FreeRTOS Countdown Timer

A FreeRTOS-based countdown timer for PIC24 microcontrollers. It supports time entry over UART, LED feedback, potentiometer-controlled brightness, and pause/resume control.

Overview

The system runs a simple state machine that controls a countdown timer. Time is entered through a UART terminal, buttons control the flow, and LEDs provide visual feedback. FreeRTOS handles tasks, timing, and concurrency.

Target: PIC24 (dsPIC33/PIC24 family)
RTOS: FreeRTOS

Hardware Requirements

Microcontroller

• PIC24 or dsPIC33
• 16 MHz input (configured to 96 MHz PLL)

Peripherals

Push Buttons:

• PB1: Enter time mode
• PB2: Used with PB3 to start
• PB3: Pause, resume, abort

LEDs:

• LED0 (RB5): Completion
• LED1 (RB6): Countdown blink
• LED2 (RB7): PWM brightness

Other:

• Potentiometer: AN5 (RA3)
• UART2: For all terminal IO

Pin Mapping

All pin definitions are in hw_config.h:

• LED0: RB5
• LED1: RB6
• LED2: RB7
• ADC: AN5 (RA3)

Features

• State-based operation
• UART time entry in MM:SS
• One-second accurate countdown
• LED animations for every state
• Software PWM for LED2
• ADC brightness control
• Pause/resume
• Abort via long press
• Optional info display (ADC + duty cycle)
• LED2 mode toggle (solid or blink)
• Backspace support

State Machine

WAITING

• LED2 pulses
• PB1 starts time entry
• Terminal shows a welcome message

ENTER_TIME

• User enters MM:SS over UART
• Input is echoed
• Validates minutes and seconds
• Enter stores time
• PB2+PB3 long press clears input

READY

• LEDs off
• PB2+PB3 click starts the countdown
• PB2+PB3 long press clears the stored time

COUNTDOWN

• LED1 blinks
• LED2 brightness controlled by ADC
• PB3 click pauses
• PB3 long press aborts
• 'i' toggles extended info
• 'b' toggles LED2 mode
• Terminal overwrites the same line with remaining time

PAUSED

• LEDs freeze
• ADC still updates LED2 brightness
• Same controls as COUNTDOWN

FINISHED

• LED0 and LED1 alternate for 5 seconds
• LED2 stays solid
• Shows completion message
• Auto-returns to WAITING

Building

Requirements

• MPLAB X
• XC16
• FreeRTOS source

Using Make

make clean
make build

Using MPLAB X

1. Open project
2. Select default config
3. Build (F11)

Output Files

• Debug: dist/default/debug/*.elf
• Production: dist/default/production/*.hex

Usage

Startup

1. Flash the hex file
2. Open UART2 at 115200 baud
3. Power the board

Starting a Timer

1. Ensure system is in WAITING (LED2 pulsing)
2. PB1 enters time input
3. Enter MM:SS
4. Press Enter
5. PB2+PB3 starts the countdown

During Countdown

• Pause: PB3 click
• Resume: PB3 click
• Abort: PB3 long press
• Change brightness: turn potentiometer
• Toggle info: press 'i'
• Toggle LED2 mode: press 'b'

Completion

• LEDs go into the finish pattern
• System resets after 5 seconds

Terminal Commands

Key	Function
MM:SS	Enter time
Enter	Submit
Backspace	Remove last char
i	Show/hide ADC + duty cycle
b	Toggle LED2 mode

Button Summary

Action	Buttons	Function
Enter time	PB1	Start input
Start countdown	PB2 + PB3	Begin countdown
Clear time	PB2 + PB3 (hold)	Reset input
Pause/Resume	PB3	Toggle pause
Abort	PB3 (hold)	Stop immediately

Project Structure

freertos-start.X 2/
├── main.c
├── app.h
├── hw_config.h
├── FreeRTOSConfig.h
│
├── adc.c / adc.h
├── buttons.c / buttons.h
├── pwm.c / pwm.h
├── uart.c / uart.h
│
├── FreeRTOS/
│   ├── include/
│   ├── portable/
│   └── *.c
│
├── build/
├── dist/
└── nbproject/

Key Files

• main.c: Tasks and state machine
• app.h: Global definitions, RTOS objects
• hw_config.h: Pin definitions and hardware macros
• adc.c: AN5 sampling and conversion
• pwm.c: Software PWM
• buttons.c: Debouncing, click/long-press detection

Technical Details

FreeRTOS

• Max priority: 3
• Tick rate: 1 kHz
• Static allocation (heap_1)
• PIC24/dsPIC33 port

Task Priorities

• 3: PWM
• 2: Countdown, Buttons
• 1: Time input, Waiting
• 0: ADC, Idle

Timing

• Countdown resolution: 1 s
• LED1 blink: 1 Hz
• LED2 PWM: defined in pwm.c
• Debounce: ~50 ms
• Long press: >1 s
• Finish sequence: 5 s

ADC

• 10-bit
• AN5
• Manual sample/convert
• ~20 microsecond conversion time

PWM

• Software-driven
• Duty cycle mapped from ADC
• Used for pulsing and brightness

Inter-Task Communication

• Queues: Button events, UART RX
• Semaphores: start signals
• Mutexes: UART printing, state, countdown value

Memory

• All tasks have fixed stack sizes
• No dynamic allocation during runtime

Troubleshooting

Countdown not starting

• Time not validated
• PB2+PB3 not pressed together
• Not in READY state

LED2 not reacting

• Check ADC wiring
• Check AN5 config
• Press 'i' to verify ADC values

UART issues

• Confirm 115200 baud
• Check TX/RX pins
• Terminal must send CR+LF

Buttons not responding

• Check debounce logic
• Verify pin setup
• Ensure button task is active