AVR5: Control Brightness with PWM

In this scenario, you will control the LED brightness using PWM. To compare results, the built-in LED will be set to 100% on while you adjust LED4's brightness.

Prerequisites
You need to book one of the AVR laboratory nodes and ensure the video stream is live.
Get familiar with the scenario: AVR1: Hello World on AVR.

Scenario
Create an application that toggles the built-in LED (GPIO 13) on permanently, and then uses a timer (Timer1, channel B) to control LED4 (GPIO 10). The LED4 should present a heartbeat pattern.

Result
Observe heart beat on LED4 (GPIO 10).

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 a template for your application that includes declarations for GPIO and a timer. We use Timer1 (16-bit) in CTC mode:

; --- Register Address Mapping ---
.equ SREG,    0x3F      ; Status Register
.equ SPH,     0x3E      ; Stack Pointer High
.equ SPL,     0x3D      ; Stack Pointer Low
 
.equ DDRB,    0x04      ; Data Direction Port B
.equ PINB,    0x03      ; Input Pins Port B (Toggle Shortcut)
.equ LED_PIN, 5         ; Built-in LED (Digital 13 / PB5)
 
.equ RAMEND,  0x08FF
; --- Timer1 Register Mapping ---
.equ TCCR1A,  0x80      ; Timer1 Control A
.equ TCCR1B,  0x81      ; Timer1 Control B
.equ OCR1AH,  0x89      ; Output Compare High
.equ OCR1AL,  0x88      ; Output Compare Low
.equ TIMSK1,  0x6F      ; Timer1 Interrupt Mask
.equ TOP_VAL, 0x3D08    ; It is for 1s, adjust for 2s
 
.equ WGM12, 3           ; WGM12 is on bit 3 of the TCCR1B
.equ CS12,  2            ; CS12 is on bit 2 of the TCCR1B
.equ CS10,  0            ; CS10 is on bit 0 of the TCCR1B
 
.section .text
.org 0x0000
    rjmp RESET          
.org 0x002C             ; Assembler treats .org as bytes, use 0x002C
    rjmp TIMER_ISR
.org 0x0034             ; Jump past vectors to start logic
 
 
RESET:
 
; --- Idle Main Loop ---
LOOP:
    rjmp LOOP

Step 2
Initialise stack (obligatory, we use ISR function calls) and configure BP5 (GPIO 13) as output:

...
    ; Prepare stack
   ldi r16, hi8(RAMEND)
   out SPH, r16
   ldi r16, lo8(RAMEND)
   out SPL, r16
 
    ; Configure PB5 (built-in LED, GPIO 13) as Output
    sbi DDRB, LED_PIN
...

Note the different method for setting GPIO 13 as an output compared to the AVR1: Hello World on AVR scenario.

Step 3
Configure timer and enable interrupts:

...
    ; Set Timer1 in CTC Mode
    ; Load the compare value for 1 second
    ldi r16, hi8(TOP_VAL)
    sts OCR1AH, r16
    ldi r16, lo8(TOP_VAL)
    sts OCR1AL, r16
 
    ; TCCR1A: Default (0x00)
    ldi r16, 0x00
    sts TCCR1A, r16
 
    ; TCCR1B: 
    ; Bit 3 (WGM12) = 1 (CTC Mode)
    ; Bit 2 (CS12)  = 1 (Prescaler 1024)
    ; Bit 0 (CS10)  = 1 (Prescaler 1024)
    ldi r16, (1 << WGM12) | (1 << CS12) | (1 << CS10)
    sts TCCR1B, r16
 
    ; 4. Enable Compare Match A Interrupt
    ldi r16, (1 << 1)   ; OCIE1A bit
    sts TIMSK1, r16
 
    ; 5. Global Interrupt Enable
    sei
...

Step 4
Your loop function remains idle, but you need an ISR implementation, e.g. like this:

TIMER_ISR:
    ; Toggle LED using the PINB write trick.
    ; Since 'sbi' does not modify SREG flags, we do not need 
    ; to save/restore context (push/pop r16, etc).
    sbi PINB, LED_PIN
    reti

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 3s.

FAQ
When using the printed version of this manual, please refer to the latest online version for 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!