What it does: This project consists of a small autonomous vehicle capable of maintaining a constant safe distance from a target object or vehicle in front of it.

Its purpose: The main goal is to implement a real-time closed-loop control system (Adaptive Cruise Control) on an 8-bit microcontroller. It demonstrates bare-metal embedded programming by utilizing hardware interrupts, hardware timers, and custom serial protocols, completely avoiding high-level abstractions like Arduino libraries.

The starting idea: The idea is inspired by modern Advanced Driver Assistance Systems (ADAS) used in the automotive industry to prevent rear-end collisions and improve driving comfort.

Why it is useful: It serves as a highly practical demonstration of low-level hardware control, bridging the gap between theoretical micro-processor architecture and real-world physical applications.

The system uses an ultrasonic sensor mounted on the front to continuously measure the distance to the obstacle ahead.

Based on the calculated distance, a proportional control algorithm calculates the necessary speed adjustments to maintain a safe gap. The system smoothly interpolates the motor speed using hardware PWM based on the target's distance. If the target is too close (under 25 cm), the vehicle applies the brakes. If the target is within the tracking range (25 cm - 85 cm), it adjusts the speed dynamically to follow it.

Block Diagram:

List of components:

• Arduino UNO (ATmega328P Microcontroller) serving as the main processing unit.
• L293D Motor Drive Shield (featuring the 74HC595 shift register).
• 1x HC-SR04 Ultrasonic Sensor.
• 2x DC TT Motors (Rear-wheel or 4-wheel drive configuration).
• 2x 18650 Li-Ion Batteries (7.4V total) with holder.
• Robot car chassis with wheels.
• Dupont connecting wires.

Electrical Schematic:

#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>
#include <stdio.h>
 
#define PM_BAUD 9600
 
#define TRIG_PIN PC0
#define ECHO_PIN PC1
 
#define LATCH_PIN PB4
#define CLK_PIN   PD4
#define EN_PIN    PD7
#define DATA_PIN  PB0
 
#define M1_A 2
#define M1_B 3
#define M2_A 1
#define M2_B 4
 
volatile uint16_t timer_val = 0;
volatile uint8_t echo_done = 0;
 
static int _usart0_putchar(char c, FILE *stream) {
    if (c == '\n') _usart0_putchar('\r', stream);
    while (!(UCSR0A & (1<<UDRE0)));
    UDR0 = c;
    return 0;
}
 
static FILE USART0_stdout = FDEV_SETUP_STREAM(_usart0_putchar, NULL, _FDEV_SETUP_WRITE);
 
void usart_init() {
    UBRR0H = (unsigned char)(103>>8); //br9600
    UBRR0L = (unsigned char)103;
    UCSR0B = (1<<RXEN0) | (1<<TXEN0); //act transmisia
    UCSR0C = (1<<USBS0) | (3<<UCSZ00); //8b
    stdout = &USART0_stdout;
}
 
void shift_out_data(uint8_t data) {
    PORTB &= ~(1<<LATCH_PIN); 
 
    for(int i=0; i<8; i++) { //pt fiecare bit
        PORTD &= ~(1<<CLK_PIN);
        if(data & (1 << (7-i))) {
            PORTB |= (1<<DATA_PIN); //punem val d
        } else {
            PORTB &= ~(1<<DATA_PIN);
        }
        PORTD |= (1<<CLK_PIN); //ridicam si coboram pin clock
    }
 
    PORTB |= (1<<LATCH_PIN); 
}
 
void motors_init() {
    DDRB |= (1<<LATCH_PIN) | (1<<DATA_PIN) | (1<<PB3);
    DDRD |= (1<<CLK_PIN) | (1<<EN_PIN) | (1<<PD3);
 
    PORTD &= ~(1<<EN_PIN);
 
    TCCR2A = (1<<COM2A1) | (1<<COM2B1) | (1<<WGM21) | (1<<WGM20);
    TCCR2B = (1<<CS22);
 
    OCR2A = 0; // punem viteza in registrii
    OCR2B = 0;
 
    shift_out_data(0);
}
 
void set_motor_dir(int m1_fwd, int m1_rev, int m2_fwd, int m2_rev) {
    uint8_t shift_data = 0;
    if (m1_fwd) shift_data |= (1<<M1_A);
    if (m1_rev) shift_data |= (1<<M1_B);
    if (m2_fwd) shift_data |= (1<<M2_A);
    if (m2_rev) shift_data |= (1<<M2_B);
    shift_out_data(shift_data);
}
 
void sonar_init() {
    DDRC |= (1<<TRIG_PIN); //pc0 output
    DDRC &= ~(1<<ECHO_PIN); 
 
    TCCR1A = 0;
    TCCR1B = (1<<CS11);
 
    PCICR |= (1<<PCIE1);
    PCMSK1 |= (1<<PCINT9);
}
 
ISR(PCINT1_vect) {
    if (PINC & (1<<ECHO_PIN)) {
        TCNT1 = 0;
    } else {
        timer_val = TCNT1;
        echo_done = 1;
    }
}
 
int get_distance() {
    echo_done = 0;
 
    PORTC &= ~(1<<TRIG_PIN);
    _delay_us(2);
    PORTC |= (1<<TRIG_PIN);
    _delay_us(10);
    PORTC &= ~(1<<TRIG_PIN);
 
    uint32_t timeout = 60000;
    while(!echo_done && timeout > 0) {
        timeout--;
    }
 
    if(timeout == 0) return 400;
 
    return timer_val / 116;
}
 
int map_speed(int x, int in_min, int in_max, int out_min, int out_max) {
    return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}
 
int main() {
    usart_init();
    motors_init();
    sonar_init();
 
    sei();
    printf("ACC System Bare Metal Started\n");
 
    int safe_dist = 25;
    int max_dist = 85;
 
    while(1) {
        int dist = get_distance();
        if(dist == 0 || dist > 400) dist = 400;
 
        printf("Dist: %d cm\n", dist);
 
        if (dist > max_dist) {
            set_motor_dir(0, 0, 0, 0);
            OCR2A = 0;
            OCR2B = 0;
        } else if (dist > safe_dist) {
            int speed = map_speed(dist, safe_dist, max_dist, 115, 220);
            set_motor_dir(1, 0, 1, 0);
            OCR2A = speed;
            OCR2B = speed;
        } else {
            set_motor_dir(0, 0, 0, 0);
            OCR2A = 0;
            OCR2B = 0;
        }
 
        _delay_ms(50);
    }
    return 0;
}

Development Environment:

• VS Code with PlatformIO (using pure C and register-level programming).

Libraries and 3rd-party sources:

• Standard AVR libc (<avr/io.h>, <avr/interrupt.h>, <util/delay.h>, <stdio.h>).
• Zero high-level Arduino libraries (like `analogWrite`, `digitalWrite`, or `pulseIn`) were used.

Algorithms and Hardware Features utilized:

• Bit-Banging for Shift Register: The L293D shield uses a 74HC595 shift register to control motor directions. A custom synchronous serial protocol was implemented (`shift_out_data`) to manually push bits to the latch, clock, and data pins using direct port manipulation (`PORTB`, `PORTD`).
• Hardware Timers (Fast PWM): Timer2 was configured at the register level (`TCCR2A`, `TCCR2B`) to generate a pure hardware Fast PWM signal on pins OC2A and OC2B. This controls the motor speed without utilizing CPU cycles.
• Pin Change Interrupts (PCINT) & Timer1: To avoid blocking the CPU while waiting for the ultrasonic echo, the Echo pin was mapped to a Pin Change Interrupt (`PCINT1_vect`). Timer1 counts the micro-seconds in the background, making the distance reading asynchronous and highly efficient.
• Custom USART Stream: The USART module was initialized manually (`UBRR0`, `UCSR0`), and the standard output stream (`stdout`) was redirected to the serial port, allowing the use of `printf()` for clean, professional real-time debugging.

The system reacts accurately and smoothly to dynamic environments.

• The custom non-blocking ultrasonic reading accurately updates the distance without hanging the main loop.
• The proportional mapping successfully overcomes the static friction of the DC motors by setting a calculated minimum PWM threshold (115), ensuring the car starts moving reliably even at low speeds.
• The car smoothly decelerates as it approaches the 25 cm safety threshold and completely halts to prevent collisions.

Final Project Photo:

Implementing this project in a bare-metal C environment provided profound insights into the internal workings of the ATmega328P microcontroller. Stripping away the Arduino framework forced a deeper understanding of memory mapping, the crucial difference between polling and hardware interrupts, and the complexity of generating custom serial signals (bit-banging). The project successfully replicates an industrial ADAS concept on an 8-bit architecture.

Project Archive - Source Code and PlatformIO Configuration

• 09.05.2026: Chosen the project topic and completed the initial OCW documentation.
• 17.05.2026: Assembled the hardware components, tested the L293D motor shield, and resolved DC motor wiring issues.
• 19.05.2026: Fully migrated to VS Code / PlatformIO. Successfully implemented bare-metal code integrating PCINT for the HC-SR04 sensor, Timer2 Fast PWM for motor speed, and custom bit-banging for directional control. Tuned the PWM thresholds to overcome static friction.

Hardware Resources:

• ATmega328P Datasheet (Microchip Technology)
• HC-SR04 Ultrasonic Sensor Specifications
• L293D Motor Shield Schematic & 74HC595 Datasheet

Software Resources:

• AVR Libc Reference Manual
• PlatformIO Documentation