Embedded Systems

What You Will Learn

The difference between a microprocessor and a microcontroller
How I2C, SPI, and UART communication protocols work
What an RTOS interrupt is and why it matters
How embedded security differs from regular software security

What Is It?

An embedded system is a dedicated computer designed to perform specific tasks within a larger system. Examples include the controller in a microwave, the ECU in a car, or the chip inside a smartcard.

Embedded systems are everywhere — and they are increasingly connected to networks, making them targets for attackers.

Why It Matters

IoT devices often run embedded Linux or bare-metal RTOS
Many embedded systems have no security updates or patching mechanism
Firmware extraction and analysis is key to finding vulnerabilities
Hardware attacks (JTAG, UART) are often the only way to gain initial access

Microprocessor vs Microcontroller

	Microprocessor	Microcontroller
Description	CPU only — requires external RAM, ROM, peripherals	CPU + RAM + ROM + peripherals on one chip
Examples	Intel Core, ARM Cortex-A	STM32, Arduino, PIC, AVR
Use case	General purpose computing	Embedded, real-time control
Cost	Higher	Lower
Power	Higher	Lower

Communication Protocols

I2C (Inter-Integrated Circuit)

I2C is a two-wire serial protocol especially used for sensors and peripheral chips:

SDA (Serial Data) — bidirectional data line
SCL (Serial Clock) — clock signal from master

Devices are addressed by a 7-bit address. A master initiates all communication.

Master → START → ADDRESS + R/W → ACK ← Slave
Master → DATA  → ACK ← Slave
Master → STOP

Common I2C devices: temperature sensors, EEPROM chips, displays.

SPI (Serial Peripheral Interface)

SPI is a four-wire full-duplex protocol for high-speed communication:

MOSI (Master Out, Slave In)
MISO (Master In, Slave Out)
SCLK (Clock)
CS/SS (Chip Select / Slave Select)

SPI is faster than I2C but uses more wires. Common for flash chips, SD cards, and ADCs.

UART (Universal Asynchronous Receiver/Transmitter)

UART is bidirectional serial communication between exactly two nodes. It is asynchronous (no shared clock). Both sides must agree on baud rate (e.g., 115200 bps).

UART is the most common debug interface on embedded Linux systems. Connect to it with a USB-to-serial adapter to get a root shell on many IoT devices.

# Connect with minicom
minicom -D /dev/ttyUSB0 -b 115200

# Or screen
screen /dev/ttyUSB0 115200

RTOS Interrupts

An interrupt signals the CPU that a hardware event has occurred (e.g., a button press, timer expiry, data received on UART). The CPU pauses its current work, runs the Interrupt Service Routine (ISR), then resumes.

Key rules for ISRs:

Must be very short — they block the scheduler
Must not block or sleep
Use deferred processing (FreeRTOS tasks) for heavier work

// FreeRTOS example: wake a task from ISR
void UART_IRQHandler(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    vTaskNotifyGiveFromISR(xHandleTask, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

Embedded Security Testing

JTAG

JTAG is a hardware debug interface on most chips. With a JTAG probe, you can:

Read and write memory
Set breakpoints and step through code
Dump flash memory (extract firmware)
Bypass secure boot in some cases

# OpenOCD — JTAG interface tool
openocd -f interface/jlink.cfg -f target/stm32f1x.cfg

# Read flash memory
flash read_bank 0 firmware.bin

UART Debug Console

Many devices expose a root shell over UART:

# Find UART pins with a logic analyzer
# Common voltage: 3.3V logic
# Identify TX, RX, GND pins
# Connect USB-to-serial adapter
screen /dev/ttyUSB0 115200

Firmware Analysis

# Extract firmware from a binary image
binwalk -e firmware.bin

# Find embedded filesystems
binwalk -A firmware.bin

# Search for passwords
grep -r "password" ./extracted_firmware/