
    out DDRB, r16            ; Set PB5 as output 
...

Step 4
Prepare your delay function and place it at the end of the code (after 'rjmp LOOP'). Also, modify it accordingly to meet your requested delay time. Note that the CPU's base frequency is 16 MHz. Refer to the chapter Introduction to the Arduino Uno programming in Assembler on how to calculate the number of loops. The function below gives you approximately a 0.5s delay (506ms).

...
delay:
    ldi r20, 43     ; Outer loop
outer_loop:
    ldi r18, 250    ; Mid loop
mid_loop:
    ldi r19, 250    ; Inner loop
inner_loop:
    dec r19
    brne inner_loop
    dec r18
    brne mid_loop
    dec r20
    brne outer_loop
    ret

Step 5
Implement the main loop:

...
LOOP:
    sbi PORTB, PB5           ; Turn LED off
    rcall delay
    cbi PORTB, PB5           ; Turn LED on
    rcall delay
    rjmp LOOP
...

Result validation
The LED should be flashing on an implemented time basis. Note that some irregularity may is observed due to the nature of video streaming over the network. It is natural and OK. If you want to measure more precise timing, increase the period, e.g. to 5s.

FAQ
When using the printed version of this manual, please refer to the latest online version of this document to obtain the most up-to-date list of FAQs.

It does not flash: Did you compile and upload to the device? Those are separate steps: it is not enough to just compile, but you also need to “flash” the MCU. Also, check your video stream if it “ticks” - the time embedded into the video stream should change. Your code may be working OK, but the video stream can be frozen, so you cannot see it working properly!

I need a longer delay: To obtain a delay function with a period of about 2.5s, you need to introduce a fourth loop (an outer loop) in the delay; 3 will not provide you enough ticks. Eventually, you can switch to 16-bit counters.