--- title: Logic Levels and Interface Physical Layers url: https://doc.liz6.com/en/hardware/06-interfaces-and-communication/01-logic-levels-and-interface-physical-layers locale: en area: hardware tags: - hardware - interfaces-and-communication date: 2026-06-30 modified: 2026-07-16 description: 'Logic Levels and Interface Physical Layers Why Logic Levels Are Needed When two chips communicate, they must agree on: What voltage counts as "1"? What counts a…' --- # Logic Levels and Interface Physical Layers ## Why Logic Levels Are Needed When two chips communicate, they must agree on: What voltage counts as "1"? What counts as "0"? What is the drive capability? What is the noise margin? Mismatches lead to communication failures or even chip damage. --- ## Single-Ended Signals ### TTL (Transistor-Transistor Logic) ``` The oldest standard (74 series) Vcc: 5V ± 10% VOH > 2.4V, VOL < 0.4V VIH > 2.0V, VIL < 0.8V Noise margin is approx. 0.4V (small!) Output drive: Weak sourcing (~0.4mA), Strong sinking (~16mA) Being replaced by CMOS, but 3.3V is still common ``` ### CMOS (Complementary Metal-Oxide-Semiconductor) ``` Modern mainstream logic levels Vcc = 5V: VOH > Vcc-0.1V, VOL < 0.1V (Almost rail-to-rail!) VIH > 0.7×Vcc, VIL < 0.3×Vcc Vcc = 3.3V: VIH > 2.0V, VIL < 0.8V Vcc = 1.8V: VIH > 0.65×Vcc, VIL < 0.35×Vcc Large noise margin → Good interference immunity Extremely low static power consumption (only consumes power during switching) Note: 3.3V CMOS output can drive 5V TTL input (VIH=2.0V) But 5V output cannot be directly connected to 3.3V CMOS input (may cause damage or latch-up) ``` --- ## Differential Signals ### Why Differential ### LVDS (Low Voltage Differential Signaling) ``` Swing: ~350mV (differential) Common-mode voltage: ~1.2V Speed: Hundreds of Mbps to several Gbps Termination: 100Ω differential termination resistor Power consumption: Extremely low (constant current drive ~3.5mA) Applications: Displays (FPD-Link), Cameras (MIPI D-PHY), Backplanes ``` ### RS-232 — Classic Serial Port ``` Single-ended, Full-duplex Logic: 1 = -3~-15V, 0 = +3~+15V (Note: Negative voltage is Logic 1!) Drive capability: Long-line drive (~15m) High voltage, large noise margin, but slow speed (≤115.2kbps) Level Shifter: MAX232 (TTL ↔ RS-232) DB9 Pins: Pin2: RXD (Receive) Pin3: TXD (Transmit) Pin5: GND ``` ### RS-485 — Mainstay of Industrial Communication ``` Differential, Half-duplex or Full-duplex Swing: > 1.5V (differential) Common-mode range: -7V ~ +12V (Large! Tolerates ground potential differences) Distance: Up to 1200m (at low speeds) Speed: Up to ~50Mbps (short distances) Multi-point: 32~256 nodes can be connected on one bus Termination: 120Ω differential termination resistor (one at each end) Applications: MODBUS, PROFIBUS, DMX512 Chips: MAX485, SN75176 ``` ### CAN Bus ``` Differential, Multi-master, Priority Arbitration Dominant (0): CANH-CANL > 0.9V Recessive (1): CANH-CANL ≈ 0V Speed: Up to 1Mbps (CAN FD up to 8Mbps) Fault Tolerance: Extremely strong (Error detection + Auto-retransmission + Fault isolation) Termination: 120Ω at each end Applications: Automotive (OBD-II), Industrial Control, Robotics ``` --- ## Open-Drain / Open-Collector ``` Vpullup │ Rpullup │ Internal ├── SDA/Signal Line ┌────┐ │ │ │───┐ │ │ GPIO│ NMOS │ │ │───┘ │ └────┘ │ GND NMOS ON → Pulls Low (Strong drive capability) NMOS OFF → Pulled High by Pull-up Resistor (Weak drive capability) Applications: - I2C (Multi-master shared bus) - Wire-AND: Multiple open-drain outputs can be directly connected; if any pulls the bus low, it is low - Level Shifting: Changing Vpullup changes the output high level ``` --- ## Level Shifting ### Scenarios ``` 3.3V MCU connecting to 5V Peripheral — Most Common! 1.8V Sensor connecting to 3.3V MCU ``` ### Solutions ``` 1. Resistor Voltage Divider (Step-down only): 3.3→1.8, 5→3.3 ──┤├───┬── Simple but weak pull-up, slow speed ├── Signal ──┤├───┘ 2. Dedicated Level Shifter Chips: TXB0104 (Auto-direction, Low Speed) TXS0108E (Auto-direction, Open-Drain Compatible) SN74LVC1T45 (Direction Control, High Speed) 3. MOSFET Solution (Bi-directional, Low Speed): VCC_LOW VCC_HIGH │ │ Rp Rp │ │ LOW ─────┤ S D ├── HIGH │ ──┬── │ │ │G │ │ │ │ GND GND GND ``` --- ## Quick Reference for Common Levels | Standard | Diff/Single | Swing | Max Speed | Distance | Application | |------|-------|------|----------|------|------| | TTL 5V | Single-Ended | 0~5V | ~50MHz | PCB | Traditional Digital | | CMOS 3.3V | Single-Ended | 0~3.3V | ~100MHz | PCB | Mainstream Digital | | CMOS 1.8V | Single-Ended | 0~1.8V | ~200MHz | PCB | Low Power | | LVDS | Differential | ±350mV | Gbps | ~10m | Display/High Speed | | RS-232 | Single-Ended | ±12V | 115kbps | 15m | Debug Serial | | RS-485 | Differential | >1.5V | 50Mbps | 1200m | Industrial | | CAN | Differential | >0.9V | 8Mbps | ~1km | Automotive | | USB 2.0 | Differential | ±400mV | 480Mbps | 5m | PC Peripherals | --- *Keywords: TTL, CMOS, LVDS, Differential Signals, Open-Drain, RS-232, RS-485, CAN, Level Shifting*