Serial Bus Protocols

Parallel vs. Serial

Parallel: Multiple lines transmit multiple bits simultaneously (8/16/32 bits at once)
          Fast but requires many wires; clock skew limits speed

Serial: Single line transmits bits sequentially
          Fewer wires, supports higher frequencies → Modern high-speed interfaces are all serial

UART (Universal Asynchronous Receiver-Transmitter)

Basic Concepts

Parameters

Applications

Debug Serial Port (printf output to PC)
GPS/Bluetooth Modules (AT Commands)
Legacy Peripheral Communication

Pros: Simple, cheap, reliable
Cons: Point-to-point only, low speed, no standard application layer

I2C (Inter-Integrated Circuit)

Basic Concepts

Synchronous, Half-Duplex, Multi-Master Multi-Slave
2 Wires: SCL (Clock) + SDA (Data)

    Vcc
     │
    Rp  Rp          ← Pull-up Resistors (Required!)
     │   │
     ├───┼────────── SDA
     │   │
    ┌┴┐ ┌┴┐
    │Mst│ │Slv1│ │Slv2│ ...
    └─┘ └─┬┘ └─┬┘
          │    │
          ├────┼──── SCL
          │    │
         GND  GND

SCL: Clock provided by Master
SDA: Bidirectional Data (Open-Drain, Wired-AND)
Each Slave has a unique 7-bit (or 10-bit) address

Communication Process

Start Condition: SDA↓ while SCL is High
  Send: 7-bit Address + R/W bit → Slave responds with ACK
  Transfer: 8-bit Data + ACK ... (can be continuous)
Stop Condition: SDA↑ while SCL is High

ACK: SDA=0 on the 9th clock cycle (Receiver Acknowledges)
NACK: SDA=1 on the 9th clock cycle (No Acknowledge)

Speed Modes:
  Standard: 100 kHz
  Fast:     400 kHz  (Most Common)
  Fast+:    1 MHz
  High Speed: 3.4 MHz

Applications

Sensors (Temperature/Humidity/IMU/Light...)
EEPROM/FRAM Memory
RTC (Real-Time Clock Chips)
PMIC (Power Management ICs)
OLED Displays
Almost all low-speed peripherals

Pros: Only 2 wires, multiple devices share bus
Cons: Slow speed, pull-up design issues (power consumption), address conflicts

Pull-up Resistor Selection

R too small → High power consumption (large sink current)
R too large → Slow rising edge → Speed limited

Rule of Thumb:
  100kHz: 4.7kΩ
  400kHz: 2.2kΩ
  1MHz:   1kΩ or smaller

Calculation: Rmax = tr / (0.8473 × Cbus)
     tr = 300ns (for 100kHz mode)
     Cbus = Bus Capacitance (Chip + PCB Traces)

SPI (Serial Peripheral Interface)

Basic Concepts

Clock Modes (CPOL, CPHA)

CPOL=0: SCK Idle Low
CPOL=1: SCK Idle High
CPHA=0: Sampling on first edge
CPHA=1: Sampling on second edge

Most Common: Mode 0 (CPOL=0, CPHA=0) and Mode 3 (CPOL=1, CPHA=1)

Both sides must match! Otherwise communication will fail.

Characteristics

Pros:
  ✓ Full-Duplex (Simultaneous TX/RX)
  ✓ High Speed (Tens of MHz, even 100MHz+)
  ✓ No Address Mechanism → No Overhead
  ✓ Flexible (Arbitrary frame length)

Cons:
  ✗ Many Wires (n Slaves = n+3 wires)
  ✗ No Standard Protocol Layer (Only Physical/Data Link defined)
  ✗ No Slave Flow Control (ACK)
  ✗ Short Distance (PCB-level)

Applications: Flash (NOR/NAND), ADC/DAC, Displays, Ethernet PHY

Comparison of the Three Buses

Quick Reference: When to Choose What

Sensor Reading (Low Speed, Many Devices) → I2C
High-Speed Data Transfer (Flash/SD/Display) → SPI
Debug Info, Wireless Modules → UART
Multi-byte Register Read/Write → I2C (Address+Data) or SPI
Battery Powered (Lowest Power) → I2C (Near-zero static power)
Anti-Interference (Long Distance Differential) → RS-485 (Physical Layer Variant of UART)

Keywords: UART, I2C, SPI, Baud Rate, SCL, SDA, MOSI, MISO, CS, Pull-up Resistor, ACK